Simultaneous expression of different transgenes in neurons and glia by combining in utero electroporation with the Tol2 transposon-mediated gene transfer system

Targeted approaches to delineate neuronal morphology during early development

Understanding the developmental changes that affect neurons is a key step in exploring the assembly and maturation of neural circuits in the brain. For decades, researchers have used a number of labeling techniques to visualize neuronal morphology at different stages of development. However, the efficiency and accuracy of neuronal labeling technologies are limited by the complexity and fragility of neonatal brains. In this review, we illustrate the various labeling techniques utilized for examining the neurogenesis and morphological changes occurring during the early stages of development. We compare the advantages and limitations of each technique from different aspects. Then, we highlight the gaps remaining in our understanding of the structure of neurons in the neonatal mouse brain.

Erratic and blood vessel-guided migration of astrocyte progenitors in the cerebral cortex

Astrocytes are one of the most abundant cell types in the mammalian brain. They play essential roles in synapse formation, maturation, and elimination. However, how astrocytes migrate into the gray matter to accomplish these processes is poorly understood. Here, we show that, by combinational analyses of in vitro and in vivo time-lapse observations and lineage traces, astrocyte progenitors move rapidly and irregularly within the developing cortex, which we call erratic migration. Astrocyte progenitors also adopt blood vessel-guided migration. These highly motile progenitors are generated in the restricted prenatal stages and differentiate into protoplasmic astrocytes in the gray matter, whereas postnatally generated progenitors do not move extensively and differentiate into fibrous astrocytes in the white matter. We found Cxcr4/7, and integrin β1 regulate the blood vessel-guided migration, and their functional blocking disrupts their positioning. This study provides insight into astrocyte development and may contribute to understanding the pathogenesis caused by their defects.

Epstein-Barr virus-based plasmid enables inheritable transgene expression in mouse cerebral cortex

Continuous development of the cerebral cortex from the prenatal to postnatal period depends on neurons and glial cells, both of which are generated from neural progenitor cells (NPCs). Owing to technical limitations regarding the transfer of genes into mouse brain, the mechanisms behind the long-term development of the cerebral cortex have not been well studied. Plasmid transfection into NPCs in embryonic mouse brains by in utero electroporation (IUE) is a widely used technique aimed at expressing transgenes in NPCs and their recent progeny neurons. Because the plasmids in NPCs are attenuated with each cell division, the transgene is not expressed in their descendants, including glial cells. The present study shows that an Epstein–Barr virus-based plasmid (EB-oriP plasmid) is helpful for studying long-term cerebral cortex development. The use of the EB-oriP plasmid for IUE allowed transgene expression even in the descendant progeny cells of adult mouse brains. Combining the EB-oriP plasmid with the shRNA expression cassette allowed examination of the genes of interest in the continuous development of the cerebral cortex. Furthermore, preferential transgene expression was achieved in combination with cell type-specific promoter-driven transgene expression. Meanwhile, introducing the EB-oriP plasmid twice into the same individual embryos during separate embryonic development stages suggested heterogeneity of NPCs. In summary, IUE using the EB-oriP plasmid is a novel option to study the long-term development of the cerebral cortex in mice.

In Utero Electroporation to Study Mouse Brain Development

In utero electroporation is a rapid and powerful technique to study the development of many brain regions. This approach presents several advantages over other methods to study specific steps of brain development in vivo, from proliferation to synaptic integration. Here, we describe in detail the individual steps necessary to carry out the technique. We also highlight the variations that can be implemented to target different cerebral structures and to study specific steps of development.

Cortical astrocytes develop in a plastic manner at both clonal and cellular levels

Astrocytes play essential roles in the neural tissue where they form a continuous network, while displaying important local heterogeneity. Here, we performed multiclonal lineage tracing using combinatorial genetic markers together with a new large volume color imaging approach to study astrocyte development in the mouse cortex. We show that cortical astrocyte clones intermix with their neighbors and display extensive variability in terms of spatial organization, number and subtypes of cells generated. Clones develop through 3D spatial dispersion, while at the individual level astrocytes acquire progressively their complex morphology. Furthermore, we find that the astroglial network is supplied both before and after birth by ventricular progenitors that scatter in the neocortex and can give rise to protoplasmic as well as pial astrocyte subtypes. Altogether, these data suggest a model in which astrocyte precursors colonize the neocortex perinatally in a non-ordered manner, with local environment likely determining astrocyte clonal expansion and final morphotype.

