Four-dimensional analysis of nucleogenesis of the pontine nucleus in the hindbrain

Uncoupling axon guidance and neuronal migration in Robo3-deficient inferior olivary neurons

Inferior olivary (IO) neurons are born in the dorsal hindbrain and migrate tangentially toward the ventral midline. During their dorsoventral migration, IO neurons extend long leading processes that cross the midline, transform into axons, and project into the contralateral cerebellum. In absence of the axon guidance receptor Robo3, IO axons fail to cross the midline and project to the ipsilateral cerebellum. Remarkably, the IO cell bodies still reach the midline where they form a nucleus of abnormal cytoarchitecture. The mechanisms underlying the migration of Robo3-deficient IO neurons are unknown. Here, we used three-dimensional imaging and transgenic mice to label subsets of IO neurons and study their migratory behavior in Robo3 knockout. We show that IO migration is delayed in absence of Robo3. Strikingly, Robo3-deficient IO neurons progress toward the midline in a direction opposite to their axons. This occurs through a change of polarity and the generation of a second leading process at the rear of the cell. These results suggest that Robo3 receptor controls the establishment of neuronal polarity and the coupling of axonogenesis and cell body migration in IO neurons.

Postmitotic Hoxa5 Expression Specifies Pontine Neuron Positional Identity and Input Connectivity of Cortical Afferent Subsets

The mammalian precerebellar pontine nucleus (PN) has a main role in relaying cortical information to the cerebellum. The molecular determinants establishing ordered connectivity patterns between cortical afferents and precerebellar neurons are largely unknown. We show that expression of Hox5 transcription factors is induced in specific subsets of postmitotic PN neurons at migration onset. Hox5 induction is achieved by response to retinoic acid signaling, resulting in Jmjd3-dependent derepression of Polycomb chromatin and 3D conformational changes. Hoxa5 drives neurons to settle posteriorly in the PN, where they are monosynaptically targeted by cortical neuron subsets mainly carrying limb somatosensation. Furthermore, Hoxa5 postmigratory ectopic expression in PN neurons is sufficient to attract cortical somatosensory inputs regardless of position and avoid visual afferents. Transcriptome analysis further suggests that Hoxa5 is involved in circuit formation. Thus, Hoxa5 coordinates postmitotic specification, migration, settling position, and sub-circuit assembly of PN neuron subsets in the cortico-cerebellar pathway.

Chemokine receptor CXCR7 non-cell-autonomously controls pontine neuronal migration and nucleus formation

Long distance tangential migration transports neurons from their birth places to distant destinations to be incorporated into neuronal circuits. How neuronal migration is guided during these long journeys is still not fully understood. We address this issue by studying the migration of pontine nucleus (PN) neurons in the mouse hindbrain. PN neurons migrate from the lower rhombic lip first anteriorly and then turn ventrally near the trigeminal ganglion root towards the anterior ventral hindbrain. Previously we showed that in mouse depleted of chemokine receptor CXCR4 or its ligand CXCL12, PN neurons make their anterior-to-ventral turn at posteriorized positions. However, the mechanism that spatiotemporally controls the anterior-to-ventral turning is still unclear. Furthermore, the role of CXCR7, the atypical receptor of CXCL12, in pontine migration has yet to be examined. Here, we find that the PN is elongated in Cxcr7 knockout due to a broadened anterior-to-ventral turning positions. Cxcr7 is not expressed in migrating PN neurons en route to their destinations, but is strongly expressed in the pial meninges. Neuroepithelium-specific knockout of Cxcr7 does not recapitulate the PN phenotype in Cxcr7 knockout, suggesting that CXCR7 acts non-cell-autonomously possibly from the pial meninges. We show further that CXCR7 regulates pontine migration by modulating CXCL12 protein levels.

