Morphogenetic and cellular movements that shape the mouse cerebellum; insights from genetic fate mapping

Transduction of Brain Neurons in Juvenile Chum Salmon (Oncorhynchus keta) with Recombinant Adeno-Associated Hippocampal Virus Injected into the Cerebellum during Long-Term Monitoring

Corpus cerebelli in juvenile chum salmon is a multiprojective region of the brain connected via afferent and efferent projections with the higher regions of the brainstem and synencephalon, as well as with multiprojection regions of the medulla oblongata and spinal cord. During the postembryonic development of the cerebellum in chum salmon, Oncorhynchus keta, the lateral part of the juvenile cerebellum gives rise to the caudomedial part of the definitive cerebellum, which is consistent with the data reported for zebrafish and mouse cerebellum. Thus, the topographic organization of the cerebellum and its efferents are similar between fish (chum salmon and zebrafish) and mammals, including mice and humans. The distributions of recombinant adeno-associated viral vectors (rAAVs) after an injection of the base vector into the cerebellum have shown highly specific patterns of transgene expression in bipolar neurons in the latero-caudal lobe of the juvenile chum tectum opticum. The distribution of rAAVs in the dorsal thalamus, epithalamus, nucleus rotundus, and pretectal complex indicates the targeted distribution of the transgene via the thalamo-cerebellar projections. The detection of GFP expression in the cells of the epiphysis and posterior tubercle of juvenile chum salmon is associated with the transgene’s distribution and with the cerebrospinal fluid flow, the brain ventricles and its outer surface. The direct delivery of the rAAV into the central nervous system by intracerebroventricular administration allows it to spread widely in the brain. Thus, the presence of special projection areas in the juvenile chum salmon cerebellum, as well as outside it, and the identification of the transgene’s expression in them confirm the potential ability of rAAVs to distribute in both intracerebellar and afferent and efferent extracerebellar projections of the cerebellum.

Molecular Segmentation of the Spinal Trigeminal Nucleus in the Adult Mouse Brain

The trigeminal column is a hindbrain structure formed by second order sensory neurons that receive afferences from trigeminal primary (ganglionic) nerve fibers. Classical studies subdivide it into the principal sensory trigeminal nucleus located next to the pontine nerve root, and the spinal trigeminal nucleus which in turn consists of oral, interpolar and caudal subnuclei. On the other hand, according to the prosomeric model, this column would be subdivided into segmental units derived from respective rhombomeres. Experimental studies have mapped the principal sensory trigeminal nucleus to pontine rhombomeres (r) r2-r3 in the mouse. The spinal trigeminal nucleus emerges as a plurisegmental formation covering several rhombomeres (r4 to r11 in mice) across pontine, retropontine and medullary hindbrain regions. In the present work we reexamined the issue of rhombomeric vs. classical subdivisions of this column. To this end, we analyzed its subdivisions in an AZIN2-lacZ transgenic mouse, known as a reference model for hindbrain topography, together with transgenic reporter lines for trigeminal fibers. We screened as well for genes differentially expressed along the axial dimension of this structure in the adult and juvenile mouse brain. This analysis yielded genes from multiple functional families that display transverse domains fitting the mentioned rhombomeric map. The spinal trigeminal nucleus thus represents a plurisegmental structure with a series of distinct neuromeric units having unique combinatorial molecular profiles.

The evolving view of thermogenic adipocytes - ontogeny, niche and function

The worldwide incidence of obesity and its sequelae, such as type 2 diabetes mellitus, have reached pandemic levels. Central to the development of these metabolic disorders is adipose tissue. White adipose tissue stores excess energy, whereas brown adipose tissue (BAT) and beige (also known as brite) adipose tissue dissipate energy to generate heat in a process known as thermogenesis. Strategies that activate and expand BAT and beige adipose tissue increase energy expenditure in animal models and offer therapeutic promise to treat obesity. A better understanding of the molecular mechanisms underlying the development of BAT and beige adipose tissue and the activation of thermogenic function is the key to creating practical therapeutic interventions for obesity and metabolic disorders. In this Review, we discuss the regulation of the tissue microenvironment (the adipose niche) and inter-organ communication between BAT and other tissues. We also cover the activation of BAT and beige adipose tissue in response to physiological cues (such as cold exposure, exercise and diet). We highlight advances in harnessing the therapeutic potential of BAT and beige adipose tissue by genetic, pharmacological and cell-based approaches in obesity and metabolic disorders.

Spatiotemporal expression of IgLON family members in the developing mouse nervous system

Differential expression of cell adhesion molecules in neuronal populations is one of the many mechanisms promoting the formation of functional neural circuits in the developing nervous system. The IgLON family consists of five cell surface immunoglobulin proteins that have been associated with various developmental disorders, such as autism spectrum disorder, schizophrenia, and major depressive disorder. However, there is still limited and fragmented information about their patterns of expression in certain regions of the developing nervous system and how their expression contributes to their function. Utilizing an in situ hybridization approach, we have analyzed the spatiotemporal expression of all IgLON family members in the developing mouse brain, spinal cord, eye, olfactory epithelium, and vomeronasal organ. At one prenatal (E16) and two postnatal (P0 and P15) ages, we show that each IgLON displays distinct expression patterns in the olfactory system, cerebral cortex, midbrain, cerebellum, spinal cord, and eye, indicating that they likely contribute to the wiring of specific neuronal circuitry. These analyses will inform future functional studies aimed at identifying additional roles for these proteins in nervous system development.

Engrailed-2 is a cell autonomous regulator of neurogenesis in cultured hippocampal neural stem cells

Abnormalities in genes that regulate early brain development are known risk factors for neurodevelopmental disorders. Engrailed-2 (En2) is a homeodomain transcription factor with established roles in cerebellar patterning. En2 is highly expressed in the developing mid-hindbrain region, and En2 knockout (KO) mice exhibit major deficits in mid-hindbrain structures. However, En2 is also expressed in forebrain regions including the hippocampus, but its function is unknown. Previous studies have shown that the hippocampus of En2-KO mice exhibits reductions in its volume and cell numbers due to aberrant neurogenesis. Aberrant neurogenesis is due, in part, to noncell autonomous effects, specifically, reductions of innervating norepinephrine fibers from the locus coeruleus. In this study, we investigate possible cell autonomous roles of En2 in hippocampal neurogenesis. We examine proliferation, survival, and differentiation using cultures of hippocampal neurospheres of P7 wild-type (WT) and En2-KO hippocampal neural progenitor cells (NPCs). At 7 days, En2-KO neurospheres were larger on average than WT spheres and exhibited 2.5-fold greater proliferation and 2-fold increase in apoptotic cells, similar to in vivo KO phenotype. Further, En2-KO cultures exhibited 40% less cells with neurite projections, suggesting decreased differentiation. Lastly, reestablishing En2 expression in En2-KO NPCs rescued excess proliferation. These results indicate that En2 functions in hippocampal NPCs by inhibiting proliferation and promoting survival and differentiation in a cell autonomous manner. More broadly, this study suggests that En2 impacts brain structure and function in diverse regions outside of the mid-hindbrain.

Adipogenesis: A Complex Interplay of Multiple Molecular Determinants and Pathways

The formation of adipocytes during embryogenesis has been largely understudied. However, preadipocytes appear to originate from multipotent mesenchymal stromal/stem cells which migrate from the mesoderm to their anatomical localization. Most studies on adipocyte formation (adipogenesis) have used preadipocytes derived from adult stem/stromal cells. Adipogenesis consists of two phases, namely commitment and terminal differentiation. This review discusses the role of signalling pathways, epigenetic modifiers, and transcription factors in preadipocyte commitment and differentiation into mature adipocytes, as well as limitations in our understanding of these processes. To date, a limited number of transcription factors, genes and signalling pathways have been described to regulate preadipocyte commitment. One reason could be that most studies on adipogenesis have used preadipocytes already committed to the adipogenic lineage, which are therefore not suitable for studying preadipocyte commitment. Conversely, over a dozen molecular players including transcription factors, genes, signalling pathways, epigenetic regulators, and microRNAs have been described to be involved in the differentiation of preadipocytes to adipocytes; however, only peroxisome proliferator-activated receptor gamma has proven to be clinically relevant. A detailed understanding of how the molecular players underpinning adipogenesis relate to adipose tissue function could provide new therapeutic approaches for addressing obesity without compromising adipose tissue function.

