Reelin mRNA expression during mouse brain development

Repurposing of the multiciliation gene regulatory network in fate specification of Cajal-Retzius neurons

Cajal-Retzius cells (CRs) are key players in cerebral cortex development, and they display a unique transcriptomic identity. Here, we use scRNA-seq to reconstruct the differentiation trajectory of mouse hem-derived CRs, and we unravel the transient expression of a complete gene module previously known to control multiciliogenesis. However, CRs do not undergo centriole amplification or multiciliation. Upon deletion of Gmnc, the master regulator of multiciliogenesis, CRs are initially produced but fail to reach their normal identity resulting in their massive apoptosis. We further dissect the contribution of multiciliation effector genes and identify Trp73 as a key determinant. Finally, we use in utero electroporation to demonstrate that the intrinsic competence of hem progenitors as well as the heterochronic expression of Gmnc prevent centriole amplification in the CR lineage. Our work exemplifies how the co-option of a complete gene module, repurposed to control a distinct process, may contribute to the emergence of novel cell identities.

Regulation of cerebellar network development by granule cells and their molecules

The well-organized cerebellar structures and neuronal networks are likely crucial for their functions in motor coordination, motor learning, cognition, and emotion. Such cerebellar structures and neuronal networks are formed during developmental periods through orchestrated mechanisms, which include not only cell-autonomous programs but also interactions between the same or different types of neurons. Cerebellar granule cells (GCs) are the most numerous neurons in the brain and are generated through intensive cell division of GC precursors (GCPs) during postnatal developmental periods. While GCs go through their own developmental processes of proliferation, differentiation, migration, and maturation, they also play a crucial role in cerebellar development. One of the best-characterized contributions is the enlargement and foliation of the cerebellum through massive proliferation of GCPs. In addition to this contribution, studies have shown that immature GCs and GCPs regulate multiple factors in the developing cerebellum, such as the development of other types of cerebellar neurons or the establishment of afferent innervations. These studies have often found impairments of cerebellar development in animals lacking expression of certain molecules in GCs, suggesting that the regulations are mediated by molecules that are secreted from or present in GCs. Given the growing recognition of GCs as regulators of cerebellar development, this review will summarize our current understanding of cerebellar development regulated by GCs and molecules in GCs, based on accumulated studies and recent findings, and will discuss their potential further contributions.

Morphological study of the postnatal hippocampal development in the TRPV1 knockout mice

Transient receptor potential vanilloid 1 (TRPV1) is a non-selective cation channel with polymodal sensory function. TRPV1 links to fever, while, according to previous studies on TRPV1 knock-out (KO) mice, the role of the channel in the generation of febrile seizure is debated. In the hippocampal formation, functional TRPV1 channels are expressed by Cajal-Retzius cells, which have a role in guidance of migrating neurons during development. Despite the developmental aspects of febrile seizure as well as of Cajal-Retzius cells, no information is available about the hippocampal development in TRPV1 KO mouse. Therefore, in the present work postnatal development of the hippocampal formation was studied in TRPV1 KO mice. Several morphological characteristics including neuronal positioning and maturation, synaptogenesis and myelination were examined with light microscopy following immunohistochemical detection of protein markers of various neurons, synapses, and myelination. Regarding the cytoarchitectonics, neuronal migration, morphological, and neurochemical maturation, no substantial difference could be detected between TRPV1 KO and wild-type control mice. Our data indicate that synapse formation and myelination occur similarly in TRPV1 KO and in control animals. We have found slightly, but not significantly larger numbers of persisting Cajal-Retzius cells in the KO mice than in controls. Our result strengthens previous suggestion concerning the role of TRPV1 channel in the postnatal apoptotic cell death of Cajal-Retzius cells. However, the fact that the hippocampus of KO mice lacks major developmental abnormalities supports the use of TRPV1 KO in various animal models of diseases and pathological conditions.

Analysis of the Expression Pattern of Cajal-Retzius Cell Markers in the Xenopus laevis Forebrain

Cajal-Retzius cells are essential for cortical development in mammals, and their involvement in the evolution of this structure has been widely postulated, but very little is known about their progenitor domains in non-mammalian vertebrates. Using in situhybridization and immunofluorescence techniques we analyzed the expression of some of the main Cajal-Retzius cell markers such as Dbx1, Ebf3, ER81, Lhx1, Lhx5, p73, Reelin, Wnt3a, Zic1, and Zic2 in the forebrain of the anuran Xenopus laevis, because amphibians are the only class of anamniote tetrapods and show a tetrapartite evaginated pallium, but no layered or nuclear organization. Our results suggested that the Cajal-Retzius cell progenitor domains were comparable to those previously described in amniotes. Thus, at dorsomedial telencephalic portions a region comparable to the cortical hem was defined in Xenopus based on the expression of Wnt3a, p73, Reelin, Zic1, and Zic2. In the septum, two different domains were observed: a periventricular dorsal septum, at the limit between the pallium and the subpallium, expressing Reelin, Zic1, and Zic2, and a related septal domain, expressing Ebf3, Zic1, and Zic2. In the lateral telencephalon, the ventral pallium next to the pallio-subpallial boundary, the lack of Dbx1 and the unique expression of Reelin during development defined this territory as the most divergent with respect to mammals. Finally, we also analyzed the expression of these markers at the prethalamic eminence region, suggested as Cajal-Retzius progenitor domain in amniotes, observing there Zic1, Zic2, ER81, and Lhx1 expression. Our data show that in anurans there are different subtypes and progenitor domains of Cajal-Retzius cells, which probably contribute to the cortical regional specification and territory-specific properties. This supports the notion that the basic organization of pallial derivatives in vertebrates follows a comparable fundamental arrangement, even in those that do not have a sophisticated stratified cortical structure like the mammalian cerebral cortex.

Selective Inactivation of Reelin in Inhibitory Interneurons Leads to Subtle Changes in the Dentate Gyrus But Leaves Cortical Layering and Behavior Unaffected

Reelin is an extracellular matrix protein, known for its dual role in neuronal migration during brain development and in synaptic plasticity at adult stages. During the perinatal phase, Reelin expression switches from Cajal-Retzius (CR) cells, its main source before birth, to inhibitory interneurons (IN), the main source of Reelin in the adult forebrain. IN-derived Reelin has been associated with schizophrenia and temporal lobe epilepsy; however, the functional role of Reelin from INs is presently unclear. In this study, we used conditional knockout mice, which lack Reelin expression specifically in inhibitory INs, leading to a substantial reduction in total Reelin expression in the neocortex and dentate gyrus. Our results show that IN-specific Reelin knockout mice exhibit normal neuronal layering and normal behavior, including spatial reference memory. Although INs are the major source of Reelin within the adult stem cell niche, Reelin from INs does not contribute substantially to normal adult neurogenesis. While a closer look at the dentate gyrus revealed some unexpected alterations at the cellular level, including an increase in the number of Reelin expressing CR cells, overall our data suggest that Reelin derived from INs is less critical for cortex development and function than Reelin expressed by CR cells.

