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, we review the diversity of astrocytes and summarize our knowledge about their production and migration.

RBFOX1 (also known as FOX1 or A2BP1) regulates alternative splicing of a variety of transcripts crucial for neuronal functions. Physiological significance of RBFOX1 during brain development is seemingly essential since abnormalities in the gene cause autism spectrum disorder (ASD) and other neurodevelopmental and neuropsychiatric disorders such as intellectual disability, epilepsy, attention deficit hyperactivity disorder, and schizophrenia. RBFOX1 was also shown to serve as a “hub” in ASD gene transcriptome network. However, the pathophysiological significance of RBFOX1 gene abnormalities remains to be clarified. To elucidate the pathophysiological relevance of Rbfox1, we performed a battery of in vivo and in vitro analyses of the brain-specific cytoplasmic isoform, Rbfox1-iso2, during mouse corticogenesis. In vivo analyses were based on in utero electroporation, and the role of Rbfox1-iso2 in cortical neuron migration, neurogenesis, and morphology was investigated by morphological methods including confocal laser microscope-assisted time-lapse imaging. In vitro analyses were carried out to examine the morphology of primary cultured mouse hippocampal neurons. Silencing of Rbfox1-iso2 in utero caused defects in the radial migration and terminal translocation of cortical neurons during corticogenesis. Time-lapse imaging revealed that radial migration was apparently impaired by dysregulated nucleokinesis. Rbfox1-iso2 also regulated neuronal network formation in vivo since axon extension to the opposite hemisphere and dendritic arborization were hampered by the knockdown. In in vitro analyses, spine density and mature spine number were reduced in Rbfox1-iso2-deficient hippocampal neurons. Impaired Rbfox1-iso2 function was found to cause abnormal corticogenesis during brain development. The abnormal process may underlie the basic pathophysiology of ASD and other neurodevelopmental disorders and may contribute to the emergence of the clinical symptoms of the patients with RBFOX1 gene abnormalities.

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.

SAD kinases regulate presynaptic vesicle clustering and neuronal polarization. A previous report demonstrated that Sada^−/− and Sadb^−/− double-mutant mice showed perinatal lethality with a severe defect in axon/dendrite differentiation, but their single mutants did not. These results indicated that they were functionally redundant. Surprisingly, we show that on a C57BL/6N background, SAD-A is essential for cortical development whereas SAD-B is dispensable. Sada^−/− mice died within a few days after birth. Their cortical lamination pattern was disorganized and radial migration of cortical neurons was perturbed. Birth date analyses with BrdU and in utero electroporation using pCAG-EGFP vector showed a delayed migration of cortical neurons to the pial surface in Sada^−/− mice. Time-lapse imaging of these mice confirmed slow migration velocity in the cortical plate. While the neurites of hippocampal neurons in Sada^−/− mice could ultimately differentiate in culture to form axons and dendrites, the average length of their axons was shorter than that of the wild type. Thus, analysis on a different genetic background than that used initially revealed a nonredundant role for SAD-A in neuronal migration and differentiation.

MACRO Domain Containing 2 (MacroD2) is a neurodevelopmental disorder-related mono-ADP-ribosylhydrolase. Molecular features of this protein in neural tissues are largely unknown. In this study, we generated a specific antibody against MacroD2, and carried out expression and morphological analyses of the molecule during mouse brain development. In Western blotting, 2 MacroD2 isoforms with molecular masses of ∼70 and ∼75 kDa started to be expressed at embryonic day 16.5, reached the maximal level at postnatal day 8, and then gradually decreased through P30. In contrast, other isoforms with molecular masses of ∼110 and ∼140 kDa gradually increased during embryonic to postnatal development. In immunohistochemical analyses, MacroD2 was strongly detected in cortical neurons in layer II–V at P0 and P7, while the protein expression decreased significantly in the neurons at P30. Immunofluorescence analyses revealed that MacroD2 was mainly distributed in the soma and to a lesser extent in the axon and dendrite of immature primary cultured mouse hippocampal neurons. On the other hand, in the matured hippocampal neurons, while MacroD2 was detected in the soma, it displayed in dendrites a punctate distribution pattern with a partial colocalization with synaptic markers, synaptophysin, and PSD95. The obtained results indicate that MacroD2 is expressed and may have a physiological role in the central nervous system during brain development.