Cerebellar nuclei excitatory neurons regulate developmental scaling of presynaptic Purkinje cell number and organ growth

For neural systems to function effectively, the numbers of each cell type must be proportioned properly during development. We found that conditional knockout of the mouse homeobox genes En1 and En2 in the excitatory cerebellar nuclei neurons (eCN) leads to reduced postnatal growth of the cerebellar cortex. A subset of medial and intermediate eCN are lost in the mutants, with an associated cell non-autonomous loss of their presynaptic partner Purkinje cells by birth leading to proportional scaling down of neuron production in the postnatal cerebellar cortex. Genetic killing of embryonic eCN throughout the cerebellum also leads to loss of Purkinje cells and reduced postnatal growth but throughout the cerebellar cortex. Thus, the eCN play a key role in scaling the size of the cerebellum by influencing the survival of their Purkinje cell partners, which in turn regulate production of granule cells and interneurons via the amount of sonic hedgehog secreted.

The Ferdinando Rossi Memorial Lecture: Zones and Stripes-Pattern Formation in the Cerebellum

The cerebellum has a complex architecture-highly reproducible and conserved through evolution. Cerebellar architecture is organized around the Purkinje cell. Purkinje cells in the mouse cerebellum come in many different subtypes, identifiable by expression markers, sensitivity to mutation, etc. These are organized first into five "transverse zones," each of which is further subdivided into dozens of reproducible "stripes." This arrangement serves as the scaffolding to organize afferent topography and restrict the distribution of excitatory and inhibitory interneurons. This brief review will survey some of the mechanisms that lead to the formation of this elaborate pattern during cerebellar development. Pattern formation in the cerebellar cortex is a multistage process that begins early in development with the generation of the various Purkinje cell subtypes, and matures through the dispersal of Purkinje cell clusters into stripes. Two developmental processes will be discussed in particular: the mechanisms that lead to Purkinje cell subtype specification (i.e., how different kinds of Purkinje cells are made) and the role played by Purkinje cell migration in pattern formation (i.e., how these Purkinje cell subtypes end up in a reproducible array of stripes).

Assembling the adipose organ: adipocyte lineage segregation and adipogenesis in vivo

Adipose tissue is composed of anatomically distinct depots that mediate several important aspects of energy homeostasis. The past two decades have witnessed increased research effort to elucidate the ontogenetic basis of adipose form and function. In this Review, we discuss advances in our understanding of adipose tissue development with particular emphasis on the embryonic patterning of depot-specific adipocyte lineages and adipocyte differentiation in vivo Micro-environmental cues and other factors that influence cell identity and cell behavior at various junctures in the adipocyte lineage hierarchy are also considered.

Specification of diverse cell types during early neurogenesis of the mouse cerebellum

We applied single-cell RNA sequencing to profile genome-wide gene expression in about 9400 individual cerebellar cells from the mouse embryo at embryonic day 13.5. Reiterative clustering identified the major cerebellar cell types and subpopulations of different lineages. Through pseudotemporal ordering to reconstruct developmental trajectories, we identified novel transcriptional programs controlling cell fate specification of populations arising from the ventricular zone and the rhombic lip, two distinct germinal zones of the embryonic cerebellum. Together, our data revealed cell-specific markers for studying the cerebellum, gene-expression cascades underlying cell fate specification, and a number of previously unknown subpopulations that may play an integral role in the formation and function of the cerebellum. Our findings will facilitate new discovery by providing insights into the molecular and cell type diversity in the developing cerebellum.

What cerebellar malformations tell us about cerebellar development

Structural birth defects of the cerebellum, or cerebellar malformations, in humans, have long been recognized. However, until recently there has been little progress in elucidating their developmental pathogenesis. Innovations in brain imaging and human genetic technologies over the last 2 decades have led to better classifications of these disorders and identification of several causative genes. In contrast, cerebellar malformations in model organisms, particularly mice, have been the focus of intense study for more than 70 years. As a result, many of the molecular, genetic and cellular programs that drive formation of the cerebellum have been delineated in mice. In this review, we overview the basic epochs and key molecular regulators of the developmental programs that build the structure of the mouse cerebellum. This mouse-centric approach has been a useful to interpret the developmental pathogenesis of human cerebellar malformations. However, it is becoming apparent that we actually know very little regarding the specifics of human cerebellar development beyond what is inferred from mice. A better understanding of human cerebellar development will not only facilitate improved diagnosis of human cerebellar malformations, but also lead to the development of treatment paradigms for these important neurodevelopmental disorders.

A de novo variant in ADGRL2 suggests a novel mechanism underlying the previously undescribed association of extreme microcephaly with severely reduced sulcation and rhombencephalosynapsis

Extreme microcephaly and rhombencephalosynapsis represent unusual pathological conditions, each of which occurs in isolation or in association with various other cerebral and or extracerebral anomalies. Unlike microcephaly for which several disease-causing genes have been identified with different modes of inheritance, the molecular bases of rhombencephalosynapsis remain unknown and rhombencephalosynapsis presents mainly as a sporadic condition consistent with de novo dominant variations. We report for the first time the association of extreme microcephaly with almost no sulcation and rhombencephalosynapsis in a fœtus for which comparative patient-parent exome sequencing strategy revealed a heterozygous de novo missense variant in the ADGRL2 gene. ADGRL2 encodes latrophilin 2, an adhesion G-protein-coupled receptor whose exogenous ligand is α-latrotoxin. Adgrl2 immunohistochemistry and in situ hybridization revealed expression in the telencephalon, mesencephalon and rhombencephalon of mouse and chicken embryos. In human brain embryos and fœtuses, Adgrl2 immunoreactivity was observed in the hemispheric and cerebellar germinal zones, the cortical plate, basal ganglia, pons and cerebellar cortex. Microfluorimetry experiments evaluating intracellular calcium release in response to α-latrotoxin binding showed significantly reduced cytosolic calcium release in the fœtus amniocytes vs amniocytes from age-matched control fœtuses and in HeLa cells transfected with mutant ADGRL2 cDNA vs wild-type construct. Embryonic lethality was also observed in constitutive Adgrl2^−/− mice. In Adgrl2^+/− mice, MRI studies revealed microcephaly and vermis hypoplasia. Cell adhesion and wound healing assays demonstrated that the variation increased cell adhesion properties and reduced cell motility. Furthermore, HeLa cells overexpressing mutant ADGRL2 displayed a highly developed cytoplasmic F-actin network related to cytoskeletal dynamic modulation. ADGRL2 is the first gene identified as being responsible for extreme microcephaly with rhombencephalosynapsis. Increased cell adhesion, reduced cell motility and cytoskeletal dynamic alterations induced by the variant therefore represent a new mechanism responsible for microcephaly.

Conditional deletion of Neurog1 in the cerebellum of postnatal mice delays inhibitory interneuron maturation

The transcriptional programs that drive the generation of diverse GABAergic neuron populations from their common progenitor pools in the developing cerebellum remain unclear. Neurog1 is a pro-neural basic helix-loop-helix transcription factor expressed in GABAergic progenitor cells in the ventricular zone (VZ) of embryos and subsequently in the presumptive white matter (pWM) tracts of developing postnatal mice. Genetic inducible fate-mapping labels Purkinje cells and all inhibitory interneuron cell types of the cerebellar cortex. As conventional Neurog1^Neo knockout (KO) mice are neonatal lethal, we generated Neurog1^loxP mutant mice to examine the effects of conditional Neurog1 deletion on the postnatal development of the cerebellum. Targeted Neurog1 loss-of-function in the developing cerebellum does not result in significant differences in cerebellar morphology or in the number of GABAergic neurons in the cerebellar cortex of mice at postnatal day 21 (P21). To determine the effects of Neurog1 deletion on GABAergic progenitors, we quantified rates of cell proliferation and cell cycle progression or re-entry in embryonic Neurog1^Neo and postnatal Neurog1^loxP mutants. The data revealed no significant effect of Neurog1 loss-of-function on embryonic day 12.5 (E12.5) VZ progenitors or on P5 and P6 progenitors in the pWM at P7. However, 4-5 day pulse-labeling of P5 and P6 progenitors revealed reductions in inhibitory interneuron dispersal from the pWM to the cerebellar cortex in P10 conditional Neurog1^loxP/loxP KO mice. Thus, our conditional Neurog1 KO approach reveals a requirement for Neurog1 activity in inhibitory interneuron cell dispersal from pWM tracts in the developing cerebellum of postnatal mice.