Early Forebrain Neurons and Scaffold Fibers in Human Embryos

Neural progenitor proliferation, neuronal migration, areal organization, and pioneer axon wiring are critical events during early forebrain development, yet remain incompletely understood, especially in human. Here, we studied forebrain development in human embryos aged 5 to 8 postconceptional weeks (WPC5-8), stages that correspond to the neuroepithelium/early marginal zone (WPC5), telencephalic preplate (WPC6 & 7), and incipient cortical plate (WPC8). We show that early telencephalic neurons are formed at the neuroepithelial stage; the most precocious ones originate from local telencephalic neuroepithelium and possibly from the olfactory placode. At the preplate stage, forebrain organization is quite similar in human and mouse in terms of areal organization and of differentiation of Cajal-Retzius cells, pioneer neurons, and axons. Like in mice, axons from pioneer neurons in prethalamus, ventral telencephalon, and cortical preplate cross the diencephalon-telencephalon junction and the pallial-subpallial boundary, forming scaffolds that could guide thalamic and cortical axons at later stages. In accord with this model, at the early cortical plate stage, corticofugal axons run in ventral telencephalon in close contact with scaffold neurons, which express CELSR3 and FZD3, two molecules that regulates formation of similar scaffolds in mice.

Reelin-Nrp1 Interaction Regulates Neocortical Dendrite Development in a Context-Specific Manner

Reelin plays versatile roles in neocortical development. The C-terminal region (CTR) of Reelin is required for the correct formation of the superficial structure of the neocortex; however, the mechanisms by which this position-specific effect occurs remain largely unknown. In this study, we demonstrate that Reelin with an intact CTR binds to neuropilin-1 (Nrp1), a transmembrane protein. Both male and female mice were used. Nrp1 is localized with very-low-density lipoprotein receptor (VLDLR), a canonical Reelin receptor, in the superficial layers of the developing neocortex. It forms a complex with VLDLR, and this interaction is modulated by the alternative splicing of VLDLR. Reelin with an intact CTR binds more strongly to the VLDLR/Nrp1 complex than to VLDLR alone. Knockdown of Nrp1 in neurons leads to the accumulation of Dab1 protein. Since the degradation of Dab1 is induced by Reelin signaling, it is suggested that Nrp1 augments Reelin signaling. The interaction between Reelin and Nrp1 is required for normal dendritic development in superficial-layer neurons. All of these characteristics of Reelin are abrogated by proteolytic processing of the six C-terminal amino acid residues of Reelin (0.17% of the whole protein). Therefore, Nrp1 is a coreceptor molecule for Reelin and, together with the proteolytic processing of Reelin, can account for context-specific Reelin function in brain development.SIGNIFICANCE STATEMENT Reelin often exhibits a context-dependent function during brain development; however, its underlying mechanism is not well understood. We found that neuropilin-1 (Nrp1) specifically binds to the CTR of Reelin and acts as a coreceptor for very-low-density lipoprotein receptor (VLDLR). The Nrp1/VLDLR complex is localized in the superficial layers of the neocortex, and its interaction with Reelin is essential for proper dendritic development in superficial-layer neurons. This study provides the first mechanistic evidence of the context-specific function of Reelin (>3400 residues) regulated by the C-terminal residues and Nrp1, a component of the canonical Reelin receptor complex.

Reelin Mediates Hippocampal Cajal-Retzius Cell Positioning and Infrapyramidal Blade Morphogenesis

We have previously described hypomorphic reelin (Reln) mutant mice, Reln^CTRdel, in which the morphology of the dentate gyrus is distinct from that seen in reeler mice. In the Reln^CTRdel mutant, the infrapyramidal blade of the dentate gyrus fails to extend, while the suprapyramidal blade forms with a relatively compact granule neuron layer. Underlying this defect, we now report several developmental anomalies in the Reln^CTRdel dentate gyrus. Most strikingly, the distribution of Cajal-Retzius cells was aberrant; Cajal-Retzius neurons were increased in the suprapyramidal blade, but were greatly reduced along the subpial surface of the prospective infrapyramidal blade. We also observed multiple abnormalities of the fimbriodentate junction. Firstly, progenitor cells were distributed abnormally; the “neurogenic cluster” at the fimbriodentate junction was absent, lacking the normal accumulation of Tbr2-positive intermediate progenitors. However, the number of dividing cells in the dentate gyrus was not generally decreased. Secondly, a defect of secondary glial scaffold formation, limited to the infrapyramidal blade, was observed. The densely radiating glial fibers characteristic of the normal fimbriodentate junction were absent in mutants. These fibers might be required for migration of progenitors, which may account for the failure of neurogenic cluster formation. These findings suggest the importance of the secondary scaffold and neurogenic cluster of the fimbriodentate junction in morphogenesis of the mammalian dentate gyrus. Our study provides direct genetic evidence showing that normal RELN function is required for Cajal-Retzius cell positioning in the dentate gyrus, and for formation of the fimbriodentate junction to promote infrapyramidal blade extension.

Reelin Functions, Mechanisms of Action and Signaling Pathways During Brain Development and Maturation

During embryonic development and adulthood, Reelin exerts several important functions in the brain including the regulation of neuronal migration, dendritic growth and branching, dendritic spine formation, synaptogenesis and synaptic plasticity. As a consequence, the Reelin signaling pathway has been associated with several human brain disorders such as lissencephaly, autism, schizophrenia, bipolar disorder, depression, mental retardation, Alzheimer’s disease and epilepsy. Several elements of the signaling pathway are known. Core components, such as the Reelin receptors very low-density lipoprotein receptor (VLDLR) and Apolipoprotein E receptor 2 (ApoER2), Src family kinases Src and Fyn, and the intracellular adaptor Disabled-1 (Dab1), are common to most but not all Reelin functions. Other downstream effectors are, on the other hand, more specific to defined tasks. Reelin is a large extracellular protein, and some aspects of the signal are regulated by its processing into smaller fragments. Rather than being inhibitory, the processing at two major sites seems to be fulfilling important physiological functions. In this review, I describe the various cellular events regulated by Reelin and attempt to explain the current knowledge on the mechanisms of action. After discussing the shared and distinct elements of the Reelin signaling pathway involved in neuronal migration, dendritic growth, spine development and synaptic plasticity, I briefly outline the data revealing the importance of Reelin in human brain disorders.