Previous studies on astrocyte development have led to controversial results due to a lack of pertinent tools. Here, authors analyze large numbers of astrocyte clones generated by nearby cortical progenitors using the MAGIC Markers strategy and ChroMS 3D imaging, and show that clonally-related astrocytes organize in a non-stereotyped manner and that cortical astrocyte subtypes are not intrinsically specified.

Spatially restricted long-term transgene expression in the developing skin used for studying the interaction of epidermal development and sensory innervation

Skin development is tightly temporally coordinated with its sensory innervation, which consists of the peripheral branches of the dorsal root ganglion (DRG) axons. Various studies suggest that the skin produces a long-range attractant for the sensory axons. However, the exact identity of the guidance cue(s) remains unclear. To reveal the detailed molecular mechanism that controls DRG axon guidance and targeting, manipulation of specific skin layers at specific time points are required. To test a variety of attractants that can be expressed in specific skin layers at specific timepoints, we combined in utero electroporation with the Tol2 transposon system to induce long-term transgene expression in the developing mouse skin, including in the highly proliferative epidermal stem cells (basal layer) and their descendants (spinous and granular layer cells). The plasmid solution was injected as close to the hindpaw plantar surface as possible. Immediately, electric pulses were passed through the embryo to transduce the plasmid DNA into hindpaw skin cells. Balancing outcome measurements including: embryo survival, transfection efficiency, and the efficiency of transgene integration into host cells, we found that IUE was best performed on E13.5, and using an electroporation voltage of 34V. After immunostaining embryonic and early postnatal skin tissue sections for keratinocyte and sensory axon markers, we observe the growth of axons into skin epidermal layers including areas expressing EGFP. Therefore, this method is useful for studying the interaction between axon growth and epidermal cell division/differentiation.

Differential potentials of neural progenitors for the generation of neurons and non-neuronal cells in the developing amniote brain

Mature mammalian brains consist of variety of neuronal and non-neuronal cell types, which are progressively generated from embryonic neural progenitors through the embryonic and postnatal periods. However, it remains unknown whether all embryonic progenitors equivalently contribute to multiple cell types, or individual neural progenitors have variable potentials to generate specific cell types in a stochastic manner. Here, we performed population-level tracing of mouse embryonic neural progenitors by using Tol2-mediated genome integration vectors. We identified that neural progenitors in early embryonic stages predominantly contribute to cortical or subcortical neurons than astrocytes, ependymal cells, and neuroblasts in the postnatal brain. Notably, neurons and astrocytes were cumulatively labeled by the increase of total labeled cells, suggesting constant neurogenic and gliogenic potentials of individual neural progenitors. On the contrary, numbers of labeled ependymal cell are more fluctuated, implicating intrinsic variability of progenitor potentials for ependymal cell generation. Differential progenitor potentials that contribute to neurons, astrocytes, and ependymal cells were also detected in the developing avian pallium. Our data suggest evolutionary conservations of coherent and variable potentials of neural progenitors that generate multiple cell types in the developing amniote brain.

NG2 expression in NG2 glia is regulated by binding of SoxE and bHLH transcription factors to a Cspg4 intronic enhancer

NG2 is a type 1 integral membrane glycoprotein encoded by the Cspg4 gene. It is expressed on glial progenitor cells known as NG2 glial cells or oligodendrocyte precursor cells that exist widely throughout the developing and mature central nervous system and vascular mural cells but not on mature oligodendrocytes, astrocytes, microglia, neurons, or neural stem cells. Hence NG2 is widely used as a marker for NG2 glia in the rodent and human. The regulatory elements of the mouse Cspg4 gene and its flanking sequences have been used successfully to target reporter and Cre recombinase to NG2 glia in transgenic mice when used in a large 200 kb bacterial artificial chromosome cassette containing the 38 kb Cspg4 gene in the center. Despite the tightly regulated cell type- and stage-specific expression of NG2 in the brain and spinal cord, the mechanisms that regulate its transcription have remained unknown. Here, we describe a 1.45 kb intronic enhancer of the mouse Cspg4 gene that directed transcription of EGFP reporter to NG2 glia but not to pericytes in vitro and in transgenic mice. The 1.45 kb enhancer contained binding sites for SoxE and basic helix-loop-helix transcription factors, and its enhancer activity was augmented cooperatively by these factors, whose respective binding elements were found in close proximity to each other. Mutations in these binding elements abrogated the enhancer activity when tested in the postnatal mouse brain.