Lateral cerebellum is preferentially sensitive to high sonic hedgehog signaling and medulloblastoma formation

The main cell of origin of the Sonic hedgehog (SHH) subgroup of medulloblastoma (MB) is granule cell precursors (GCPs), a SHH-dependent transient amplifying population in the developing cerebellum. SHH-MBs can be further subdivided based on molecular and clinical parameters, as well as location because SHH-MBs occur preferentially in the lateral cerebellum (hemispheres). Our analysis of adult patient data suggests that tumors with Smoothened (SMO) mutations form more specifically in the hemispheres than those with Patched 1 (PTCH1) mutations. Using sporadic mouse models of SHH-MB with the two mutations commonly seen in adult MB, constitutive activation of Smo (SmoM2) or loss-of-Ptch1, we found that regardless of timing of induction or type of mutation, tumors developed primarily in the hemispheres, with SmoM2-mutants indeed showing a stronger specificity. We further uncovered that GCPs in the hemispheres are more susceptible to high-level SHH signaling compared with GCPs in the medial cerebellum (vermis), as more SmoM2 or Ptch1-mutant hemisphere cells remain undifferentiated and show increased tumorigenicity when transplanted. Finally, we identified location-specific GCP gene-expression profiles, and found that deletion of the genes most highly expressed in the hemispheres (Nr2f2) or vermis (Engrailed1) showed opposing effects on GCP differentiation. Our studies thus provide insights into intrinsic differences within GCPs that impact on SHH-MB progression.

Cerebellar Vermis and Midbrain Hypoplasia Upon Conditional Deletion of Chd7 from the Embryonic Mid-Hindbrain Region

Reduced fibroblast growth factor (FGF) signaling from the mid-hindbrain or isthmus organizer (IsO) during early embryonic development results in hypoplasia of the midbrain and cerebellar vermis. We previously reported evidence for reduced Fgf8 expression and FGF signaling in the mid-hindbrain region of embryos heterozygous for Chd7, the gene mutated in CHARGE (Coloboma, Heart defects, choanal Atresia, Retarded growth and development, Genitourinary anomalies and Ear defects) syndrome. However, Chd7^+/- animals only exhibit mild cerebellar vermis anomalies. As homozygous deletion of Chd7 is embryonic lethal, we conditionally deleted Chd7 from the early embryonic mid-hindbrain region to identify the function of CHD7 in mid-hindbrain development. Using a combination of high resolution structural MRI and histology, we report striking midbrain and cerebellar vermis hypoplasia in the homozygous conditional mutants. We show that cerebellar vermis hypoplasia is associated with reduced embryonic Fgf8 expression and an expanded roof plate in rhombomere 1 (r1). These findings identify an essential role for Chd7 in regulating mid-hindbrain development via Fgf8.

Novel MYC-driven medulloblastoma models from multiple embryonic cerebellar cells

Group3 medulloblastoma (MBG3) that predominantly occur in young children are usually associated with MYC amplification and/or overexpression, frequent metastasis and a dismal prognosis. Physiologically relevant MBG3 models are currently lacking, making inferences related to their cellular origin thus far limited. Using in utero electroporation, we here report that MBG3 mouse models can be developed in situ from different multipotent embryonic cerebellar progenitor cells via conditional expression of Myc and loss of Trp53 function in several Cre driver mouse lines. The Blbp-Cre driver that targets embryonic neural progenitors induced tumors exhibiting a large-cell/anaplastic histopathology adjacent to the fourth ventricle, recapitulating human MBG3. Enforced co-expression of luciferase together with Myc and a dominant-negative form of Trp53 revealed that GABAergic neuronal progenitors as well as cerebellar granule cells give rise to MBG3 with their distinct growth kinetics. Cross-species gene expression analysis revealed that these novel MBG3 models shared molecular characteristics with human MBG3, irrespective of their cellular origin. We here developed MBG3 mouse models in their physiological environment and we show that oncogenic insults drive this MB subgroup in different cerebellar lineages rather than in a specific cell of origin.

An expandable embryonic stem cell-derived Purkinje neuron progenitor population that exhibits in vivo maturation in the adult mouse cerebellum

The directed differentiation of patient-derived induced pluripotent stem cells into cell-type specific neurons has inspired the development of therapeutic discovery for neurodegenerative diseases. Many forms of ataxia result from degeneration of cerebellar Purkinje cells, but thus far it has not been possible to efficiently generate Purkinje neuron (PN) progenitors from human or mouse pluripotent stem cells, let alone to develop a methodology for in vivo transplantation in the adult cerebellum. Here, we present a protocol to obtain an expandable population of cerebellar neuron progenitors from mouse embryonic stem cells. Our protocol is characterized by applying factors that promote proliferation of cerebellar progenitors. Cerebellar progenitors isolated in culture from cell aggregates contained a stable subpopulation of PN progenitors that could be expanded for up to 6 passages. When transplanted into the adult cerebellum of either wild-type mice or a strain lacking Purkinje cells (L7cre-ERCC1 knockout), GFP-labeled progenitors differentiated in vivo to establish a population of calbindin-positive cells in the molecular layer with dendritic trees typical of mature PNs. We conclude that this protocol may be useful for the generation and maturation of PNs, highlighting the potential for development of a regenerative medicine approach to the treatment of cerebellar neurodegenerative diseases.

The Temporal Contribution of the Gbx2 Lineage to Cerebellar Neurons

The cerebellum (Cb) is an exquisite structure that controls elaborate motor behaviors and is essential for sensory-motor learning. During development, the Cb is derived from rhombomere 1 (r1). Within this embryonic compartment, precursors in r1 are patterned by signaling cues originating from the isthmus organizer (IsO) and subsequently undergo complex morphogenic movements to establish their final position in the mature Cb. The transcription factor Gbx2 is expressed in the developing Cb and is intimately involved in organizing and patterning the Cb. Nevertheless, how precursors expressing Gbx2 at specific embryonic time points contribute to distinct cell types in the adult Cb is unresolved. In this study, we used Genetic Inducible Fate Mapping (GIFM) to mark Gbx2-expressing precursors with fine temporal resolution and to subsequently track this lineage through embryogenesis. We then determined the terminal neuronal fate of the Gbx2 lineage in the adult Cb. Our analysis demonstrates that the Gbx2 lineage contributes to the Cb with marking over the course of five stages: Embryonic day 7.5 (E7.5) through E11.5. The Gbx2 lineage gives rise to Purkinje cells, granule neurons, and deep cerebellar neurons across these marking stages. Notably, the contribution of the Gbx2 lineage shifts as development proceeds with each marking stage producing a distinct profile of mature neurons in the adult Cb. These findings demonstrate the relationship between the temporal expression of Gbx2 and the terminal cell fate of neurons in the Cb. Based on these results, Gbx2 is critical to Cb development, not only for its well-defined role in positioning and maintaining the IsO, but also for guiding the development of Cb precursors and determining the identity of Cb neurons.

Consensus Paper: Cerebellar Development

The development of the mammalian cerebellum is orchestrated by both cell-autonomous programs and inductive environmental influences. Here, we describe the main processes of cerebellar ontogenesis, highlighting the neurogenic strategies used by developing progenitors, the genetic programs involved in cell fate specification, the progressive changes of structural organization, and some of the better-known abnormalities associated with developmental disorders of the cerebellum.