Motor Performances of Spontaneous and Genetically Modified Mutants with Cerebellar Atrophy

Chance discovery of spontaneous mutants with atrophy of the cerebellar cortex has unearthed genes involved in optimizing motor coordination. Rotorod, stationary beam, and suspended wire tests are useful in delineating behavioral phenotypes of spontaneous mutants with cerebellar atrophy such as Grid2^Lc, Grid2^ho, Rora^sg, Agtpbp1^pcd, Reln^rl, and Dab1^scm. Likewise, transgenic or null mutants serving as experimental models of spinocerebellar ataxia (SCA) are phenotyped with the same tests. Among experimental models of autosomal dominant SCA, rotorod deficits were reported in SCA1 to 3, SCA5 to 8, SCA14, SCA17, and SCA27 and stationary beam deficits in SCA1 to 3, SCA5, SCA6, SCA13, SCA17, and SCA27. Beam tests are sensitive to experimental therapies of various kinds including molecules affecting glutamate signaling, mesenchymal stem cells, anti-oligomer antibodies, lentiviral vectors carrying genes, interfering RNAs, or neurotrophic factors, and interbreeding with other mutants.

Comparative gene expression analysis of the engulfment and cell motility (ELMO) protein family in the mouse brain

Engulfment and cell motility (ELMO) proteins bind to Dock180, a guanine nucleotide exchange factor (GEF) of the Rac family, and regulate GEF activity. The resultant ELMO/Dock180/Rac module regulates cytoskeletal reorganization responsible for the engulfment of apoptotic cells, cell migration, and neurite extension. The expression and function of Elmo family proteins in the nervous system, however, are not yet fully understood. Here, we characterize the comparative gene expression profiles of three Elmo family members (Elmo1, Elmo2, and Elmo3) in the brain of C57BL/6J mice, a widely used inbred strain, together with reeler mutant mice to understand gene expression in normal laminated brain areas compared with abnormal areas. Although all three Elmo genes showed widespread mRNA expression over various mouse tissues tested, Elmo1 and Elmo2 were the major types expressed in the brain, and three Elmo genes were up-regulated between the first postnatal week (infant stage) and the third postnatal week (juvenile, weaning stage). In addition, the mRNAs of Elmo genes showed distinct distribution patterns in various brain areas and cell-types; such as neurons including inhibitory interneurons as well as some non-neuronal cells. In the cerebral cortex, the three Elmo genes were widely expressed over many cortical regions, but the predominant areas of Elmo1 and Elmo2 expression tended to be distributed unevenly in the deep (a lower part of the VI) and superficial (II/III) layers, respectively, which also changed depending on the cortical areas and postnatal stages. In the dentate gyrus of the hippocampus, Elmo2 was expressed in dentate granule cells more in the mature stage rather than the immature-differentiating stage. In the thalamus, Elmo1 but not the other members was highly expressed in many nuclei. In the medial habenula, Elmo2 and Elmo3 were expressed at intermediate levels. In the cerebellar cortex, Elmo1 and Elmo2 were expressed in differentiating-mature granule cells and mature granule cells, respectively. In the Purkinje cell layer, Elmo1 and Elmo2 were expressed in Purkinje cells and Bergmann glia, respectively. Disturbed cellular distributions and laminar structures caused by the reeler mutation did not severely change expression in these cell types despite the disturbed cellular distributions and laminar structures, including those of the cerebrum, hippocampus, and cerebellum. Taken together, these results suggested that these three Elmo family members share their functional roles in various brain regions during prenatal-postnatal development.

Cerebellar Abiotrophy Across Domestic Species

Cerebellar abiotrophy (CA) is a neurodegenerative disorder affecting the cerebellum and occurs in multiple species. Although CA is well researched in humans and mice, domestic species such as the dog, cat, sheep, cow, and horse receive little recognition. This may be due to few studies addressing the mechanism of CA in these species. However, valuable information can still be extracted from these cases. A review of the clinicohistologic phenotype of CA in these species and determining the various etiologies of CA may aid in determining conserved and required pathways necessary for proper cerebellar development and function. This review outlines research approaches of studies of CA in domestic species, compared to the approaches used in mice, with the objective of comparing CA in domestic species while identifying areas for further research efforts.

Role of Reelin in cell positioning in the cerebellum and the cerebellum-like structure in zebrafish

The cerebellum and the cerebellum-like structure in the mesencephalic tectum in zebrafish contain multiple cell types, including principal cells (i.e., Purkinje cells and type I neurons) and granule cells, that form neural circuits in which the principal cells receive and integrate inputs from granule cells and other neurons. It is largely unknown how these cells are positioned and how neural circuits form. While Reelin signaling is known to play an important role in cell positioning in the mammalian brain, its role in the formation of other vertebrate brains remains elusive. Here we found that zebrafish with mutations in Reelin or in the Reelin-signaling molecules Vldlr or Dab1a exhibited ectopic Purkinje cells, eurydendroid cells (projection neurons), and Bergmann glial cells in the cerebellum, and ectopic type I neurons in the tectum. The ectopic Purkinje cells and type I neurons received aberrant afferent fibers in these mutants. In wild-type zebrafish, reelin transcripts were detected in the internal granule cell layer, while Reelin protein was localized to the superficial layer of the cerebellum and the tectum. Laser ablation of the granule cell axons perturbed the localization of Reelin, and the mutation of both kif5aa and kif5ba, which encode major kinesin I components in the granule cells, disrupted the elongation of granule cell axons and the Reelin distribution. Our findings suggest that in zebrafish, (1) Reelin is transported from the granule cell soma to the superficial layer by axonal transport; (2) Reelin controls the migration of neurons and glial cells from the ventricular zone; and (3) Purkinje cells and type I neurons attract afferent axons during the formation of the cerebellum and the cerebellum-like structure.

Deep hypothermia reverses behavioral and histological alterations in a rat model of perinatal asphyxia

The consequences of perinatal asphyxia (PA) include alterations which may manifest as schizophrenia. Characteristic features of this disease include a decrease in specific subpopulations of GABAergic cells and deterioration of social interaction. The purpose of this study is to assess if a deep and short-hypothermic treatment can ameliorate this damage in a model of PA. Rats offsprings were exposed to 19 min of asphyxia by immersing the uterus horns in water at 37 °C followed by 30 min in air at 10 °C that resulted in 15 °C body temperature. At postnatal day 36-38, the rats were tested in the open field and social interaction paradigms and processed for immunostaining of calbindin and reelin. A brief exposure to deep hypothermia reversed the deterioration produced by PA in play soliciting. PA decreased the density of calbindin neurons in layer II of the Anterior Insular Cortex, while deep hypothermia reversed this effect. Paradoxically, in AIC, there was a significant increase in the number of reelin-secreting neurons in layers II and III generated by PA and this increase was reversed by hypothermia. This suggests a compensatory mechanism, where reelin neurons trend to compensate for the loss of calbindin neurons, at least within Anterior Insular Cortex. Finally, the deep hypothermic shock might represent a valuable therapeutic alternative to treat PA.