Opportunities and challenges for using the zebrafish to study neuronal connectivity as an endpoint of developmental neurotoxicity

Chemical exposures have been implicated as environmental risk factors that interact with genetic susceptibilities to influence individual risk for complex neurodevelopmental disorders, including autism spectrum disorder, schizophrenia, attention deficit hyperactivity disorder and intellectual disabilities. Altered patterns of neuronal connectivity represent a convergent mechanism of pathogenesis for these and other neurodevelopmental disorders, and growing evidence suggests that chemicals can interfere with specific signaling pathways that regulate the development of neuronal connections. There is, therefore, a growing interest in developing screening platforms to identify chemicals that alter neuronal connectivity. Cell-cell, cell-matrix interactions and systemic influences are known to be important in defining neuronal connectivity in the developing brain, thus, a systems-based model offers significant advantages over cell-based models for screening chemicals for effects on neuronal connectivity. The embryonic zebrafish represents a vertebrate model amenable to higher throughput chemical screening that has proven useful in characterizing conserved mechanisms of neurodevelopment. Moreover, the zebrafish is readily amenable to gene editing to integrate genetic susceptibilities. Although use of the zebrafish model in toxicity testing has increased in recent years, the diverse tools available for imaging structural differences in the developing zebrafish brain have not been widely applied to studies of the influence of gene by environment interactions on neuronal connectivity in the developing zebrafish brain. Here, we discuss tools available for imaging of neuronal connectivity in the developing zebrafish, review what has been published in this regard, and suggest a path forward for applying this information to developmental neurotoxicity testing.

OTX1 regulates cell cycle progression of neural progenitors in the developing cerebral cortex

The progenitor cells in the cerebral cortex coordinate proliferation and mitotic exit to generate the correct number of neurons and glial cells during development. However, mechanisms for regulating the mitotic cycle of cortical progenitors are not fully understood. Otx1 is one of the homeobox-containing transcription factors frequently implicated in the development of the central nervous system. Mice bearing a targeted deletion of Otx1 exhibit brain hypoplasia and a decrease in the number of cortical neurons. We hypothesized that Otx1 might be crucial to the proliferation and differentiation of cortical progenitors. Otx1 knockdown by in utero electroporation in the mouse brain reduced the proportion of the G₁ phase while increasing the S and M phases of progenitor cells. The knockdown diminished Tbr1+ neurons but increased GFAP+ astrocytes in the early postnatal cortex as revealed by lineage tracing study. Tbr2+ basal progenitors lacking Otx1 were held at the transit-amplifying stage. In contrast, overexpression of wildtype Otx1 but not an astrocytoma-related mutant Y320C inhibited proliferation of the progenitor cells in embryonic cortex. This study demonstrates that Otx1 is one of the key elements regulating cortical neurogenesis, and a loss-of-function in Otx1 may contribute to the overproduction of astrocytes in vivo.

Transposons As Tools for Functional Genomics in Vertebrate Models

Genetic tools and mutagenesis strategies based on transposable elements are currently under development with a vision to link primary DNA sequence information to gene functions in vertebrate models. By virtue of their inherent capacity to insert into DNA, transposons can be developed into powerful tools for chromosomal manipulations. Transposon-based forward mutagenesis screens have numerous advantages including high throughput, easy identification of mutated alleles, and providing insight into genetic networks and pathways based on phenotypes. For example, the Sleeping Beauty transposon has become highly instrumental to induce tumors in experimental animals in a tissue-specific manner with the aim of uncovering the genetic basis of diverse cancers. Here, we describe a battery of mutagenic cassettes that can be applied in conjunction with transposon vectors to mutagenize genes, and highlight versatile experimental strategies for the generation of engineered chromosomes for loss-of-function as well as gain-of-function mutagenesis for functional gene annotation in vertebrate models, including zebrafish, mice, and rats.