Fetal Cerebellar Vermis Circumference Measured by 2-Dimensional Ultrasound Scan: Reference Range, Feasibility and Reproducibility

Purpose: To provide 2-dimensional ultrasonographic (2D-US) normograms of cerebellar vermis biometry, as well as to evaluate the feasibility and the reproducibility of these measurements in clinical practice. Materials and Methods: A prospective cross-sectional study of 328 normal singleton pregnancies between 18 and 33 weeks of gestation. Measurements of the fetal cerebellar vermis circumference (VC) in the mid-sagittal plane were performed by both a senior and a junior operator using 2D-US. VC as a function of gestational age (GA) was expressed by regression equations. In 24 fetuses 3-dimensional (3D) reconstructed planes were obtained in order to allow comparisons with 2D-US measurements. The agreement between 2D and 3D measurements and the interobserver variability were assessed by interclass correlation coefficients (ICC). Results: Satisfactory vermis measurements could be obtained in 89.9% of cases. The VC (constant= - 12.21; slope=2.447; r=0.887, p<0.0001) correlated linearly with GA. A high degree of consistency was observed between 2D and 3D ultrasound measurements (ICC=0.846 95% CI 679-0.930) as well as between measurements obtained by different examiners (ICC=0.890 95% CI 989-0.945). Conclusion: 2-dimensional ultrasonographic measurements of cerebellar vermis throughout gestation in the mid-sagittal view seem to be feasible and reproducible enough to be potentially used in clinical practice. Such measurements may supply a tool for accurate identification of posterior fossa anomalies, providing the basis for proper counseling and management and of these conditions.

A knock-in allele of En1 expressing dre recombinase

Engrailed 1 (En1) is a homeobox-containing transcription factor expressed during development in diverse tissues, including the embryonic midbrain and anterior hindbrain. To facilitate investigation of genetic and developmental heterogeneity among cells with a history of En1 expression, we have generated En1(Dre) , a knock-in allele expressing Dre recombinase. En1(Dre) can be used with existing Cre and Flp recombinase lines for genetic intersectional labeling, fate mapping, and functional manipulation of subpopulations of cells characterized by transient expression of En1. To avoid disrupting En1 function, the Dre cDNA is inserted at the 3' end of the En1 coding sequence, together with a viral 2A peptide to mediate translation of separate EN1 and Dre proteins. Consequently, viable and fertile En1(Dre) homozygotes can be used to increase the proportion of useful genotypes produced in complex crosses. The pattern of Dre expression from En1(Dre) is indistinguishable from wild-type En1 expression in mid-gestation mouse embryos, and En1(Dre) controls Dre-responsive indicator alleles by efficiently recombining rox sites in vivo. Through the application of genetic tools that allow manipulation of cells based on combinatorial expression of multiple distinct recombinases, En1(Dre) will significantly extend the ability to target important subpopulations of neurons and other cells within the broader En1 expression domain. genesis 54:447-454, 2016. Published 2016. This article is a US Government work and is in the public domain in the USA.

Adult Neurogenesis in Fish

Teleost fish have a remarkable neurogenic and regenerative capacity in the adult throughout the rostrocaudal axis of the brain. The distribution of proliferation zones shows a remarkable conservation, even in distantly related teleost species, suggesting a common teleost ground plan of proliferation zones. There are different progenitor populations in the neurogenic niches-progenitors positive for radial glial markers (dorsal telencephalon, hypothalamus) and progenitors with neuroepithelial-like characteristics (ventral telencephalon, optic tectum, cerebellum). Definition of these progenitors has allowed studying their role in normal growth of the adult brain, but also when challenged following a lesion. From these studies, important roles have emerged for intrinsic mechanisms and extrinsic signals controlling the activation of adult neurogenesis that enable regeneration of the adult brain to occur, opening up new perspectives on rekindling regeneration also in the context of the mammalian brain.

Neurog1 Genetic Inducible Fate Mapping (GIFM) Reveals the Existence of Complex Spatiotemporal Cyto-Architectures in the Developing Cerebellum

Neurog1 is a pro-neural basic helix-loop-helix (bHLH) transcription factor expressed in progenitor cells located in the ventricular zone and subsequently the presumptive white matter tracts of the developing mouse cerebellum. We used genetic inducible fate mapping (GIFM) with a transgenic Neurog1-CreER allele to characterize the contributions of Neurog1 lineages to cerebellar circuit formation in mice. GIFM reveals Neurog1-expressing progenitors are fate-mapped to become Purkinje cells and all GABAergic interneuron cell types of the cerebellar cortex but not glia. The spatiotemporal sequence of GIFM is unique to each neuronal cell type. GIFM on embryonic days (E) 10.5 to E12.5 labels Purkinje cells with different medial-lateral settling patterns depending on the day of tamoxifen delivery. GIFM on E11.5 to P7 labels interneurons and the timing of tamoxifen administration correlates with the final inside-to-outside resting position of GABAergic interneurons in the cerebellar cortex. Proliferative status and long-term BrdU retention of GIFM lineages reveals Purkinje cells express Neurog1 around the time they become post-mitotic. In contrast, GIFM labels mitotic and post-mitotic interneurons. Neurog1-CreER GIFM reveals a correlation between the timing of Neurog1 expression and the spatial organization of GABAergic neurons in the cerebellar cortex with possible implications for cerebellar circuit assembly.

Emerging Complexities in Adipocyte Origins and Identity

The global incidence of obesity and its comorbidities continues to rise along with a demand for novel therapeutic interventions. Brown adipose tissue (BAT) is attracting attention as a therapeutic target because of its presence in adult humans and high capacity to dissipate energy as heat, and thus burn excess calories, when stimulated. Another potential avenue for therapeutic intervention is to induce, within white adipose tissue (WAT), the formation of brown-like adipocytes called brite (brown-like-in-white) or beige adipocytes. However, understanding how to harness the potential of these thermogenic cells requires a deep understanding of their developmental origins and regulation. Recent cell-labeling and lineage-tracing experiments are beginning to shed light on this emerging area of adipocyte biology. We review here adipocyte development, giving particular attention to thermogenic adipocytes.

A Mathematical Model of Granule Cell Generation During Mouse Cerebellum Development

Determining the cellular basis of brain growth is an important problem in developmental neurobiology. In the mammalian brain, the cerebellum is particularly amenable to studies of growth because it contains only a few cell types, including the granule cells, which are the most numerous neuronal subtype. Furthermore, in the mouse cerebellum granule cells are generated from granule cell precursors (gcps) in the external granule layer (EGL), from 1 day before birth until about 2 weeks of age. The complexity of the underlying cellular processes (multiple cell behaviors, three spatial dimensions, time-dependent changes) requires a quantitative framework to be fully understood. In this paper, a differential equation-based model is presented, which can be used to estimate temporal changes in granule cell numbers in the EGL. The model includes the proliferation of gcps and their differentiation into granule cells, as well as the process by which granule cells leave the EGL. Parameters describing these biological processes were derived from fitting the model to histological data. This mathematical model should be useful for understanding altered gcp and granule cell behaviors in mouse mutants with abnormal cerebellar development and cerebellar cancers.

Engrailed 1 shapes the dopaminergic and serotonergic landscape through proper isthmic organizer maintenance and function

The isthmic organizer (IsO) is a signaling center that specifies the correct and distinct embryonic development of the dopaminergic midbrain and serotonergic hindbrain. The IsO is a linear boundary between the two brain regions, emerging at around embryonic day 7-8 of murine embryonic development, that shapes its surroundings through the expression of instructive signals such as Wnt and growth factors. Homeobox transcription factor engrailed 1 (En1) is present in midbrain and rostral hindbrain (i.e. rhombomere 1, R1). Its expression spans the IsO, and it is known to be an important survival factor for both dopaminergic and serotonergic neurons. Erroneous composition of dopaminergic neurons in the midbrain or serotonergic neurons in the hindbrain is associated with severe pathologies such as Parkinson's disease, depression or autism. Here we investigated the role of En1 in early mid-hindbrain development, using multiple En1-ablated mouse models as well as lineage-tracing techniques, and observed the appearance of ectopic dopaminergic neurons, indistinguishable from midbrain dopaminergic neurons based on molecular profile and intrinsic electrophysiological properties. We propose that this change is the direct result of a caudal relocation of the IsO as represented by ectopic presence of Fgf8, Otx2, Wnt1 and canonical Wnt-signalling. Our work suggests a newly-discovered role for En1: the repression of Otx2, Wnt1 and canonical Wnt-signaling in R1. Overall, our results suggest that En1 is essential for proper IsO maintenance and function.

Summary: Local molecular coding under the influence of En1 is essential for proper spatiotemporal expression of key factors involved in the maintenance and function of the isthmic organizer.