Bergmann glial Sonic hedgehog signaling activity is required for proper cerebellar cortical expansion and architecture

Neuronal-glial relationships play a critical role in the maintenance of central nervous system architecture and neuronal specification. A deeper understanding of these relationships can elucidate cellular cross-talk capable of sustaining proper development of neural tissues. In the cerebellum, cerebellar granule neuron precursors (CGNPs) proliferate in response to Purkinje neuron-derived Sonic hedgehog (Shh) before ultimately exiting the cell cycle and migrating radially along Bergmann glial fibers. However, the function of Bergmann glia in CGNP proliferation remains not well defined. Interestingly, the Hh pathway is also activated in Bergmann glia, but the role of Shh signaling in these cells is unknown. In this study, we show that specific ablation of Shh signaling using the tamoxifen-inducible TNC^YFP-CreER line to eliminate Shh pathway activator Smoothened in Bergmann glia is sufficient to cause severe cerebellar hypoplasia and a significant reduction in CGNP proliferation. TNC^YFP-CreER; Smo^F/- (Smo^CKO) mice demonstrate an obvious reduction in cerebellar size within two days of ablation of Shh signaling. Mutant cerebella have severely reduced proliferation and increased differentiation of CGNPs due to a significant decrease in Shh activity and concomitant activation of Wnt signaling in Smo^CKO CGNPs, suggesting that this pathway is involved in cross-talk with the Shh pathway in regulating CGNP proliferation. In addition, Purkinje cells are ectopically located, their dendrites stunted, and the Bergmann glial network disorganized. Collectively, these data demonstrate a previously unappreciated role for Bergmann glial Shh signaling activity in the proliferation of CGNPs and proper maintenance of cerebellar architecture.

C-Terminal Region Truncation of RELN Disrupts an Interaction with VLDLR, Causing Abnormal Development of the Cerebral Cortex and Hippocampus

We discovered a hypomorphic reelin (Reln) mutant with abnormal cortical lamination and no cerebellar hypoplasia. This mutant, Reln^CTRdel, carries a chemically induced splice-site mutation that truncates the C-terminal region (CTR) domain of RELN protein and displays remarkably distinct phenotypes from reeler The mutant does not have an inverted cortex, but cortical neurons overmigrate and invade the marginal zone, which are characteristics similar to a phenotype seen in the cerebral cortex of Vldlr^null mice. The dentate gyrus shows a novel phenotype: the infrapyramidal blade is absent, while the suprapyramidal blade is present and laminated. Genetic epistasis analysis showed that Reln^CTRdel/Apoer2^null double homozygotes have phenotypes akin to those of reeler mutants, while Reln^CTRdel/Vldlr^null mice do not. Given that the receptor double knock-out mice resemble reeler mutants, we infer that Reln^CTRdel/Apoer2^null double homozygotes have both receptor pathways disrupted. This suggests that CTR-truncation disrupts an interaction with VLDLR (very low-density lipoprotein receptor), while the APOER2 signaling pathway remains active, which accounts for the hypomorphic phenotype in Reln^CTRdel mice. A RELN-binding assay confirms that CTR truncation significantly decreases RELN binding to VLDLR, but not to APOER2. Together, the in vitro and in vivo results demonstrate that the CTR domain confers receptor-binding specificity of RELN.

SIGNIFICANCE STATEMENT

Reelin signaling is important for brain development and is associated with human type II lissencephaly. Reln mutations in mice and humans are usually associated with cerebellar hypoplasia. A new Reln mutant with a truncation of the C-terminal region (CTR) domain shows that Reln mutation can cause abnormal phenotypes in the cortex and hippocampus without cerebellar hypoplasia. Genetic analysis suggested that CTR truncation disrupts an interaction with the RELN receptor VLDLR (very low-density lipoprotein receptor); this was confirmed by a RELN-binding assay. This result provides a mechanistic explanation for the hypomorphic phenotype of the CTR-deletion mutant, and further suggests that Reln mutations may cause more subtle forms of human brain malformation than classic lissencephalies.

Cofilin Activation Is Temporally Associated with the Cessation of Growth in the Developing Hippocampus

Dendritic extension and synaptogenesis proceed at high rates in rat hippocampus during early postnatal life but markedly slow during the third week of development. The reasons for the latter, fundamental event are poorly understood. Here, we report that levels of phosphorylated (inactive) cofilin, an actin depolymerizing factor, decrease by 90% from postnatal days (pnds) 10 to 21. During the same period, levels of total and phosphorylated Arp2, which nucleates actin branches, increase. A search for elements that could explain the switch from inactive to active cofilin identified reductions in β1 integrin, TrkB, and LIM domain kinase 2b, upstream proteins that promote cofilin phosphorylation. Moreover, levels of slingshot 3, which dephosphorylates cofilin, increase during the period in which growth slows. Consistent with the cofilin results, in situ phalloidin labeling of F-actin demonstrated that spines and dendrites contained high levels of dynamic actin filaments during Week 2, but these fell dramatically by pnd 21. The results suggest that the change from inactive to constitutively active cofilin leads to a loss of dynamic actin filaments needed for process extension and thus the termination of spine formation and synaptogenesis. The relevance of these events to the emergence of memory-related synaptic plasticity is described.

Neurochemical Markers in the Mammalian Brain: Structure, Roles in Synaptic Communication, and Pharmacological Relevance

BACKGROUND

Knowledge of molecular marker (typically protein or mRNA) expression in neural systems can provide insight to the chemical blueprint of signal processing and transmission, assist in tracking developmental or pathological progressions, and yield key information regarding potential medicinal targets. These markers are particularly relevant in the mammalian brain in the light of its unsurpassed cellular diversity. Accordingly, molecular expression profiling is rapidly becoming a major approach to classify neuron types. Despite a profusion of research, however, the biological functions of molecular markers commonly used to distinguish neuron types remain incompletely understood. Furthermore, most molecular markers of mammalian neuron types are also present in other organs, therefore complicating considerations of their potential pharmacological interactions.

OBJECTIVE

Here, we survey 15 prominent neurochemical markers from five categories, namely membrane transporters, calcium-binding proteins, neuropeptides, receptors, and extracellular matrix proteins, explaining their relation and relevance to synaptic communication.

METHOD

For each marker, we summarize fundamental structural features, cellular functionality, distributions within and outside the brain, as well as known drug effectors and mechanisms of action.

CONCLUSION

This essential primer thus links together the cellular complexity of the brain, the chemical properties of key molecular players in neurotransmission, and possible biomedical opportunities.

Reelin Expression in Creutzfeldt-Jakob Disease and Experimental Models of Transmissible Spongiform Encephalopathies

Reelin is an extracellular glycoprotein involved in key cellular processes in developing and adult nervous system, including regulation of neuronal migration, synapse formation, and plasticity. Most of these roles are mediated by the intracellular phosphorylation of disabled-1 (Dab1), an intracellular adaptor molecule, in turn mediated by binding Reelin to its receptors. Altered expression and glycosylation patterns of Reelin in cerebrospinal and cortical extracts have been reported in Alzheimer's disease. However, putative changes in Reelin are not described in natural prionopathies or experimental models of prion infection or toxicity. With this is mind, in the present study, we determined that Reelin protein and mRNA levels increased in CJD human samples and in mouse models of human prion disease in contrast to murine models of prion infection. However, changes in Reelin expression appeared only at late terminal stages of the disease, which prevent their use as an efficient diagnostic biomarker. In addition, increased Reelin in CJD and in in vitro models does not correlate with Dab1 phosphorylation, indicating failure in its intracellular signaling. Overall, these findings widen our understanding of the putative changes of Reelin in neurodegeneration.