Building a lineage from single cells: genetic techniques for cell lineage tracking

Resolving lineage relationships between cells in an organism is a fundamental interest of developmental biology. Furthermore, investigating lineage can drive understanding of pathological states, including cancer, as well as understanding of developmental pathways that are amenable to manipulation by directed differentiation. Although lineage tracking through the injection of retroviral libraries has long been the state of the art, a recent explosion of methodological advances in exogenous labelling and single-cell sequencing have enabled lineage tracking at larger scales, in more detail, and in a wider range of species than was previously considered possible. In this Review, we discuss these techniques for cell lineage tracking, with attention both to those that trace lineage forwards from experimental labelling, and those that trace backwards across the life history of an organism.

Evidence That the Laminar Fate of LGE/CGE-Derived Neocortical Interneurons Is Dependent on Their Progenitor Domains

Neocortical interneurons show tremendous diversity in terms of their neurochemical marker expressions, morphology, electrophysiological properties, and laminar fate. Allocation of interneurons to their appropriate regions and layers in the neocortex is thought to play important roles for the emergence of higher functions of the neocortex. Neocortical interneurons mainly originate from the medial ganglionic eminence (MGE) and the caudal ganglionic eminence (CGE). The diversity and the laminar fate of MGE-derived interneurons depend on the location of their birth and birthdate, respectively. However, this relationship does not hold for CGE-derived interneurons. Here, using the method of in utero electroporation, which causes arbitrary occurrence of labeled progenitor domains, we tracked all descendants of the lateral ganglionic eminence (LGE)/CGE progenitors in mice. We provide evidence that neocortical interneurons with distinct laminar fate originate from distinct progenitor domains within the LGE/CGE. We find layer I interneurons are predominantly labeled in a set of animals, whereas other upper layer neurons are predominantly labeled in another set. We also find distinct subcortical structures labeled between the two sets. Further, interneurons labeled in layer I show distinct neurochemical properties from those in other layers. Together, these results suggest that the laminar fate of LGE/CGE-derived interneurons depends on their spatial origin.

SIGNIFICANCE STATEMENT

Diverse types of neocortical interneurons have distinct laminar fate, neurochemical marker expression, morphology, and electrophysiological properties. Although the specifications and laminar fate of medial ganglionic eminence-derived neocortical interneurons depend on their location of embryonic origin and birthdate, no similar causality of lateral/caudal ganglionic eminence (LGE/CGE)-derived neocortical interneurons is known. Here, we performed in utero electroporation on mouse LGE/CGE and found two groups of animals, one with preferential labeling of layer I and the other with preferential labeling of other layers. Interneurons labeled in these two groups show distinct neurochemical properties and morphologies and are associated with labeling of distinct subcortical structures. These findings suggest that the laminar fate of LGE/CGE-derived neocortical interneurons depends on their spatial origin.

Association of astrocytes with neurons and astrocytes derived from distinct progenitor domains in the subpallium

Astrocytes play pivotal roles in metabolism and homeostasis as well as in neural development and function in a manner thought to depend on their region-specific diversity. In the mouse spinal cord, astrocytes and neurons, which are derived from a common progenitor domain (PD) and controlled by common PD-specific transcription factors, migrate radially and share their final positions. However, whether astrocytes can only interact with neurons from common PDs in the brain remains unknown. Here, we focused on subpallium-derived cells, because the subpallium generates neurons that show a diverse mode of migration. We tracked their fate by in utero electroporation of plasmids that allow for chromosomal integration of transgenes or of a Cre recombinase expression vector to reporter mice. We also used an Nkx2.1(Cre) mouse line to fate map the cells originating from the medial ganglionic eminence and preoptic area. We find that although neurons and astrocytes are labeled in various regions, only neurons are labeled in the neocortex, hippocampus and olfactory bulb. Furthermore, we find astrocytes derived from an Nkx 2.1-negative PD are associated with neurons from the Nkx2.1(+) PD. Thus, forebrain astrocytes can associate with neurons as well as astrocytes derived from a distinct PD.

