Glial cell type-specific gene expression in the mouse cerebrum using the piggyBac system and in utero electroporation

Astrocyte diversity in the ferret cerebrum revealed with astrocyte-specific genetic manipulation

Astrocytes in the cerebrum play important roles such as the regulation of synaptic functions, homeostasis, water transport, and the blood-brain barrier. It has been proposed that astrocytes in the cerebrum acquired diversity and developed functionally during evolution. Here, we show that like human astrocytes, ferret astrocytes in the cerebrum exhibit various morphological subtypes which mice do not have. We found that layer 1 of the ferret cerebrum contained not only protoplasmic astrocytes but also pial interlaminar astrocytes and subpial interlaminar astrocytes. Morphologically polarized astrocytes, which have a long unbranched process, were found in layer 6. Like human white matter, ferret white matter exhibited four subtypes of astrocytes. Furthermore, our quantification showed that ferret astrocytes had a larger territory size and a longer radius length than mouse astrocytes. Thus, our results indicate that, similar to the human cerebrum, the ferret cerebrum has a well-developed diversity of astrocytes. Ferrets should be useful for investigating the molecular and cellular mechanisms leading to astrocyte diversity, the functions of each astrocyte subtype and the involvement of different astrocyte subtypes in various neurological diseases.

Brain Sample Preparation After Performing in Utero Electroporation to Proliferating and Differentiated Cells in the Developing Mouse Neocortex

In utero electroporation (IUE) enables labeling and manipulating specific types of cells by introducing DNA plasmids with desired promoters. After the surgery, mouse brains are fixed at any stage and analyzed after staining using specific antibodies. Here, we describe the flow of the IUE experiment from the preparation to microscopic observations.

Advanced Techniques Using In Vivo Electroporation to Study the Molecular Mechanisms of Cerebral Development Disorders

The mammalian cerebral cortex undergoes a strictly regulated developmental process. Detailed in situ visualizations, imaging of these dynamic processes, and in vivo functional gene studies significantly enhance our understanding of brain development and related disorders. This review introduces basic techniques and recent advancements in in vivo electroporation for investigating the molecular mechanisms underlying cerebral diseases. In utero electroporation (IUE) is extensively used to visualize and modify these processes, including the forced expression of pathological mutants in human diseases; thus, this method can be used to establish animal disease models. The advent of advanced techniques, such as genome editing, including de novo knockout, knock-in, epigenetic editing, and spatiotemporal gene regulation, has further expanded our list of investigative tools. These tools include the iON expression switch for the precise control of timing and copy numbers of exogenous genes and TEMPO for investigating the temporal effects of genes. We also introduce the iGONAD method, an improved genome editing via oviductal nucleic acid delivery approach, as a novel genome-editing technique that has accelerated brain development exploration. These advanced in vivo electroporation methods are expected to provide valuable insights into pathological conditions associated with human brain disorders.

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.

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.

Orchestrated neuronal migration and cortical folding: A computational and experimental study

Brain development involves precisely orchestrated genetic, biochemical, and mechanical events. At the cellular level, neuronal proliferation in the innermost zone of the brain followed by migration towards the outermost layer results in a rapid increase in brain surface area, outpacing the volumetric growth of the brain, and forming the highly folded cortex. This work aims to provide mechanistic insights into the process of brain development and cortical folding using a biomechanical model that couples cell division and migration with volumetric growth. Unlike phenomenological growth models, our model tracks the spatio-temporal development of cohorts of neurons born at different times, with each cohort modeled separately as an advection-diffusion process and the total cell density determining the extent of volume growth. We numerically implement our model in Abaqus/Standard (2020) by writing user-defined element (UEL) subroutines. For model calibration, we apply in utero electroporation (IUE) to ferret brains to visualize and track cohorts of neurons born at different stages of embryonic development. Our calibrated simulations of cortical folding align qualitatively with the ferret experiments. We have made our experimental data and finite-element implementation available online to offer other researchers a modeling platform for future study of neurological disorders associated with atypical neurodevelopment and cortical malformations.

Brain development and cortical folding is a highly dynamic process that results from the interaction between gene expression, cellular mechanisms, and mechanical forces. Here, we expand on existing mathematical models of brain development and cortical folding to capture the behavior of multiple different subpopulations of neurons. By calibrating our biomechanical model to our novel experiments on ferrets, we can track the distribution of neurons over time and observe how the brain grows and develops its characteristic folds. Our calibrated model captures interactions between cell behavior and tissue deformation and offers more detailed information about the orchestrated migration of neuronal subpopulations. This work offers new mechanistic insights into brain development and opens the door to future investigations of atypical brain development caused by disrupted neuronal activities, particularly those alterations associated with injury, exposure, or treatment at a specific location or time during development. Finally, our experimental data and numerical implementations are provided as a resource online for the use of other researchers.

In Utero Electroporation for Manipulation of Specific Neuronal Populations

The complexity of brain functions is supported by the heterogeneity of brain tissue and millisecond-scale information processing. Understanding how complex neural circuits control animal behavior requires the precise manipulation of specific neuronal subtypes at high spatiotemporal resolution. In utero electroporation, when combined with optogenetics, is a powerful method for precisely controlling the activity of specific neurons. Optogenetics allows for the control of cellular membrane potentials through light-sensitive ion channels artificially expressed in the plasma membrane of neurons. Here, we first review the basic mechanisms and characteristics of in utero electroporation. Then, we discuss recent applications of in utero electroporation combined with optogenetics to investigate the functions and characteristics of specific regions, layers, and cell types. These techniques will pave the way for further advances in understanding the complex neuronal and circuit mechanisms that underlie behavioral outputs.

Visualizing Cortical Development and Evolution: A Toolkit Update

Visualizing the process of neural circuit formation during neurogenesis, using genetically modified animals or somatic transgenesis of exogenous plasmids, has become a key to decipher cortical development and evolution. In contrast to the establishment of transgenic animals, the designing and preparation of genes of interest into plasmids are simple and easy, dispensing with time-consuming germline modifications. These advantages have led to neuron labeling based on somatic transgenesis. In particular, mammalian expression plasmid, CRISPR-Cas9, and DNA transposon systems, have become widely used for neuronal visualization and functional analysis related to lineage labeling during cortical development. In this review, we discuss the advantages and limitations of these recently developed techniques.

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.

Potential of Multiscale Astrocyte Imaging for Revealing Mechanisms Underlying Neurodevelopmental Disorders

Astrocytes provide trophic and metabolic support to neurons and modulate circuit formation during development. In addition, astrocytes help maintain neuronal homeostasis through neurovascular coupling, blood-brain barrier maintenance, clearance of metabolites and nonfunctional proteins via the glymphatic system, extracellular potassium buffering, and regulation of synaptic activity. Thus, astrocyte dysfunction may contribute to a myriad of neurological disorders. Indeed, astrocyte dysfunction during development has been implicated in Rett disease, Alexander's disease, epilepsy, and autism, among other disorders. Numerous disease model mice have been established to investigate these diseases, but important preclinical findings on etiology and pathophysiology have not translated into clinical interventions. A multidisciplinary approach is required to elucidate the mechanism of these diseases because astrocyte dysfunction can result in altered neuronal connectivity, morphology, and activity. Recent progress in neuroimaging techniques has enabled noninvasive investigations of brain structure and function at multiple spatiotemporal scales, and these technologies are expected to facilitate the translation of preclinical findings to clinical studies and ultimately to clinical trials. Here, we review recent progress on astrocyte contributions to neurodevelopmental and neuropsychiatric disorders revealed using novel imaging techniques, from microscopy scale to mesoscopic scale.