The zinc-finger transcription factor GLI3 is a regulator of precerebellar neuronal migration

Hindbrain precerebellar neurons arise from progenitor pools at the dorsal edge of the embryonic hindbrain: the caudal rhombic lip. These neurons follow distinct migratory routes to establish nuclei that provide climbing or mossy fiber inputs to the cerebellum. Gli3, a zinc-finger transcription factor in the Sonic hedgehog signaling pathway, is an important regulator of dorsal brain development. We demonstrate that in Gli3-null mutant mice, disrupted neuronal migratory streams lead to a disorganization of precerebellar nuclei. Precerebellar progenitors are properly established in Gli3-null embryos and, using conditional gene inactivation, we provide evidence that Gli3 does not play a cell-autonomous role in migrating precerebellar neurons. Thus, GLI3 likely regulates the development of other hindbrain structures, such as non-precerebellar nuclei or cranial ganglia and their respective projections, which may in turn influence precerebellar migration. Although the organization of non-precerebellar hindbrain nuclei appears to be largely unaffected in absence of Gli3, trigeminal ganglia and their central descending tracts are disrupted. We show that rostrally migrating precerebellar neurons are normally in close contact with these tracts, but are detached in Gli3-null embryos.

H3.3K27M Cooperates with Trp53 Loss and PDGFRA Gain in Mouse Embryonic Neural Progenitor Cells to Induce Invasive High-Grade Gliomas

Gain-of-function mutations in histone 3 (H3) variants are found in a substantial proportion of pediatric high-grade gliomas (pHGG), often in association with TP53 loss and platelet-derived growth factor receptor alpha (PDGFRA) amplification. Here, we describe a somatic mouse model wherein H3.3^K27M and Trp53 loss alone are sufficient for neoplastic transformation if introduced in utero. H3.3^K27M-driven lesions are clonal, H3K27me3 depleted, Olig2 positive, highly proliferative, and diffusely spreading, thus recapitulating hallmark molecular and histopathological features of pHGG. Addition of wild-type PDGFRA decreases latency and increases tumor invasion, while ATRX knockdown is associated with more circumscribed tumors. H3.3^K27M-tumor cells serially engraft in recipient mice, and preliminary drug screening reveals mutation-specific vulnerabilities. Overall, we provide a faithful H3.3^K27M-pHGG model which enables insights into oncohistone pathogenesis and investigation of future therapies.

The Long Journey of Pontine Nuclei Neurons: From Rhombic Lip to Cortico-Ponto-Cerebellar Circuitry

The pontine nuclei (PN) are the largest of the precerebellar nuclei, neuronal assemblies in the hindbrain providing principal input to the cerebellum. The PN are predominantly innervated by the cerebral cortex and project as mossy fibers to the cerebellar hemispheres. Here, we comprehensively review the development of the PN from specification to migration, nucleogenesis and circuit formation. PN neurons originate at the posterior rhombic lip and migrate tangentially crossing several rhombomere derived territories to reach their final position in ventral part of the pons. The developing PN provide a classical example of tangential neuronal migration and a study system for understanding its molecular underpinnings. We anticipate that understanding the mechanisms of PN migration and assembly will also permit a deeper understanding of the molecular and cellular basis of cortico-cerebellar circuit formation and function.

Distinct migratory behaviors of striosome and matrix cells underlying the mosaic formation in the developing striatum

The striatum, the largest nucleus of the basal ganglia controlling motor and cognitive functions, can be characterized by a labyrinthine mosaic organization of striosome/matrix compartments. It is unclear how striosome/matrix mosaic formation is spatially and temporally controlled at the cellular level during striatal development. Here, by combining in vivo electroporation and brain slice cultures, we set up a prospective experimental system in which we differentially labeled striosome and matrix cells from the time of birth and followed their distributions and migratory behaviors. Our results showed that, at an initial stage of striosome/matrix mosaic formation, striosome cells were mostly stationary, whereas matrix cells actively migrated in multiple directions regardless of the presence of striosome cells. The mostly stationary striosome cells were still able to associate to form patchy clusters via attractive interactions. Our results suggest that the restricted migratory capability of striosome cells may allow them to cluster together only when they happen to be located in close proximity to each other and are not separated by actively migrating matrix cells. The way in which the mutidirectionally migrating matrix cells intermingle with the mostly stationary striosome cells may therefore determine the topographic features of striosomes. At later stages, the actively migrating matrix cells began to repulse the patchy clusters of striosomes, presumably enhancing the striosome cluster formation and the segregation and eventual formation of dichotomous homogeneous striosome/matrix compartments. Overall, our study reveals temporally distinct migratory behaviors of striosome/matrix cells, which may underlie the sequential steps of mosaic formation in the developing striatum. J. Comp. Neurol. 525:794-817, 2017. © 2016 Wiley Periodicals, Inc.

