Developmental changes in interkinetic nuclear migration dynamics with respect to cell‐cycle progression in the mouse cerebral cortex ventricular zone

Forebrain Eml1 depletion reveals early centrosomal dysfunction causing subcortical heterotopia

Subcortical heterotopia is a cortical malformation associated with epilepsy, intellectual disability, and an excessive number of cortical neurons in the white matter. Echinoderm microtubule-associated protein like 1 (EML1) mutations lead to subcortical heterotopia, associated with abnormal radial glia positioning in the cortical wall, prior to malformation onset. This perturbed distribution of proliferative cells is likely to be a critical event for heterotopia formation; however, the underlying mechanisms remain unexplained. This study aimed to decipher the early cellular alterations leading to abnormal radial glia. In a forebrain conditional Eml1 mutant model and human patient cells, primary cilia and centrosomes are altered. Microtubule dynamics and cell cycle kinetics are also abnormal in mouse mutant radial glia. By rescuing microtubule formation in Eml1 mutant embryonic brains, abnormal radial glia delamination and heterotopia volume were significantly reduced. Thus, our new model of subcortical heterotopia reveals the causal link between Eml1's function in microtubule regulation and cell position, both critical for correct cortical development.

Control of G2 Phase Duration by CDC25B Modulates the Switch from Direct to Indirect Neurogenesis in the Neocortex

During development, cortical neurons are produced in a temporally regulated sequence from apical progenitors, directly or indirectly, through the production of intermediate basal progenitors. The balance between these major progenitor types is critical for the production of the proper number and types of neurons, and it is thus important to decipher the cellular and molecular cues controlling this equilibrium. Here we address the role of a cell cycle regulator, the CDC25B phosphatase, in this process. We show that, in the developing mouse neocortex of both sex, deleting CDC25B in apical progenitors leads to a transient increase in the production of TBR1+ neurons at the expense of TBR2+ basal progenitors. This phenotype is associated with lengthening of the G2 phase of the cell cycle, the total cell cycle length being unaffected. Using in utero electroporation and cortical slice cultures, we demonstrate that the defect in TBR2+ basal progenitor production requires interaction with CDK1 and is because of the G2 phase lengthening in CDC25B mutants. Together, this study identifies a new role for CDC25B and G2 phase length in direct versus indirect neurogenesis at early stages of cortical development.SIGNIFICANCE STATEMENT This study is the first analysis of the function of CDC25B, a G2/M regulator, in the developing neocortex. We show that removing CDC25B function leads to a transient increase in neuronal differentiation at early stages, occurring simultaneously with a decrease in basal intermediate progenitors (bIPs). Conversely, a CDC25B gain of function promotes production of bIPs, and this is directly related to CDC25B's ability to regulate CDK1 activity. This imbalance of neuron/progenitor production is linked to a G2 phase lengthening in apical progenitors; and using pharmacological treatments on cortical slice cultures, we show that shortening the G2 phase is sufficient to enhance bIP production. Our results reveal the importance of G2 phase length regulation for neural progenitor fate determination.

Single-cell imaging of the cell cycle reveals CDC25B-induced heterogeneity of G1 phase length in neural progenitor cells

Although lengthening of the cell cycle and G1 phase is a generic feature of tissue maturation during development, the underlying mechanism remains poorly understood. Here, we develop a time-lapse imaging strategy to measure the four cell cycle phases in single chick neural progenitor cells in their endogenous environment. We show that neural progenitors are widely heterogeneous with respect to cell cycle length. This variability in duration is distributed over all phases of the cell cycle, with the G1 phase contributing the most. Within one cell cycle, each phase duration appears stochastic and independent except for a correlation between S and M phase duration. Lineage analysis indicates that the majority of daughter cells may have a longer G1 phase than mother cells, suggesting that, at each cell cycle, a mechanism lengthens the G1 phase. We identify that the CDC25B phosphatase known to regulate the G2/M transition indirectly increases the duration of the G1 phase, partly through delaying passage through the restriction point. We propose that CDC25B increases the heterogeneity of G1 phase length, revealing a previously undescribed mechanism of G1 lengthening that is associated with tissue development.

Conservation of neural progenitor identity and the emergence of neocortical neuronal diversity

One paramount challenge for neuroscientists over the past century has been to identify the embryonic origins of the enormous diversity of cortical neurons found in the adult human neocortex and to unravel the developmental processes governing their emergence. In all mammals, including humans, the radial glia lining the ventricles of the embryonic telencephalon, more recently reclassified as apical radial glia (aRGs), have been identified as the neural progenitors giving rise to all excitatory neurons and inhibitory interneurons of the six-layered cortex. In this review, we explore the fundamental molecular and cellular mechanisms that regulate aRG function and the generation of neuronal diversity in the dorsal telencephalon. We survey the key structural features essential for the retention of the highly polarized aRG morphology and therefore impose aRG identity after cytokinesis. We discuss how these structures and associated molecular signaling complexes influence aRG proliferative capacity and the decision to undergo proliferative self-renewing symmetric or neurogenic asymmetric divisions. We also explore the intriguing and complex question of how the extensive neuronal diversity within the adult neocortex arises from the small aRG population located within the cortical proliferative zone. We further highlight the recent clonal lineage tracing and single-cell transcriptomic profiling studies providing compelling evidence that individual neuronal identity emerges as a consequence of exposure to temporally regulated extrinsic cues which coordinate waves of transcriptional activity that evolve over time to drive neuronal commitment and maturation.