Reference Charts for Fetal Cerebellar Vermis Height: A Prospective Cross-Sectional Study of 10605 Fetuses

OBJECTIVE

To establish reference charts for fetal cerebellar vermis height in an unselected population.

METHODS

A prospective cross-sectional study between September 2009 and December 2014 was carried out at ALTAMEDICA Fetal-Maternal Medical Centre, Rome, Italy. Of 25203 fetal biometric measurements, 12167 (48%) measurements of the cerebellar vermis were available. After excluding 1562 (12.8%) measurements, a total of 10605 (87.2%) fetuses were considered and analyzed once only. Parametric and nonparametric quantile regression models were used for the statistical analysis. In order to evaluate the robustness of the proposed reference charts regarding various distributional assumptions on the ultrasound measurements at hand, we compared the gestational age-specific reference curves we produced through the statistical methods used. Normal mean height based on parametric and nonparametric methods were defined for each week of gestation and the regression equation expressing the height of the cerebellar vermis as a function of gestational age was calculated. Finally the correlation between dimension/gestation was measured.

RESULTS

The mean height of the cerebellar vermis was 12.7mm (SD, 1.6mm; 95% confidence interval, 12.7-12.8mm). The regression equation expressing the height of the CV as a function of the gestational age was: height (mm) = -4.85+0.78 x gestational age. The correlation between dimension/gestation was expressed by the coefficient r = 0.87.

CONCLUSION

This is the first prospective cross-sectional study on fetal cerebellar vermis biometry with such a large sample size reported in literature. It is a detailed statistical survey and contains new centile-based reference charts for fetal height of cerebellar vermis measurements.

Engrailed-2 (En2) deletion produces multiple neurodevelopmental defects in monoamine systems, forebrain structures and neurogenesis and behavior

Many genes involved in brain development have been associated with human neurodevelopmental disorders, but underlying pathophysiological mechanisms remain undefined. Human genetic and mouse behavioral analyses suggest that ENGRAILED-2 (EN2) contributes to neurodevelopmental disorders, especially autism spectrum disorder. In mouse, En2 exhibits dynamic spatiotemporal expression in embryonic mid-hindbrain regions where monoamine neurons emerge. Considering their importance in neuropsychiatric disorders, we characterized monoamine systems in relation to forebrain neurogenesis in En2-knockout (En2-KO) mice. Transmitter levels of serotonin, dopamine and norepinephrine (NE) were dysregulated from Postnatal day 7 (P7) to P21 in En2-KO, though NE exhibited the greatest abnormalities. While NE levels were reduced ∼35% in forebrain, they were increased 40 -: 75% in hindbrain and cerebellum, and these patterns paralleled changes in locus coeruleus (LC) fiber innervation, respectively. Although En2 promoter was active in Embryonic day 14.5 -: 15.5 LC neurons, expression diminished thereafter and gene deletion did not alter brainstem NE neuron numbers. Significantly, in parallel with reduced NE levels, En2-KO forebrain regions exhibited reduced growth, particularly hippocampus, where P21 dentate gyrus granule neurons were decreased 16%, suggesting abnormal neurogenesis. Indeed, hippocampal neurogenic regions showed increased cell death (+77%) and unexpectedly, increased proliferation. Excess proliferation was restricted to early Sox2/Tbr2 progenitors whereas increased apoptosis occurred in differentiating (Dcx) neuroblasts, accompanied by reduced newborn neuron survival. Abnormal neurogenesis may reflect NE deficits because intra-hippocampal injections of β-adrenergic agonists reversed cell death. These studies suggest that disruption of hindbrain patterning genes can alter monoamine system development and thereby produce forebrain defects that are relevant to human neurodevelopmental disorders.

Development of external surfaces of human cerebellar lobes in the fetal period

In the fetal period, development of cerebellar lobes may proceed dissimilarly due to possible differentiated origins of the cells and diversified times of their migration to certain cerebellum regions. This can cause various growth trajectories for the external surfaces of cerebellar lobes. The goal of the study was to describe the development of the external surface of cerebellum lobes and fissures delineating them in the fetal period. The material consisted of 101 fetuses (48 males and 53 females)-crown rump length 89-229 mm corresponding to 15-28 weeks of fetal life. The methods were based on anthropometric measurements and preparation techniques combined with elicited image computer analysis. At the largest values of the cerebellum posterior lobe surface, the most dynamic growth rate was observed in the case of the anterior lobe. Among the cerebellar lobes, proportional change was observed as well as a gradual increase in anterior lobe surface area and a simultaneous decrease in the surface area of the flocculonodular lobe part of the cerebellum total external surface. This paper presents the different growth trajectories of cerebellar lobes and demonstrates the importance of the primary fissure as a delineating mark for two regions with different dynamics of development.

Sox9 is critical for suppression of neurogenesis but not initiation of gliogenesis in the cerebellum

Background

The high mobility group (HMG) family transcription factor Sox9 is critical for induction and maintenance of neural stem cell pool in the central nervous system (CNS). In the spinal cord and retina, Sox9 is also the master regulator that defines glial fate choice by mediating the neurogenic-to-gliogenic fate switch. On the other hand, the genetic repertoire governing the maintenance and fate decision of neural progenitor pool in the cerebellum has remained elusive.

Results

By employing the Cre/loxP strategy, we specifically inactivated Sox9 in the mouse cerebellum. Unexpectedly, the self-renewal capacity and multipotency of neural progenitors at the cerebellar ventricular zone (VZ) were not perturbed upon Sox9 ablation. Instead, the mutants exhibited an increased number of VZ-derived neurons including Purkinje cells and GABAergic interneurons. Simultaneously, we observed continuous neurogenesis from Sox9-null VZ at late gestation, when normally neurogenesis ceases to occur and gives way for gliogenesis. Surprisingly, glial cell specification was not affected upon Sox9 ablation.

Conclusion

Our findings suggest Sox9 may mediate the neurogenic-to-gliogenic fate switch in mouse cerebellum by modulating the termination of neurogenesis, and therefore indicate a functional discrepancy of Sox9 between the development of cerebellum and other major neural tissues.

Cellular commitment in the developing cerebellum

The mammalian cerebellum is located in the posterior cranial fossa and is critical for motor coordination and non-motor functions including cognitive and emotional processes. The anatomical structure of cerebellum is distinct with a three-layered cortex. During development, neurogenesis and fate decisions of cerebellar primordium cells are orchestrated through tightly controlled molecular events involving multiple genetic pathways. In this review, we will highlight the anatomical structure of human and mouse cerebellum, the cellular composition of developing cerebellum, and the underlying gene expression programs involved in cell fate commitments in the cerebellum. A critical evaluation of the cell death literature suggests that apoptosis occurs in ~5% of cerebellar cells, most shortly after mitosis. Apoptosis and cellular autophagy likely play significant roles in cerebellar development, we provide a comprehensive discussion of their role in cerebellar development and organization. We also address the possible function of unfolded protein response in regulation of cerebellar neurogenesis. We discuss recent advancements in understanding the epigenetic signature of cerebellar compartments and possible connections between DNA methylation, microRNAs and cerebellar neurodegeneration. Finally, we discuss genetic diseases associated with cerebellar dysfunction and their role in the aging cerebellum.

In vivo analysis of Purkinje cell firing properties during postnatal mouse development

Purkinje cell activity is essential for controlling motor behavior. During motor behavior Purkinje cells fire two types of action potentials: simple spikes that are generated intrinsically and complex spikes that are induced by climbing fiber inputs. Although the functions of these spikes are becoming clear, how they are established is still poorly understood. Here, we used in vivo electrophysiology approaches conducted in anesthetized and awake mice to record Purkinje cell activity starting from the second postnatal week of development through to adulthood. We found that the rate of complex spike firing increases sharply at 3 wk of age whereas the rate of simple spike firing gradually increases until 4 wk of age. We also found that compared with adult, the pattern of simple spike firing during development is more irregular as the cells tend to fire in bursts that are interrupted by long pauses. The regularity in simple spike firing only reached maturity at 4 wk of age. In contrast, the adult complex spike pattern was already evident by the second week of life, remaining consistent across all ages. Analyses of Purkinje cells in alert behaving mice suggested that the adult patterns are attained more than a week after the completion of key morphogenetic processes such as migration, lamination, and foliation. Purkinje cell activity is therefore dynamically sculpted throughout postnatal development, traversing several critical events that are required for circuit formation. Overall, we show that simple spike and complex spike firing develop with unique developmental trajectories.