How does Reelin signaling regulate the neuronal cytoskeleton during migration?

Neuronal migration is an essential step in the formation of laminated brain structures. In the developing cerebral cortex, pyramidal neurons migrate toward the Reelin-containing marginal zone. Reelin is an extracellular matrix protein synthesized by Cajal-Retzius cells. In this review, we summarize our recent results and hypotheses on how Reelin might regulate neuronal migration by acting on the actin and microtubule cytoskeleton. By binding to ApoER2 receptors on the migrating neurons, Reelin induces stabilization of the leading processes extending toward the marginal zone, which involves Dab1 phosphorylation, adhesion molecule expression, cofilin phosphorylation and inhibition of tau phosphorylation. By binding to VLDLR and integrin receptors, Reelin interacts with Lis1 and induces nuclear translocation, accompanied by the ubiquitination of phosphorylated Dab1. Eventually Reelin induces clustering of its receptors resulting in the endocytosis of a Reelin/receptor complex (particularly VLDLR). The resulting decrease in Reelin contributes to neuronal arrest at the marginal zone.

Structural Insights into Reelin Function: Present and Future

Reelin is a neuronal glycoprotein secreted by the Cajal-Retzius cells in marginal regions of the cerebral cortex and the hippocampus where it plays important roles in the control of neuronal migration and the formation of cellular layers during brain development. This 3461 residue-long protein is composed of a signal peptide, an F-spondin-like domain, eight Reelin repeats (RR1-8), and a positively charged sequence at the C-terminus. Biochemical data indicate that the central region of Reelin binds to the low-density lipoprotein receptors apolipoprotein E receptor 2 (ApoER2) and the very-low-density lipoprotein receptor (VLDLR), leading to the phosphorylation of the intracellular adaptor protein Dab1. After secretion, Reelin is rapidly degraded in three major fragments, but the functional significance of this degradation is poorly understood. Probably due to its large mass and the complexity of its architecture, the high-resolution, three-dimensional structure of Reelin has never been determined. However, the crystal structures of some of the RRs have been solved, providing important insights into their fold and the interaction with the ApoER2 receptor. This review discusses the current findings on the structure of Reelin and its binding to the ApoER2 and VLDLR receptors, and we discuss some areas where proteomics and structural biology can help understanding Reelin function in brain development and human health.

Celsr3 and Fzd3 Organize a Pioneer Neuron Scaffold to Steer Growing Thalamocortical Axons

Celsr3 and Fzd3 regulate the development of reciprocal thalamocortical projections independently of their expression in cortical or thalamic neurons. To understand this cell non autonomous mechanism further, we tested whether Celsr3 and Fzd3 could act via Isl1-positive guidepost cells. Isl1-positive cells appear in the forebrain at embryonic day (E) 9.5-E10.5 and, from E12.5, they form 2 contingents in ventral telencephalon and prethalamus. In control mice, corticothalamic axons run in the ventral telencephalic corridor in close contact with Isl1-positive cells. When Celsr3 or Fzd3 is inactivated in Isl1-expressing cells, corticofugal fibers stall and loop in the ventral telencephalic corridor of high Isl1 expression, and thalamic axons fail to cross the diencephalon–telencephalon junction (DTJ). At E12.5, before thalamic and cortical axons emerge, pioneer projections from Isl1-positive cells cross the DTJ from both sides in control but not mutant embryos. These early projections appear to act like a bridge to guide later growing thalamic axons through the DTJ. Our data suggest that Celsr3 and Fzd3 orchestrate the formation of a scaffold of pioneer neurons and their axons. This scaffold extends from prethalamus to ventral telencephalon and subcortex, and steers reciprocal corticothalamic fibers.

Reelin Signaling in the Migration of Ventral Brain Stem and Spinal Cord Neurons

The extracellular matrix protein Reelin is an important orchestrator of neuronal migration during the development of the central nervous system. While its role and mechanism of action have been extensively studied and reviewed in the formation of dorsal laminar brain structures like the cerebral cortex, hippocampus, and cerebellum, its functions during the neuronal migration events that result in the nuclear organization of the ventral central nervous system are less well understood. In an attempt to delineate an underlying pattern of Reelin action in the formation of neuronal cell clusters, this review highlights the role of Reelin signaling in the migration of neuronal populations that originate in the ventral brain stem and the spinal cord.

Neurochemical Phenotype of Reelin Immunoreactive Cells in the Piriform Cortex Layer II

Reelin, a glycoprotein expressed by Cajal-Retzius neurons throughout the marginal layer of developing neocortex, has been extensively shown to play an important role during brain development, guiding neuronal migration and detachment from radial glia. During the adult life, however, many studies have associated Reelin expression to enhanced neuronal plasticity. Although its mechanism of action in the adult brain remains mostly unknown, Reelin is expressed mainly by a subset of mature interneurons. Here, we confirm the described phenotype of this subpopulation in the adult neocortex. We show that these mature interneurons, although being in close proximity, lack polysialylated neural cell adhesion molecule (PSA-NCAM) expression, a molecule expressed by a subpopulation of mature interneurons, related to brain development and involved in neuronal plasticity of the adult brain as well. However, in the layer II of Piriform cortex there is a high density of cells expressing Reelin whose neurochemical phenotype and connectivity has not been described before. Interestingly, in close proximity to these Reelin expressing cells there is a numerous subpopulation of immature neurons expressing PSA-NCAM and doublecortin (DCX) in this layer of the Piriform cortex. Here, we show that Reelin cells express the neuronal marker Neuronal Nuclei (NeuN), but however the majority of neurons lack markers of mature excitatory or inhibitory neurons. A detail analysis of its morphology indicates these that some of these cells might correspond to semilunar neurons. Interestingly, we found that the majority of these cells express T-box brain 1 (TBR-1) a transcription factor found not only in post-mitotic neurons that differentiate to glutamatergic excitatory neurons but also in Cajal-Retzius cells. We suggest that the function of these Reelin expressing cells might be similar to that of the Cajal-Retzius cells during development, having a role in the maintenance of the immature phenotype of the PSA-NCAM/DCX neurons through its receptors apolipoprotein E receptor 2 (ApoER2) and very low density lipoprotein receptor (VLDLR) in the Piriform cortex layer II during adulthood.