Imaging activity in astrocytes and neurons with genetically encoded calcium indicators following in utero electroporation

Complex interactions between networks of astrocytes and neurons are beginning to be appreciated, but remain poorly understood. Transgenic mice expressing fluorescent protein reporters of cellular activity, such as the GCaMP family of genetically encoded calcium indicators (GECIs), have been used to explore network behavior. However, in some cases, it may be desirable to use long-established rat models that closely mimic particular aspects of human conditions such as Parkinson's disease and the development of epilepsy following status epilepticus. Methods for expressing reporter proteins in the rat brain are relatively limited. Transgenic rat technologies exist but are fairly immature. Viral-mediated expression is robust but unstable, requires invasive injections, and only works well for fairly small genes (<5 kb). In utero electroporation (IUE) offers a valuable alternative. IUE is a proven method for transfecting populations of astrocytes and neurons in the rat brain without the strict limitations on transgene size. We built a toolset of IUE plasmids carrying GCaMP variants 3, 6s, or 6f driven by CAG and targeted to the cytosol or the plasma membrane. Because low baseline fluorescence of GCaMP can hinder identification of transfected cells, we included the option of co-expressing a cytosolic tdTomato protein. A binary system consisting of a plasmid carrying a piggyBac inverted terminal repeat (ITR)-flanked CAG-GCaMP-IRES-tdTomato cassette and a separate plasmid encoding for expression of piggyBac transposase was employed to stably express GCaMP and tdTomato. The plasmids were co-electroporated on embryonic days 13.5-14.5 and astrocytic and neuronal activity was subsequently imaged in acute or cultured brain slices prepared from the cortex or hippocampus. Large spontaneous transients were detected in slices obtained from rats of varying ages up to 127 days. In this report, we demonstrate the utility of this toolset for interrogating astrocytic and neuronal activity in the rat brain.

Diverse subtypes of astrocytes and their development during corticogenesis

Astrocytes are one of the most abundant cell types in the mammalian central nervous system, and are known to have a wide variety of physiological functions, including maintenance of neurons, formation of the blood brain barrier, and regulation of synapse functions. Although the migration and positioning of neurons has been extensively studied over the last several decades and many aspects have been uncovered, the process underlying glial development was largely unknown until recently due to the existence of multiple subtypes of glia and the sustained proliferative ability of these cells through adulthood. To overcome these difficulties, new gene transfer techniques and genetically modified mice were developed, and have been gradually revealing when and how astrocytes develop during corticogenesis. In this paper, I review the diversity of astrocytes and summarize our knowledge about their production and migration.

Spatiotemporal analyses of neural lineages after embryonic and postnatal progenitor targeting combining different reporters

Genetic lineage tracing with electroporation is one of the most powerful techniques to target neural progenitor cells and their progeny. However, the spatiotemporal relationship between neural progenitors and their final phenotype remain poorly understood. One critical factor to analyze the cell fate of progeny is reporter integration into the genome of transfected cells. To address this issue, we performed postnatal and in utero co-electroporations of different fluorescent reporters to label, in both cerebral cortex and olfactory bulb, the progeny of subventricular zone neural progenitors. By comparing fluorescent reporter expression in the adult cell progeny, we show a differential expression pattern within the same cell lineage, depending on electroporation stage and cell identity. Further, while neuronal lineages arise from many progenitors in proliferative zones after few divisions, glial lineages come from fewer progenitors that accomplish many cell divisions. Together, these data provide a useful guide to select a strategy to track the cell fate of a specific cell population and to address whether a different proliferative origin might be correlated with functional heterogeneity.

Induction of protein deletion through in utero electroporation to define deficits in neuronal migration in transgenic models

Genetic deletion using the Cre-Lox system in transgenic mouse lines is a powerful tool used to study protein function. However, except in very specific Cre models, deletion of a protein throughout a tissue or cell population often leads to complex phenotypes resulting from multiple interacting mechanisms. Determining whether a phenotype results from disruption of a cell autonomous mechanism, which is intrinsic to the cell in question, or from a non-cell autonomous mechanism, which would result from impairment of that cell's environment, can be difficult to discern. To gain insight into protein function in an in vivo context, in utero electroporation (IUE) enables gene deletion in a small subset of cells within the developing cortex or some other selected brain region. IUE can be used to target specific brain areas, including the dorsal telencephalon, medial telencephalon, hippocampus, or ganglionic eminence. This facilitates observation of the consequences of cell autonomous gene deletion in the context of a healthy environment. The goal of this protocol is to show how IUE can be used to analyze a defect in radial migration in a floxed transgenic mouse line, with an emphasis on distinguishing between the cell autonomous and non-cell autonomous effects of protein deletion. By comparing the phenotype resulting from gene deletion within the entire cortex versus IUE-mediated gene deletion in a limited cell population, greater insight into protein function in brain development can be obtained than by using either technique in isolation.