Global pattern of interkinetic nuclear migration in tracheoesophageal epithelia of the mouse embryo: Interorgan and intraorgan regional differences

Interkinetic nuclear migration (INM) is an apicobasal (AB) polarity-based regulatory mechanism of proliferation/differentiation in epithelial stem/progenitor cells. We previously documented INM in the endoderm-derived tracheal/esophageal epithelia at embryonic day (E) 11.5 and suggested that INM is involved in the development of both organs. We here investigated interorgan (trachea vs esophagus) and intraorgan regional (ventral vs dorsal) differences in the INM mode in the tracheal and esophageal epithelia of the mouse embryo. We also analyzed convergent extension (CE) and planar cell movement (PCM) in the epithelia based on cell distribution. The pregnant C57BL/6J mice were intraperitoneally injected with 5-ethynyl-2'-deoxyuridine at E11.5 and E12.5 and were sacrificed 1, 4, 6, 8, and 12 hours later to obtain the embryos. The distribution of labeled cell nuclei along the AB axis was chronologically analyzed in the total, ventral, and dorsal sides of the epithelia. The percentage distribution of the nuclei population was represented by histogram and the chronological change was analyzed statistically using multidimensional scaling. The interorgan comparison of the INM mode during E11.5-E12.0, but not E12.5-E13.0, showed a significant difference. During E11.5-E12.0 the trachea, but not the esophagus, showed a significant difference between ventral and dorsal sides. During E12.5-E13.0 neither organ showed regional differences. CE appeared to occur in both organs during E11.5-E12.0 while PCM was unclear in both organs. These findings suggest a difference between the trachea and esophagus, and a regional difference in the trachea, not in the esophagus, in the INM mode, which may be related with the later differential organogenesis/histogenesis of these organs.

A novel mouse model of glucagon-like peptide-1 receptor expression: A look at the brain

Glucagon-like peptide-1 (GLP-1) is an incretin hormone with a number of functions to maintain energy homeostasis and contribute to motivated behavior, both peripherally and within the central nervous system (CNS). These functions, which include insulin secretion, gastric emptying, satiety, and the hedonic aspects of food and drug intake, are primarily mediated through stimulation of the GLP-1 receptor. While this receptor plays an important role in a variety of physiological outcomes, data regarding its CNS expression has been primarily limited to regional receptor binding and single-label transcript expression studies. We thus developed a bacterial artificial chromosome transgenic mouse, in which expression of a red fluorescent protein (mApple) is driven by the GLP-1R promoter. Using this reporter mouse, we characterized the regional and cellular expression patterns of GLP-1R expressing cells in the CNS, using double-label immunohistochemistry and in situ hybridization. GLP-1R-expressing cells were enriched in several key brain regions and circuits, including the lateral septum, hypothalamus, amygdala, bed nucleus of the stria terminalis, hippocampus, ventral midbrain, periaqueductal gray, and cerebral cortex. In most regions, GLP-1R primarily colocalized with GABAergic neurons, except within some regions such as the hippocampus, where it was co-expressed in glutamatergic neurons. GLP-1R-mApple cells were highly co-expressed with 5-HT3 receptor-containing neurons within the cortex and striatum, as well as with dopamine receptor- and calbindin-expressing cells within the lateral septum, the brain region in which GLP-1R is most highly expressed. In this manuscript, we provide detailed images of GLP-1R-mApple expression and distribution within the brain and characterization of these neurons.

Dynein activating adaptor BICD2 controls radial migration of upper-layer cortical neurons in vivo

For the proper organization of the six-layered mammalian neocortex it is required that neurons migrate radially from their place of birth towards their designated destination. The molecular machinery underlying this neuronal migration is still poorly understood. The dynein-adaptor protein BICD2 is associated with a spectrum of human neurological diseases, including malformations of cortical development. Previous studies have shown that knockdown of BICD2 interferes with interkinetic nuclear migration in radial glial progenitor cells, and that Bicd2-deficient mice display an altered laminar organization of the cerebellum and the neocortex. However, the precise in vivo role of BICD2 in neocortical development remains unclear. By comparing cell-type specific conditional Bicd2 knock-out mice, we found that radial migration in the cortex predominantly depends on BICD2 function in post-mitotic neurons. Neuron-specific Bicd2 cKO mice showed severely impaired radial migration of late-born upper-layer neurons. BICD2 depletion in cortical neurons interfered with proper Golgi organization, and neuronal maturation and survival of cortical plate neurons. Single-neuron labeling revealed a specific role of BICD2 in bipolar locomotion. Rescue experiments with wildtype and disease-related mutant BICD2 constructs revealed that a point-mutation in the RAB6/RANBP2-binding-domain, associated with cortical malformation in patients, fails to restore proper cortical neuron migration. Together, these findings demonstrate a novel, cell-intrinsic role of BICD2 in cortical neuron migration in vivo and provide new insights into BICD2-dependent dynein-mediated functions during cortical development.