Reelin and cofilin cooperate during the migration of cortical neurons: a quantitative morphological analysis

In reeler mutant mice, which are deficient in reelin (Reln), the lamination of the cerebral cortex is disrupted. Reelin signaling induces phosphorylation of LIM kinase 1, which phosphorylates the actin-depolymerizing protein cofilin in migrating neurons. Conditional cofilin mutants show neuronal migration defects. Thus, both reelin and cofilin are indispensable during cortical development. To analyze the effects of cofilin phosphorylation on neuronal migration we used in utero electroporation to transfect E14.5 wild-type cortical neurons with pCAG-EGFP plasmids encoding either a nonphosphorylatable form of cofilin 1 (cofilin(S3A)), a pseudophosphorylated form (cofilin(S3E)) or wild-type cofilin 1 (cofilin(WT)). Wild-type controls and reeler neurons were transfected with pCAG-EGFP. Real-time microscopy and histological analyses revealed that overexpression of cofilin(WT) and both phosphomutants induced migration defects and morphological abnormalities of cortical neurons. Of note, reeler neurons and cofilin(S3A)- and cofilin(S3E)-transfected neurons showed aberrant backward migration towards the ventricular zone. Overexpression of cofilin(S3E), the pseudophosphorylated form, partially rescued the migration defect of reeler neurons, as did overexpression of Limk1. Collectively, the results indicate that reelin and cofilin cooperate in controlling cytoskeletal dynamics during neuronal migration.

Tangential migration of glutamatergic neurons and cortical patterning during development: Lessons from Cajal-Retzius cells

Tangential migration is a mode of cell movement, which in the developing cerebral cortex, is defined by displacement parallel to the ventricular surface and orthogonal to the radial glial fibers. This mode of long-range migration is a strategy by which distinct neuronal classes generated from spatially and molecularly distinct origins can integrate to form appropriate neural circuits within the cortical plate. While it was previously believed that only GABAergic cortical interneurons migrate tangentially from their origins in the subpallial ganglionic eminences to integrate in the cortical plate, it is now known that transient populations of glutamatergic neurons also adopt this mode of migration. These include Cajal-Retzius cells (CRs), subplate neurons (SPs), and cortical plate transient neurons (CPTs), which have crucial roles in orchestrating the radial and tangential development of the embryonic cerebral cortex in a noncell-autonomous manner. While CRs have been extensively studied, it is only in the last decade that the molecular mechanisms governing their tangential migration have begun to be elucidated. To date, the mechanisms of SPs and CPTs tangential migration remain unknown. We therefore review the known signaling pathways, which regulate parameters of CRs migration including their motility, contact-redistribution and adhesion to the pial surface, and discuss this in the context of how CR migration may regulate their signaling activity in a spatial and temporal manner. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 76: 847-881, 2016.

An in silico agent-based model demonstrates Reelin function in directing lamination of neurons during cortical development

The characteristic six-layered appearance of the neocortex arises from the correct positioning of pyramidal neurons during development and alterations in this process can cause intellectual disabilities and developmental delay. Malformations in cortical development arise when neurons either fail to migrate properly from the germinal zones or fail to cease migration in the correct laminar position within the cortical plate. The Reelin signalling pathway is vital for correct neuronal positioning as loss of Reelin leads to a partially inverted cortex. The precise biological function of Reelin remains controversial and debate surrounds its role as a chemoattractant or stop signal for migrating neurons. To investigate this further we developed an in silico agent-based model of cortical layer formation. Using this model we tested four biologically plausible hypotheses for neuron motility and four biologically plausible hypotheses for the loss of neuron motility (conversion from migration). A matrix of 16 combinations of motility and conversion rules was applied against the known structure of mouse cortical layers in the wild-type cortex, the Reelin-null mutant, the Dab1-null mutant and a conditional Dab1 mutant. Using this approach, many combinations of motility and conversion mechanisms can be rejected. For example, the model does not support Reelin acting as a repelling or as a stopping signal. In contrast, the study lends very strong support to the notion that the glycoprotein Reelin acts as a chemoattractant for neurons. Furthermore, the most viable proposition for the conversion mechanism is one in which conversion is affected by a motile neuron sensing in the near vicinity neurons that have already converted. Therefore, this model helps elucidate the function of Reelin during neuronal migration and cortical development.

Reelin Induces Branching of Neurons and Radial Glial Cells during Corticogenesis

Newborn neurons migrate along the processes of radial glial cells (RGCs) to reach their final positions in the cortex. Here, we visualized individual migrating neurons and RGCs using in utero electroporation. We show that branching of migrating neurons and RGCs is closely correlated spatiotemporally with the distribution of Reelin. Time-lapse imaging revealed that the leading processes of migrating neurons gave rise to increasingly more branches once their growth cones contacted the Reelin-containing marginal zone. This was accompanied by translocation of the nucleus and gradual shortening of the leading process. Absence of Reelin in reeler mice altered these processes resulting in misorientation, loss of bipolarity, and aberrant migration of cortical neurons. Moreover, in reeler, the branching of the basal processes of RGCs in the marginal zone was severely disrupted. Consistent with previous reports, we show that in dissociated reeler cortical cultures, exposure to recombinant Reelin enhanced dendritic complexity and glial branching. Our results suggest that Reelin induces branching of the leading processes of migrating neurons and that of basal processes of RGCs when they arrive at the Reelin-containing marginal zone. Branching of these processes may be crucial for the termination of nuclear translocation during the migratory process and for correct neuronal positioning.

Retinal distribution of Disabled-1 in a diurnal murine rodent, the Nile grass rat Arvicanthis niloticus

We sought to study the expression pattern of Disabled-1 (Dab1; an adaptor protein in the reelin pathway) in the cone-rich retina of a diurnal murine rodent. Expression was examined by western blotting and immunohistochemistry using well-established antibodies against Dab1 and various markers of retinal neurons. Western blots revealed the presence of Dab1 (80 kDa) in brain and retina of the Nile grass rat. Retinal immunoreactivity was predominant in soma and dendrites of horizontal cells as well as in amacrine cell bodies aligned at the INL/IPL border. Dab1(+) neurons in the inner retina do not stain for parvalbumin, calbindin, protein kinase C-alpha, choline acetyltransferase, glutamic acid decarboxylase, or tyrosine hydroxylase. They express, however, the glycine transporter GlyT1. They have small ovoid cell bodies (7.1 ± 1.06 μm in diameter) and bistratified terminal plexii in laminas a and b of the IPL. Dab1(+) amacrine cells are evenly distributed across the retina (2600 cells/mm(2)) in a fairly regular mosaic (regularity indexes ≈3.3-5.5). We conclude that retinal Dab1 in the adult Nile grass rat exhibits a dual cell patterning similar to that found in human. It is expressed in horizontal cells as well as in a subpopulation of glycinergic amacrine cells undetectable with antibodies against calcium-binding proteins. These amacrine cells are likely of the AII type.