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.

In vivo imaging of cortical interneurons migrating in the intermediate/subventricular zones

We developed an imaging system that enables migrating cortical interneurons (CIs) through the lower intermediate zone/subventricular zone (IZ/SVZ) in mouse embryos. CIs were labeled by in utero electroporation performed at embryonic day (E) 11.5 and were observed, through the skull of living embryos, detached from the dam with the umbilical cord remain attached. To identify imaged cell locations, we used GAD67-GFP mice and GFP fluorescence was photo-bleached after the recording. We found that CIs in the IZ/SVZ migrated medially straight toward the midline on the tangential plane, while those in the marginal zone migrated in all directions.

Specific labeling of climbing fibers shows early synaptic interactions with immature Purkinje cells in the prenatal cerebellum

During development, growing axons must locate target cells to form synapses. This is not easy, since target cells are also growing and even actively migrating. In some brain regions, such axons have been reported to wait for the timing when target cells become mature, without invading their target region. However, in the cerebellum climbing fibers (CFs), major afferent axons, arrive near their target neurons, Purkinje cells, when the neurons are still actively migrating. We, therefore, examined whether synaptic contacts are established at such early stages. To specifically label CFs, we introduced by in utero electroporation a mixture of genes encoding for Ptf1a-enhancer-driven Cre recombinase and Cre-dependent fluorescent protein into the mouse hindbrain at embryonic day (E) 10.5 and observed them during development. The earliest stages at which labeled CFs were observed in the cerebellar primordium were E15.5-E16.5. These fibers were fasciculated in the dorsal region and entered the cerebellar primordium. Some fibers defasciculated and reached the caudal region. At E17.5 and E18.5, fasciculated fibers were also found in the mantle region, and some grew toward the surface of the primordium to penetrate a mass of Purkinje cells. Interestingly, as early as E16.5, labeled fibers were found to run in close apposition to Purkinje cell dendrites and to express a presynaptic marker. These observations suggest that CFs form synapses with Purkinje cells as soon as the fibers enter the cerebellum.

Neuronal polarization in the developing cerebral cortex

Cortical neurons consist of excitatory projection neurons and inhibitory GABAergic interneurons, whose connections construct highly organized neuronal circuits that control higher order information processing. Recent progress in live imaging has allowed us to examine how these neurons differentiate during development in vivo or in in vivo-like conditions. These analyses have revealed how the initial steps of polarization, in which neurons establish an axon, occur. Interestingly, both excitatory and inhibitory cortical neurons establish neuronal polarity de novo by undergoing a multipolar stage reminiscent of the manner in which polarity formation occurs in hippocampal neurons in dissociated culture. In this review, we focus on polarity formation in cortical neurons and describe their typical morphology and dynamic behavior during the polarization period. We also discuss cellular and molecular mechanisms underlying polarization, with reference to polarity formation in dissociated hippocampal neurons in vitro.

Calm1 signaling pathway is essential for the migration of mouse precerebellar neurons

The calcium ion regulates many aspects of neuronal migration, which is an indispensable process in the development of the nervous system. Calmodulin (CaM) is a multifunctional calcium ion sensor that transduces much of the signal. To better understand the role of Ca(2+)-CaM in neuronal migration, we investigated mouse precerebellar neurons (PCNs), which undergo stereotyped, long-distance migration to reach their final position in the developing hindbrain. In mammals, CaM is encoded by three non-allelic CaM (Calm) genes (Calm1, Calm2 and Calm3), which produce an identical protein with no amino acid substitutions. We found that these CaM genes are expressed in migrating PCNs. When the expression of CaM from this multigene family was inhibited by RNAi-mediated acute knockdown, inhibition of Calm1 but not the other two genes caused defective PCN migration. Many PCNs treated with Calm1 shRNA failed to complete their circumferential tangential migration and thus failed to reach their prospective target position. Those that did reach the target position failed to invade the depth of the hindbrain through the required radial migration. Overall, our results suggest the participation of CaM in both the tangential and radial migration of PCNs.