Neonatal pial surface electroporation

Over the past several years the pial surface has been identified as a germinal niche of importance during embryonic, perinatal and adult neuro- and gliogenesis, including after injury. However, methods for genetically interrogating these progenitor populations and tracking their lineages had been limited owing to a lack of specificity or time consuming production of viruses. Thus, progress in this region has been relatively slow with only a handful of investigations of this location. Electroporation has been used for over a decade to study neural stem cell properties in the embryo, and more recently in the postnatal brain. Here we describe an efficient, rapid, and simple technique for the genetic manipulation of pial surface progenitors based on an adapted electroporation approach. Pial surface electroporation allows for facile genetic labeling and manipulation of these progenitors, thus representing a time-saving and economical approach for studying these cells.

Contribution of tumor heterogeneity in a new animal model of CNS tumors

The etiology of central nervous system (CNS) tumor heterogeneity is unclear. To clarify this issue, a novel animal model was developed of glioma and atypical teratoid/rhabdoid-like tumor (ATRT) produced in rats by nonviral cellular transgenesis initiated in utero. This model system affords the opportunity for directed oncogene expression, clonal labeling, and addition of tumor-modifying transgenes. By directing HRasV12 and AKT transgene expression in different cell populations with promoters that are active ubiquitously (CAG promoter), astrocyte-selective (glial fibrillary acidic protein promoter), or oligodendrocyte-selective (myelin basic protein promoter) we generated glioblastoma multiforme and anaplastic oligoastrocytoma, respectively. Importantly, the glioblastoma multiforme and anaplastic oligoastrocytoma tumors were distinguishable at both the cellular and molecular level. Furthermore, proneural basic helix-loop-helix (bHLH) transcription factors, Ngn2 (NEUROG2) or NeuroD1, were expressed along with HRasV12 and AKT in neocortical radial glia, leading to the formation of highly lethal ATRT like tumors. This study establishes a unique model in which determinants of CNS tumor diversity can be parsed out and reveals that both mutation and expression of neurogenic bHLH transcription factors contribute to CNS tumor diversity.

IMPLICATIONS

A novel CNS tumor model reveals that oncogenic events occurring in disparate cell types and/or molecular contexts lead to different tumor types; these findings shed light on the sources of brain tumor heterogeneity.

Recent molecular approaches to understanding astrocyte function in vivo

Astrocytes are a predominant glial cell type in the nervous systems, and are becoming recognized as important mediators of normal brain function as well as neurodevelopmental, neurological, and neurodegenerative brain diseases. Although numerous potential mechanisms have been proposed to explain the role of astrocytes in the normal and diseased brain, research into the physiological relevance of these mechanisms in vivo is just beginning. In this review, we will summarize recent developments in innovative and powerful molecular approaches, including knockout mouse models, transgenic mouse models, and astrocyte-targeted gene transfer/expression, which have led to advances in understanding astrocyte biology in vivo that were heretofore inaccessible to experimentation. We will examine the recently improved understanding of the roles of astrocytes – with an emphasis on astrocyte signaling – in the context of both the healthy and diseased brain, discuss areas where the role of astrocytes remains debated, and suggest new research directions.

Increased dosage of DYRK1A and DSCR1 delays neuronal differentiation in neocortical progenitor cells

Down's syndrome (DS), a major genetic cause of mental retardation, arises from triplication of genes on human chromosome 21. Here we show that DYRK1A (dual-specificity tyrosine-phosphorylated and -regulated kinase 1A) and DSCR1 (DS critical region 1), two genes lying within human chromosome 21 and encoding for a serine/threonine kinase and calcineurin regulator, respectively, are expressed in neural progenitors in the mouse developing neocortex. Increasing the dosage of both proteins in neural progenitors leads to a delay in neuronal differentiation, resulting ultimately in alteration of their laminar fate. This defect is mediated by the cooperative actions of DYRK1A and DSCR1 in suppressing the activity of the transcription factor NFATc. In Ts1Cje mice, a DS mouse model, dysregulation of NFATc in conjunction with increased levels of DYRK1A and DSCR1 was observed. Furthermore, counteracting the dysregulated pathway ameliorates the delayed neuronal differentiation observed in Ts1Cje mice. In sum, our findings suggest that dosage of DYRK1A and DSCR1 is critical for proper neurogenesis through NFATc and provide a potential mechanism to explain the neurodevelopmental defects in DS.