Alterations in the hippocampal and striatal catecholaminergic fiber densities of heterozygous reeler mice

The heterozygous reeler mouse (HRM), haploinsufficient for reelin, shares several neurochemical and behavioral similarities with patients suffering from schizophrenia. It has been shown that defective reelin signaling influences the mesolimbic dopaminergic pathways in a specific manner. However, there is only little information about the impact of reelin haploinsufficiency on the monoaminergic innervation of different brain areas, known to be involved in the pathophysiology of schizophrenia. In the present study using immunocytochemical procedures, we investigated HRM and wild-type mice (WT) for differences in the densities of tyrosine hydroxylase (TH)-immunoreactive (IR) and serotonin (5-HT)-IR fibers in prefrontal cortex, ventral and dorsal hippocampal formation, amygdala and ventral and dorsal striatum. We found that HRM, compared to WT, shows a significant increase in TH-IR fiber densities in dorsal hippocampal CA1, CA3 and ventral CA1. In contrast, HRM exhibits a significant decrease of TH-IR in the shell of the nucleus accumbens (AcbShell), but no differences in the other brain areas investigated. Overall, no genotype differences were found in the 5-HT-IR fiber densities. In conclusion, these results support the view that reelin haploinsufficiency differentially influences the catecholaminergic (esp. dopaminergic) systems in brain areas associated with schizophrenia. The reelin haploinsufficient mouse may provide a useful model for studying the role of reelin in hippocampal dysfunction and its effect on the dopaminergic system as related to schizophrenia.

Reelin in the Years: Controlling Neuronal Migration and Maturation in the Mammalian Brain

The extracellular protein Reelin was initially identified as an essential factor in the control of neuronal migration and layer formation in the developing mammalian brain. In the years following its discovery, however, it became clear that Reelin is a multifunctional protein that controls not only the positioning of neurons in the developing brain, but also their growth, maturation, and synaptic activity in the adult brain. In this review, we will highlight the major discoveries of the biological activities of Reelin and the underlying molecular mechanisms that affect the development and function of the mammalian brain, from embryonic ages to adulthood.

Molecular pathways controlling the sequential steps of cortical projection neuron migration

Coordinated migration of newly-born neurons to their target territories is essential for correct neuronal circuit assembly in the developing brain. Although a cohort of signaling pathways has been implicated in the regulation of cortical projection neuron migration, the precise molecular mechanisms and how a balanced interplay of cell-autonomous and non-autonomous functions of candidate signaling molecules controls the discrete steps in the migration process, are just being revealed. In this chapter, I will focally review recent advances that improved our understanding of the cell-autonomous and possible cell-nonautonomous functions of the evolutionarily conserved LIS1/NDEL1-complex in regulating the sequential steps of cortical projection neuron migration. I will then elaborate on the emerging concept that the Reelin signaling pathway, acts exactly at precise stages in the course of cortical projection neuron migration. Lastly, I will discuss how finely tuned transcriptional programs and downstream effectors govern particular aspects in driving radial migration at discrete stages and how they regulate the precise positioning of cortical projection neurons in the developing cerebral cortex.

Origin, lineage and function of cerebellar glia

The glial cells of the cerebellum, and particularly astrocytes and oligodendrocytes, are characterized by a remarkable phenotypic variety, in which highly peculiar morphological features are associated with specific functional features, unique among the glial cells of the entire CNS. Here, we provide a critical report about the present knowledge of the development of cerebellar glia, including lineage relationships between cerebellar neurons, astrocytes and oligodendrocytes, the origins and the genesis of the repertoire of glial types, and the processes underlying their acquisition of mature morphological and functional traits. In parallel, we describe and discuss some fundamental roles played by specific categories of glial cells during cerebellar development. In particular, we propose that Bergmann glia exerts a crucial scaffolding activity that, together with the organizing function of Purkinje cells, is necessary to achieve the normal pattern of foliation and layering of the cerebellar cortex. Moreover, we discuss some of the functional tasks of cerebellar astrocytes and oligodendrocytes that are distinctive of cerebellar glia throughout the CNS. Notably, we report about the regulation of synaptic signalling in the molecular and granular layer mediated by Bergmann glia and parenchymal astrocytes, and the functional interaction between oligodendrocyte precursor cells and neurons. On the whole, this review provides an extensive overview of the available literature and some novel insights about the origin and differentiation of the variety of cerebellar glial cells and their function in the developing and mature cerebellum.

N-acetyl-cysteine prevents pyramidal cell disarray and reelin-immunoreactive neuron deficiency in CA3 after prenatal immune challenge in rats

BACKGROUND

Prenatal infection is a major risk factor for the occurrence of neuropsychiatric disorders. These have been associated with hippocampal neuroanatomical and functional abnormalities. In the present study, we evaluated the occurrence of pyramidal cell disarray and reelin neuronal deficit in the hippocampus, and the protective role of N-acetyl-cysteine (NAC) in a rodent experimental model of prenatal immune challenge.

METHODS

Sprague-Dawley rats received either 500 μg/kg of endotoxin (lipopolysaccharide, LPS) or 2 ml/kg of isotonic saline by i.p. injection on day 19 of gestation. After LPS injection, rats were or were not maintained on a preventive treatment of NAC (5 g/l in tap water), up to delivery. The pyramidal cell orientation and the number and type of reelin-expressing neurons were determined in male offspring.

RESULTS

Prenatal LPS challenge led to permanent pyramidal cell disarray and to an early and transient decreased density of reelin-immunoreactive neurons. These disorders, more pronounced in the CA3 area, were prevented by NAC.

CONCLUSION

Hippocampal cytoarchitectural alterations and reelin deficiency may be involved in the development of remote cognitive impairments in this model. The antioxidant NAC is an efficient neuroprotective drug that underlines the role of oxidative stress in prenatal infection and associated neurodevelopmental damage.

Dual origins of the mammalian accessory olfactory bulb revealed by an evolutionarily conserved migratory stream

The accessory olfactory bulb (AOB) is a critical olfactory structure that has been implicated in mediating social behavior. It receives input from the vomeronasal organ and projects to targets in the amygdaloid complex. Its anterior and posterior components (aAOB and pAOB) display molecular, connectional and functional segregation in processing reproductive and defensive and aggressive behaviors, respectively. We observed a dichotomy in the development of the projection neurons of the aAOB and pAOB in mice. We found that they had distinct sites of origin and that different regulatory molecules were required for their specification and migration. aAOB neurons arose locally in the rostral telencephalon, similar to main olfactory bulb neurons. In contrast, pAOB neurons arose caudally, from the neuroepithelium of the diencephalic-telencephalic boundary, from which they migrated rostrally to reach their destination. This unusual origin and migration is conserved in Xenopus, providing an insight into the origin of a key component of this system in evolution.