Functional regionalization of the teleost cerebellum analyzed in vivo

There has been accumulating evidence for a regionalized organization of the cerebellum, which was mostly deduced from anatomical mapping of axonal projections of cerebellar afferents. A likewise regionalization of the cerebellar output has been suggested from lesion studies and dye-tracer experiments, but its physiological targets as well as the functional relevance of such an output regionalization are less clear. Ideally, such functional regionalization should be proven noninvasively in vivo. We here provide evidence for such a regionalization of the output from the cerebellar cortex by genetically encoded transneuronal mapping of efferent circuits of zebrafish Purkinje neurons. These identified circuits correspond to distinct regionalized Purkinje cell activity patterns in freely behaving zebrafish larvae during the performance of cerebellar-dependent behaviors. Furthermore, optogenetic interrogation of selected Purkinje cell regions during animal behavior confirms the functional regionalization of Purkinje cell efferents and reveals their contribution to behavior control as well as their function in controlling lateralized behavioral output. Our findings reveal how brain compartments serve to fulfill a multitude of functions by dedicating specialized efferent circuits to distinct behavioral tasks.

Hypoxic preconditioning differentially affects GABAergic and glutamatergic neuronal cells in the injured cerebellum of the neonatal rat

In this study we examined cerebellar alterations in a neonatal rat model of hypoxic-ischemic brain injury with or without hypoxic preconditioning (Pc). Between postnatal days 7 and 15, the cerebellum is still undergoing intense cellular proliferation, differentiation and migration, dendritogenesis and synaptogenesis. The expression of glutamate decarboxylase 1 (GAD67) and the differentiation factor NeuroD1 were examined as markers of Purkinje and granule cells, respectively. We applied quantitative immunohistochemistry to sagittal cerebellar slices, and Western blot analysis of whole cerebella obtained from control (C) rats and rats submitted to Pc, hypoxia-ischemia (L) and a combination of both treatments (PcL). We found that either hypoxia-ischemia or Pc perturbed the granule cells in the posterior lobes, affecting their migration and final placement in the internal granular layer. These effects were partially attenuated when the Pc was delivered prior to the hypoxia-ischemia. Interestingly, whole nuclear NeuroD1 levels in Pc animals were comparable to those in the C rats. However, a subset of Purkinje cells that were severely affected by the hypoxic-ischemic insult—showing signs of neuronal distress at the levels of the nucleus, cytoplasm and dendritic arborization—were not protected by Pc. A monoclonal antibody specific for GAD67 revealed a three-band pattern in cytoplasmic extracts from whole P15 cerebella. A ∼110 kDa band, interpreted as a potential homodimer of a truncated form of GAD67, was reduced in Pc and L groups while its levels were close to the control animals in PcL rats. Additionally we demonstrated differential glial responses depending on the treatment, including astrogliosis in hypoxiated cerebella and a selective effect of hypoxia-ischemia on the vimentin-immunolabeled intermediate filaments of the Bergmann glia. Thus, while both glutamatergic and GABAergic cerebellar neurons are compromised by the hypoxic-ischemic insult, the former are protected by a preconditioning hypoxia while the latter are not.

Generation and characterization of an Nse-CreERT2 transgenic line suitable for inducible gene manipulation in cerebellar granule cells

We created an Nse-CreER^T2 mouse line expressing the tamoxifen-inducible CreER^T2 recombinase under the control of the neuron-specific enolase (Nse) promoter. By using Cre reporter lines we could show that this Nse-CreER^T2 line has recombination activity in the granule cells of all cerebellar lobules as well as in postmitotic granule cell precursors in the external granular layer of the developing cerebellum. A few hippocampal dentate gyrus granule cells showed Cre-mediated recombination as well. Cre activity could be induced in both the developing and adult mouse brain. The established mouse line constitutes a valuable tool to study the function of genes expressed by cerebellar granule cells in the developing and adult brain. In combination with reporter lines it is a useful model to analyze the development and maintenance of the cerebellar architecture including granule cell distribution, migration, and the extension of granule cell fibers in vivo.

Strategies to achieve conditional gene mutation in mice

The laboratory mouse is an ideal model organism for studying disease because it is physiologically similar to human and also because its genome is readily manipulated. Genetic engineering allows researchers to introduce specific loss-of-function or gain-of-function mutations into genes and then to study the resulting phenotypes in an in vivo context. One drawback of using traditional transgenic and knockout mice to study human diseases is that many mutations passed through the germline can profoundly affect development, thus impeding the study of disease phenotypes in adults. New technology has made it possible to generate conditional mutations that can be introduced in a spatially and/or temporally restricted manner. Mouse strains carrying conditional mutations represent valuable experimental models for the study of human diseases and they can be used to develop strategies for prevention and treatment of these diseases. In this article, we will describe the most widely used DNA recombinase systems used to achieve conditional gene mutation in mouse models and discuss how these systems can be employed in vivo.

Adipocyte lineages: tracing back the origins of fat

The obesity epidemic has intensified efforts to understand the mechanisms controlling adipose tissue development. Adipose tissue is generally classified as white adipose tissue (WAT), the major energy storing tissue, or brown adipose tissue (BAT), which mediates non-shivering thermogenesis. It is hypothesized that brite adipocytes (brown in white) may represent a third adipocyte class. The recent realization that brown fat exist in adult humans suggests increasing brown fat energy expenditure could be a therapeutic strategy to combat obesity. To understand adipose tissue development, several groups are tracing the origins of mature adipocytes back to their adult precursor and embryonic ancestors. From these studies emerged a model that brown adipocytes originate from a precursor shared with skeletal muscle that expresses Myf5-Cre, while all white adipocytes originate from a Myf5-negative precursors. While this provided a rational explanation to why BAT is more metabolically favorable than WAT, recent work indicates the situation is more complex because subsets of white adipocytes also arise from Myf5-Cre expressing precursors. Lineage tracing studies further suggest that the vasculature may provide a niche supporting both brown and white adipocyte progenitors; however, the identity of the adipocyte progenitor cell is under debate. Differences in origin between adipocytes could explain metabolic heterogeneity between depots and/or influence body fat patterning particularly in lipodystrophy disorders. Here, we discuss recent insights into adipose tissue origins highlighting lineage-tracing studies in mice, how variations in metabolism or signaling between lineages could affect body fat distribution, and the questions that remain unresolved. This article is part of a Special Issue entitled: Modulation of Adipose Tissue in Health and Disease.

Independently specified Atoh1 domains define novel developmental compartments in rhombomere 1

The rhombic lip gives rise to neuronal populations that contribute to cerebellar, proprioceptive and interoceptive networks. Cell production depends on the expression of the basic helix-loop-helix (bHLH) transcription factor Atoh1. In rhombomere 1, Atoh1-positive cells give rise to both cerebellar neurons and extra-cerebellar nuclei in ventral hindbrain. The origin of this cellular diversity has previously been attributed to temporal signals rather than spatial patterning. Here, we show that in both chick and mouse the cerebellar Atoh1 precursor pool is partitioned into initially cryptic spatial domains that reflect the activity of two different organisers: an isthmic Atoh1 domain, which gives rise to isthmic nuclei, and the rhombic lip, which generates deep cerebellar nuclei and granule cells. We use a combination of in vitro explant culture, genetic fate mapping and gene overexpression and knockdown to explore the role of isthmic signalling in patterning these domains. We show that an FGF-dependent isthmic Atoh1 domain is the origin of distinct populations of Lhx9-positive neurons in the extra-cerebellar isthmic nuclei. In the cerebellum, ectopic FGF induces proliferation while blockade reduces the length of the cerebellar rhombic lip. FGF signalling is not required for the specification of cerebellar cell types from the rhombic lip and its upregulation inhibits their production. This suggests that although the isthmus regulates the size of the cerebellar anlage, the downregulation of isthmic FGF signals is required for induction of rhombic lip-derived cerebellar neurons.