New tools for investigating astrocyte-to-neuron communication

Gray matter protoplasmic astrocytes extend very thin processes and establish close contacts with synapses. It has been suggested that the release of neuroactive gliotransmitters at the tripartite synapse contributes to information processing. However, the concept of calcium (Ca²⁺)-dependent gliotransmitter release from astrocytes, and the release mechanisms are being debated. Studying astrocytes in their natural environment is challenging because: (i) astrocytes are electrically silent; (ii) astrocytes and neurons express an overlapping repertoire of transmembrane receptors; (iii) the size of astrocyte processes in contact with synapses are below the resolution of confocal and two-photon microscopes (iv) bulk-loading techniques using fluorescent Ca²⁺ indicators lack cellular specificity. In this review, we will discuss some limitations of conventional methodologies and highlight the interest of novel tools and approaches for studying gliotransmission. Genetically encoded Ca²⁺ indicators (GECIs), light-gated channels, and exogenous receptors are being developed to selectively read out and stimulate astrocyte activity. Our review discusses emerging perspectives on: (i) the complexity of astrocyte Ca²⁺ signaling revealed by GECIs; (ii) new pharmacogenetic and optogenetic approaches to activate specific Ca²⁺ signaling pathways in astrocytes; (iii) classical and new techniques to monitor vesicle fusion in cultured astrocytes; (iv) possible strategies to express specifically reporter genes in astrocytes.

Development of cerebellar neurons and glias revealed by in utero electroporation: Golgi-like labeling of cerebellar neurons and glias

Cerebellar cortical functions rely on precisely arranged cytoarchitectures composed of several distinct types of neurons and glias. Studies have indicated that cerebellar excitatory and inhibitory neurons have distinct spatial origins, the upper rhombic lip (uRL) and ventricular zone (VZ), respectively, and that different types of neurons have different birthdates. However, the spatiotemporal relationship between uRL/VZ progenitors and their final phenotype remains poorly understood due to technical limitations. To address this issue, we performed in utero electroporation (IUE) of fluorescent protein plasmids using mouse embryos to label uRL/VZ progenitors at specific developmental stages, and observed labeled cells at maturity. To overcome any potential dilution of the plasmids caused by progenitor division, we also utilized constructs that enable permanent labeling of cells. Cerebellar neurons and glias were labeled in a Golgi-like manner enabling ready identification of labeled cells. Five types of cerebellar neurons, namely Purkinje, Golgi, Lugaro and unipolar brush cells, large-diameter deep nuclei (DN) neurons, and DN astrocytes were labeled by conventional plasmids, whereas plasmids that enable permanent labeling additionally labeled stellate, basket, and granule cells as well as three types of glias. IUE allows us to label uRL/VZ progenitors at different developmental stages. We found that the five types of neurons and DN astrocytes were labeled in an IUE stage-dependent manner, while stellate, basket, granule cells and three types of glias were labeled regardless of the IUE stage. Thus, the results indicate the IUE is an efficient method to track the development of cerebellar cells from uRL/VZ progenitors facing the ventricular lumen. They also indicate that while the generation of the five types of neurons by uRL/VZ progenitors is regulated in a time-dependent manner, the progenitor pool retains multipotency throughout embryonic development.

Rapid genetic targeting of pial surface neural progenitors and immature neurons by neonatal electroporation

Background

Recent findings have indicated the presence of a progenitor domain at the marginal zone/layer 1 of the cerebral cortex, and it has been suggested that these progenitors have neurogenic and gliogenic potential. However, their contribution to the histogenesis of the cortex remains poorly understood due to difficulties associated with genetically manipulating these unique cells in a population-specific manner.

Results

We have adapted the electroporation technique to target pial surface cells for rapid genetic manipulation at postnatal day 2. In vivo data show that most of these cells proliferate and progressively differentiate into both neuronal and glial subtypes. Furthermore, these cells localize to the superficial layers of the optic tectum and cerebral cortex prior to migration away from the surface.

Conclusions

We provide a foundation upon which future studies can begin to elucidate the molecular controls governing neural progenitor fate, migration, differentiation, and contribution to cortical and tectal histogenesis. Furthermore, specific genetic targeting of such neural progenitor populations will likely be of future clinical interest.

A method for stable transgenesis of radial glia lineage in rat neocortex by piggyBac mediated transposition

Methods that combine lineage tracing with cellular transgenesis are needed in order to determine mechanisms that specify neural cell types. Currently available methods include viral infection and Cre-mediated recombination. In utero electroporation (IUE) has been used in multiple species to deliver multiple transgenes simultaneously into neural progenitors. In standard IUE, most plasmids remain episomal, are lost during cell division, and so transgenes are not expressed in the complete neural lineage. Here we combine IUE with a binary piggyBac transposon system (PB-IUE), and show that unlike conventional IUE, a single embryonic transfection of neocortical radial glia with a piggyBac transposon system results in stable transgene expression in the neural lineage of radial glia: cortical neurons, astrocytes, oligodendrocytes, and olfactory bulb interneurons. We also developed a modular toolkit of donor and helper plasmids with different promoters that allows for shRNA, bicistronic expression, and trangenesis in subsets of progenitors. As a demonstration of the utility of the toolkit we show that transgenesis of epidermal growth factor receptor (EGFR) expands the number of astrocytes and oligodendrocyrtes generated from progenitors. The relative ease of implementation and experimental flexibility should make the piggyBac IUE method a valuable new tool for tracking and manipulating neural lineages.

A tightly controlled conditional knockdown system using the Tol2 transposon-mediated technique

Background

Gene knockdown analyses using the in utero electroporation method have helped reveal functional aspects of genes of interest in cortical development. However, the application of this method to analyses in later stages of brain development or in the adult brain is still difficult because the amount of injected plasmids in a cell decreases along with development due to dilution by cell proliferation and the degradation of the plasmids. Furthermore, it is difficult to exclude the influence of earlier knockdown effects.

Methodology/Principal Findings

We developed a tightly controlled conditional knockdown system using a newly constructed vector, pT2K-TBI-shRNAmir, based on a Tol2 transposon-mediated gene transfer methodology with the tetracycline-inducible gene expression technique, which allows us to maintain a transgene for a long period of time and induce the knockdown of the gene of interest. We showed that expression of the endogenous amyloid precursor protein (APP) was sharply decreased by our inducible, stably integrated knockdown system in PC12 cells. Moreover, we induced an acute insufficiency of Dab1 with our system and observed that radial migration was impaired in the developing cerebral cortex. Such inhibitory effects on radial migration were not observed without induction, indicating that our system tightly controlled the knockdown, without any expression leakage in vivo.

Conclusions/Significance

Our system enables us to investigate the brain at any of the later stages of development or in the adult by utilizing a knockdown technique with the aid of the in utero electroporation gene transfer methodology. Furthermore, we can perform knockdown analyses free from the influence of undesired earlier knockdown effects.

In utero electroporation as a tool for genetic manipulation in vivo to study psychiatric disorders: from genes to circuits and behaviors

Many genetic risk factors for major mental disorders have key roles in brain development. Thus, exploring the roles for these genetic factors for brain development at the molecular, cellular, and neuronal circuit level is crucial for discovering how genetic disturbances affect high brain functions, which ultimately lead to disease pathologies. However, it is a tremendously difficult task, given that most mental disorders have genetic complexities in which many genetic risk factors have multiple roles in different cell types and brain regions over a time-course dependent manner. Furthermore, some genetic risk factors are likely to act epistatically in common molecular pathways. For this reason, a technique for spatial and temporal manipulation of multiple genes is necessary for understanding how genetic disturbances contribute to disease etiology. Here, the authors will review the said technique, in utero electroporation, which investigates the molecular disease pathways in rodent models for major mental disorders. This technique is also useful to examine the effect of genetic risks at the behavioral level. Furthermore, the authors will discuss the recent progress of this technology, such as inducible and cell type-specific targeting, as well as nonepisomal genetic manipulation, which provide further availability of this technique for research on major mental disorders.