Mouse model reveals the role of RERE in cerebellar foliation and the migration and maturation of Purkinje cells

Nuclear receptors and their coregulators play a critical role in brain development by regulating the spatiotemporal expression of their target genes. The arginine-glutamic acid dipeptide repeats gene (Rere) encodes a nuclear receptor coregulator previously known as Atrophin 2. In the developing cerebellum, RERE is expressed in the molecular layer, the Purkinje cell layer and the granule cell layer but not in granule cell precursors. To study RERE's role in cerebellar development, we used RERE-deficient embryos bearing a null allele (om) and a hypomorphic allele (eyes3) of Rere (Rere^om/eyes3). In contrast to wild-type embryos, formation of the principal fissures in these RERE-deficient embryos was delayed and the proliferative activity of granule cell precursors (GCPs) was reduced at E18.5. This reduction in proliferation was accompanied by a decrease in the expression of sonic hedgehog (SHH), which is secreted from Purkinje cells and is required for normal GCP proliferation. The maturation and migration of Purkinje cells in Rere^om/eyes3 embryos was also delayed with decreased numbers of post-migratory Purkinje cells in the cerebellum. During the postnatal period, RERE depletion caused incomplete division of lobules I/II and III due to truncated development of the precentral fissure in the cerebellar vermis, abnormal development of lobule crus I and lobule crus II in the cerebellar hemispheres due to attenuation of the intercrural fissure, and decreased levels of Purkinje cell dendritic branching. We conclude that RERE-deficiency leads to delayed development of the principal fissures and delayed maturation and migration of Purkinje cells during prenatal cerebellar development and abnormal cerebellar foliation and Purkinje cell maturation during postnatal cerebellar development.

Deregulated FGF and homeotic gene expression underlies cerebellar vermis hypoplasia in CHARGE syndrome

Mutations in CHD7 are the major cause of CHARGE syndrome, an autosomal dominant disorder with an estimated prevalence of 1/15,000. We have little understanding of the disruptions in the developmental programme that underpin brain defects associated with this syndrome. Using mouse models, we show that Chd7 haploinsufficiency results in reduced Fgf8 expression in the isthmus organiser (IsO), an embryonic signalling centre that directs early cerebellar development. Consistent with this observation, Chd7 and Fgf8 loss-of-function alleles interact during cerebellar development. CHD7 associates with Otx2 and Gbx2 regulatory elements and altered expression of these homeobox genes implicates CHD7 in the maintenance of cerebellar identity during embryogenesis. Finally, we report cerebellar vermis hypoplasia in 35% of CHARGE syndrome patients with a proven CHD7 mutation. These observations provide key insights into the molecular aetiology of cerebellar defects in CHARGE syndrome and link reduced FGF signalling to cerebellar vermis hypoplasia in a human syndrome.

DOI:http://dx.doi.org/10.7554/eLife.01305.001

CHARGE syndrome is a rare genetic condition that causes various developmental abnormalities, including heart defects, deafness and neurological defects. In most cases, it is caused by mutations in a human gene called CHD7. CHD7 is known to control the expression of other genes during embryonic development, but the molecular mechanisms by which mutations in CHD7 lead to the neural defects found in CHARGE syndrome are unclear.

During embryonic development, the neural tube—the precursor to the nervous system—is divided into segments, which give rise to different neural structures. The r1 segment, for example, forms the cerebellum, and the secretion of a protein called FGF8 (short for fibroblast growth factor 8) by a nearby structure called the isthmus organiser has an important role in this process. Since a reduction in FGF8 causes defects similar to those found in CHARGE syndrome, Yu et al. decided to investigate if the FGF signalling pathway was involved in this syndrome.

Mice should have two working copies of the Chd7 gene, and mice that lack one of these suffer from symptoms similar to those of humans with CHARGE syndrome. Yu et al. examined the embryos of these mice and found that the isthmus organiser produced less FGF8. Embryos with no working copies of the gene completely lost the r1 segment. The loss of this segment appeared to be caused by changes in the expression of homeobox genes (the genes that determine the identity of brain segments).

Embryos that did not have any working copies of the Chd7 gene died early in development, which made further studies impossible. However, embryos that had one working copy of the Chd7 gene survived, and Yu et al. took advantage of this to study the effects of reduced FGF8 expression on these mice. These experiments showed that mice with just one working copy of the Fgf8 gene and one working copy of the Chd7 gene had a small cerebellar vermis. This part of the cerebellum is known to be very sensitive to changes in FGF8 signalling. Yu et al. then used an MRI scanner to look at the cerebellar vermis in patients with CHARGE syndrome, and found that more than half of the patients had abnormal cerebella.

In addition to confirming that studies on mouse embryos can provide insights into human disease, the work of Yu et al. add defects in the cerebellar vermis to the list of developmental abnormalities associated with CHARGE syndrome. The next step will be to test if any mutations in the human FGF8 gene can contribute to cerebellar defects in CHARGE syndrome, and to investigate if any other developmental defects in CHARGE syndrome are associated with abnormal FGF8 levels.

DOI:http://dx.doi.org/10.7554/eLife.01305.002

Cerebellar development in the absence of Gbx function in zebrafish

The midbrain-hindbrain boundary (MHB) is a well-known organizing center during vertebrate brain development. The MHB forms at the expression boundary of Otx2 and Gbx2, mutually repressive homeodomain transcription factors expressed in the midbrain/forebrain and anterior hindbrain, respectively. The genetic hierarchy of gene expression at the MHB is complex, involving multiple positive and negative feedback loops that result in the establishment of non-overlapping domains of Wnt1 and Fgf8 on either side of the boundary and the consequent specification of the cerebellum. The cerebellum derives from the dorsal part of the anterior-most hindbrain segment, rhombomere 1 (r1), which undergoes a distinctive morphogenesis to give rise to the cerebellar primordium within which the various cerebellar neuron types are specified. Previous studies in the mouse have shown that Gbx2 is essential for cerebellar development. Using zebrafish mutants we show here that in the zebrafish gbx1 and gbx2 are required redundantly for morphogenesis of the cerebellar primordium and subsequent cerebellar differentiation, but that this requirement is alleviated by knocking down Otx. Expression of fgf8, wnt1 and the entire MHB genetic program is progressively lost in gbx1-;gbx2- double mutants but is rescued by Otx knock-down. This rescue of the MHB genetic program depends on rescued Fgf signaling, however the rescue of cerebellar primordium morphogenesis is independent of both Gbx and Fgf. Based on our findings we propose a revised model for the role of Gbx in cerebellar development.

Congenital hypoplasia of the cerebellum: developmental causes and behavioral consequences

Over the last 60 years, the spotlight of research has periodically returned to the cerebellum as new techniques and insights have emerged. Because of its simple homogeneous structure, limited diversity of cell types and characteristic behavioral pathologies, the cerebellum is a natural home for studies of cell specification, patterning, and neuronal migration. However, recent evidence has extended the traditional range of perceived cerebellar function to include modulation of cognitive processes and implicated cerebellar hypoplasia and Purkinje neuron hypo-cellularity with autistic spectrum disorder. In the light of this emerging frontier, we review the key stages and genetic mechanisms behind cerebellum development. In particular, we discuss the role of the midbrain hindbrain isthmic organizer in the development of the cerebellar vermis and the specification and differentiation of Purkinje cells and granule neurons. These developmental processes are then considered in relation to recent insights into selected human developmental cerebellar defects: Joubert syndrome, Dandy–Walker malformation, and pontocerebellar hypoplasia. Finally, we review current research that opens up the possibility of using the mouse as a genetic model to study the role of the cerebellum in cognitive function.

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.

Cellular and molecular basis of cerebellar development

Historically, the molecular and cellular mechanisms of cerebellar development were investigated through structural descriptions and studying spontaneous mutations in animal models and humans. Advances in experimental embryology, genetic engineering, and neuroimaging techniques render today the possibility to approach the analysis of molecular mechanisms underlying histogenesis and morphogenesis of the cerebellum by experimental designs. Several genes and molecules were identified to be involved in the cerebellar plate regionalization, specification, and differentiation of cerebellar neurons, as well as the establishment of cellular migratory routes and the subsequent neuronal connectivity. Indeed, pattern formation of the cerebellum requires the adequate orchestration of both key morphogenetic signals, arising from distinct brain regions, and local expression of specific transcription factors. Thus, the present review wants to revisit and discuss these morphogenetic and molecular mechanisms taking place during cerebellar development in order to understand causal processes regulating cerebellar cytoarchitecture, its highly topographically ordered circuitry and its role in brain function.

What underlies the diversity of brain tumors?

Glioma and medulloblastoma represent the most commonly occurring malignant brain tumors in adults and in children, respectively. Recent genomic and transcriptional approaches present a complex group of diseases and delineate a number of molecular subgroups within tumors that share a common histopathology. Differences in cells of origin, regional niches, developmental timing, and genetic events all contribute to this heterogeneity. In an attempt to recapitulate the diversity of brain tumors, an increasing array of genetically engineered mouse models (GEMMs) has been developed. These models often utilize promoters and genetic drivers from normal brain development and can provide insight into specific cells from which these tumors originate. GEMMs show promise in both developmental biology and developmental therapeutics. This review describes numerous murine brain tumor models in the context of normal brain development and the potential for these animals to impact brain tumor research.

Common partner Smad-independent canonical bone morphogenetic protein signaling in the specification process of the anterior rhombic lip during cerebellum development

Bone morphogenetic protein (BMP) signaling is critical for cerebellum development. However, the details of receptor regulated-Smad (R-Smad) and common partner Smad (Co-Smad, or Smad4) involvement are unclear. Here, we report that cerebellum-specific double conditional inactivation of Smad1 and Smad5 (Smad1/5) results in cerebellar hypoplasia, reduced granule cell numbers, and disorganized Purkinje neuron migration during embryonic development. However, single conditional inactivation of either Smad1 or Smad5 did not result in cerebellar abnormalities. Surprisingly, conditional inactivation of Smad4, which is considered to be the central mediator of canonical BMP-Smad signaling, resulted only in very mild cerebellar defects. Conditional inactivation of Smad1/5 led to developmental defects in the anterior rhombic lip (ARL), as shown by reduced cell proliferation and loss of Pax6 and Atoh1 expression. These defects subsequently caused the loss of the nuclear transitory zone and a region of the deep cerebellar nuclei. The normal maturation of the remaining granule cell precursors in the external granular layer (EGL) suggests Smad1/5 signaling is required for the specification process in ARL but not for the subsequent EGL development. Our results demonstrate functional redundancy for Smad1 and Smad5 but functional discrepancy between Smad1/5 and Smad4 during cerebellum development.

Embryonic origins of the mouse superior olivary complex

Many areas of the central nervous system are organized into clusters of cell groups, with component cell groups exhibiting diverse but related functions. One such cluster, the superior olivary complex (SOC), is located in the ventral auditory brainstem in mammals. The SOC is an obligatory contact point for most projection neurons of the ventral cochlear nucleus and plays central roles in many aspects of monaural and binaural information processing. Despite their important interrelated functions, little is known about the embryonic origins of SOC nuclei, due in part to a paucity of developmental markers to distinguish individual cell groups. In this report, we present a collection of novel markers for the developing SOC nuclei in mice, including the transcription factors FoxP1, MafB, and Sox2, and the lineage-marking transgenic line En1-Cre. We use these definitive markers to examine the rhombic lip and rhombomeric origins of SOC nuclei and demonstrate that they can serve to uniquely identify SOC nuclei and subnuclei in newborn pups. The markers are also useful in identifying distinct nuclear domains within the presumptive SOC as early as embryonic day (E) 14.5, well before morphological distinction of individual nuclei is evident. These findings indicate that the mediolateral and dorsoventral position of SOC nuclei characteristic of the adult brainstem is established during early neurogenesis.

MRI analysis of cerebellar and vestibular developmental phenotypes in Gbx2 conditional knockout mice

PURPOSE

Our aim in this study was to apply three-dimensional MRI methods to analyze early postnatal morphological phenotypes in a Gbx2 conditional knockout (Gbx2-CKO) mouse that has variable midline deletions in the central cerebellum, reminiscent of many human cerebellar hypoplasia syndromes.

METHODS

In vivo three-dimensional manganese-enhanced MRI at 100-µm isotropic resolution was used to visualize mouse brains between postnatal days 3 and 11, when cerebellum morphology undergoes dramatic changes. Deformation-based morphometry and volumetric analysis of manganese-enhanced MRI images were used to, respectively, detect and quantify morphological phenotypes in Gbx2-CKO mice. Ex vivo micro-MRI was performed after perfusion-fixation with supplemented gadolinium for higher resolution (50-µm) analysis.

RESULTS

In vivo manganese-enhanced MRI and deformation-based morphometry correctly identified known cerebellar defects in Gbx2-CKO mice, and novel phenotypes were discovered in the deep cerebellar nuclei and the vestibulo-cerebellum, both validated using histology. Ex vivo micro-MRI revealed subtle phenotypes in both the vestibulo-cerebellum and the vestibulo-cochlear organ, providing an interesting example of complementary phenotypes in a sensory organ and its associated brain region.

CONCLUSION

These results show the potential of three-dimensional MRI for detecting and analyzing developmental defects in mouse models of neurodevelopmental diseases.

Clustered fine compartmentalization of the mouse embryonic cerebellar cortex and its rearrangement into the postnatal striped configuration

Compartmentalization is essential for a brain area to be involved in different functions through topographic afferent and efferent connections that reflect this organization. The adult cerebellar cortex is compartmentalized into longitudinal stripes, in which Purkinje cells (PCs) have compartment-specific molecular expression profiles. How these compartments form during development is generally not understood. To investigate this process, we focused on the late developmental stages of the cerebellar compartmentalization that occur from embryonic day 17.5 (E17.5), when embryonic compartmentalization is evidently observed, to postnatal day 6 (P6), when adult-type compartmentalization begins to be established. The transformation between these compartmentalization patterns was analyzed by mapping expression patterns of several key molecular markers in serial cerebellar sections in the mouse. A complete set of 54 clustered PC subsets, which had different expression profiles of FoxP2, PLCβ4, EphA4, Pcdh10, and a reporter molecule of the 1NM13 transgenic mouse strain, were distinguished in three-dimensional space in the E17.5 cerebellum. Following individual PC subsets during development indicated that these subsets were rearranged from a clustered and multilayered configuration to a flattened, single-layered and striped configuration by means of transverse slide, longitudinal split, or transverse twist spatial transformations during development. The Purkinje cell-free spaces that exist between clusters at E17.5 become granule cell raphes that separate striped compartments at P6. The results indicate that the ∼50 PC clusters of the embryonic cerebellum will ultimately become the longitudinal compartments of the adult cerebellum after undergoing various peri- and postnatal transformations that alter their relative spatial relationships.

Purkinje cell compartmentalization in the cerebellum of the spontaneous mutant mouse dreher

The cerebellar morphological phenotype of the spontaneous neurological mutant mouse dreher (Lmx1a(dr-J)) results from cell fate changes in dorsal midline patterning involving the roof plate and rhombic lip. Positional cloning revealed that the gene Lmx1a, which encodes a LIM homeodomain protein, is mutated in dreher, and is expressed in the developing roof plate and rhombic lip. Loss of Lmx1a causes reduction of the roof plate, an important embryonic signaling center, and abnormal cell fate specification within the embryonic cerebellar rhombic lip. In adult animals, these defects result in variable, medial fusion of the cerebellar vermis and posterior cerebellar vermis hypoplasia. It is unknown whether deleting Lmx1a results in displacement or loss of specific lobules in the vermis. To distinguish between an ectopic and absent vermis, the expression patterns of two Purkinje cell-specific compartmentation antigens, zebrin II/aldolase C and the small heat shock protein HSP25 were analyzed in dreher cerebella. The data reveal that despite the reduction in volume and abnormal foliation of the cerebellum, the transverse zones and parasagittal stripe arrays characteristic of the normal vermis are present in dreher, but may be highly distorted. In dreher mutants with a severe phenotype, zebrin II stripes are fragmented and distributed non-symmetrically about the cerebellar midline. We conclude that although Purkinje cell agenesis or selective Purkinje cell death may contribute to the dreher phenotype, our data suggest that aberrant anlage patterning and granule cell development lead to Purkinje cell ectopia, which ultimately causes abnormal cerebellar architecture in dreher.