Extracellular matrix functions during neuronal migration and lamination in the mammalian central nervous system

Extracellular matrix (ECM) glycoproteins are expressed in the central nervous system (CNS) in complex and developmentally regulated patterns. The ECM provides a number of critical functions in the CNS, contributing both to the overall structural organization of the CNS and to control of individual cells. At the cellular level, the ECM affects its functions by a wide range of mechanisms, including providing structural support to cells, regulating the activity of second messenger systems, and controlling the distribution and local concentration of growth and differentiation factors. Perhaps the most well known role of the ECM is as a substrate on which motile cells can migrate. Genetic, cell biological, and biochemical studies provide strong evidence that ECM glycoproteins such as laminins, tenascins, and proteoglycans control neuronal migration and positioning in several regions of the developing and adult brain. Recent findings have also shed important new insights into the cellular and molecular mechanisms by which reelin regulates migration. Here we will summarize these findings, emphasizing the emerging concept that ECM glycoproteins promote different modes of neuronal migration such as radial, tangential, and chain migration. We also discuss several studies demonstrating that mutations in ECM glycoproteins can alter neuronal positioning by cell nonautonomous mechanisms that secondarily affect migrating neurons.

Postnatal characterization of cells in the accessory olfactory bulb of wild type and reeler mice

Olfaction is the most relevant chemosensory sense of the rodents. General odors are primarily detected by the main olfactory system while most pheromonal signals are received by the accessory olfactory system. The first relay in the brain occurs in the olfactory bulb, which is subdivided in the main and accessory olfactory bulb (MOB/AOB). Given that the cell generation time is different between AOB and MOB, and the cell characterization of AOB remains limited, the goal of this work was first, the definition of the layering of AOB/MOB and second, the determination of cellular phenotypes in the AOB in a time window corresponding to the early postnatal development. Moreover, since reelin (Reln) deficiency has been related to olfactory learning deficits, we analyzed reeler mice. First, we compared the layering between AOB and MOB at early embryonic stages. Then, cell phenotypes were established using specific neuronal and glial markers as well as the Reln adaptor protein Dab1 to analyse differences in both genetic backgrounds. There was no apparent difference in the cell phenotypes among AOB and MOB or between wild type (wt) and reeler animals. However, a disruption in the granular cell layer of reeler with respect to wt mice was observed. In conclusion, the AOB in Reln-deficient mice showed similar neuronal and glial cell types being only affected the organization of granular neurons.

Reelin sets the pace of neocortical neurogenesis

Migration of neurons during cortical development is often assumed to rely on purely post-proliferative reelin signaling. However, Notch signaling, long known to regulate neural precursor formation and maintenance, is required for the effects of reelin on neuronal migration. Here, we show that reelin gain-of-function causes a higher expression of Notch target genes in radial glia and accelerates the production of both neurons and intermediate progenitor cells. Converse alterations correlate with reelin loss-of-function, consistent with reelin controlling Notch signaling during neurogenesis. Ectopic expression of reelin in isolated clones of progenitors causes a severe reduction in neuronal differentiation. In mosaic cell cultures, reelin-primed progenitor cells respond to wild-type cells by further decreasing neuronal differentiation, consistent with an increased sensitivity to lateral inhibition. These results indicate that reelin and Notch signaling cooperate to set the pace of neocortical neurogenesis, a prerequisite for proper neuronal migration and cortical layering.

Sall1 regulates cortical neurogenesis and laminar fate specification in mice: implications for neural abnormalities in Townes-Brocks syndrome

Progenitor cells in the cerebral cortex undergo dynamic cellular and molecular changes during development. Sall1 is a putative transcription factor that is highly expressed in progenitor cells during development. In humans, the autosomal dominant developmental disorder Townes-Brocks syndrome (TBS) is associated with mutations of the SALL1 gene. TBS is characterized by renal, anal, limb and auditory abnormalities. Although neural deficits have not been recognized as a diagnostic characteristic of the disease, ∼10% of patients exhibit neural or behavioral abnormalities. We demonstrate that, in addition to being expressed in peripheral organs, Sall1 is robustly expressed in progenitor cells of the central nervous system in mice. Both classical- and conditional-knockout mouse studies indicate that the cerebral cortex is particularly sensitive to loss of Sall1. In the absence of Sall1, both the surface area and depth of the cerebral cortex were decreased at embryonic day 18.5 (E18.5). These deficiencies are associated with changes in progenitor cell properties during development. In early cortical progenitor cells, Sall1 promotes proliferative over neurogenic division, whereas, at later developmental stages, Sall1 regulates the production and differentiation of intermediate progenitor cells. Furthermore, Sall1 influences the temporal specification of cortical laminae. These findings present novel insights into the function of Sall1 in the developing mouse cortex and provide avenues for future research into potential neural deficits in individuals with TBS.

Dab1 (Disable homolog-1) reelin adaptor protein is overexpressed in the olfactory bulb at early postnatal stages

Dab1 mediates reelin signalling and plays critical roles in early brain development such as the stereotypical positioning of neurons in the brain. The olfactory bulb undergoes a prominent layering reorganization, but shows not apparent differences between wild type and reeler in the layer organization. Therefore, an accurate regional and cellular simultaneous analysis of these molecules becomes essential to clarify the role played by Dab1 upon Reelin effect. The present study reveals a strong and consistent Dab1 mRNA and protein expressions, throughout the olfactory bulb layers in both wild type and reeler mice. In addition, noteworthy is the pattern of Dab1 location within cell nuclei in both strains. Furthermore, a temporal increment of Dab1 expression levels is detected from P0 to P15 in both strains, being the protein quantity higher in reeler than in wild type mice. Altogether, our results revealed that Reln acts directly from projection neurons via the production of different Reln fragments. Changes in the pattern of Dab1 expression could reflect an alternative Reln function in postnatal and adult stages, besides a possible regulation of Dab1 by other molecules distinct to Reln.

Cell-type-specific consequences of Reelin deficiency in the mouse neocortex, hippocampus, and amygdala

The disrupted cortical lamination phenotype in reeler mice and subsequent identification of the Reelin signaling pathway have strongly informed models of cortical development. We describe here a marker-based phenotyping approach to reexamine the cytoarchitectural consequences of Reelin deficiency, using high-throughput histology and newly identified panels of highly specific molecular markers. The resulting cell-type-level cytoarchitectural analysis revealed novel features of abnormal patterning in the male reeler mouse not obvious with less specific markers or histology. The reeler cortex has been described as a rough laminar inversion, but the data presented here are not compatible with this model. The reeler cortex is disrupted in a more complex fashion, with some regions showing a mirror-image laminar phenotype. Major rostrocaudal and cell-type-specific differences in the laminar phenotype between cortical areas are detailed. These and similar findings in hippocampus and amygdala have implications for mechanisms of normal brain development and abnormalities in neurodevelopmental disorders.

Reelin inhibits migration of sympathetic preganglionic neurons in the spinal cord of the chick

The present study examined the effects of Reelin in the migration of sympathetic preganglionic neurons (SPN) in the spinal cord of the chick. SPN in the chick first migrate from the neuroepithelium to the ventrolateral spinal cord. They then undergo a secondary migration to cluster adjacent to the central canal, forming the column of Terni (CT). During secondary migration, abundant Reelin is found in large areas of the ventral spinal cord; the only areas devoid of Reelin are areas occupied by SPN or somatic motor neurons and the pathway along which SPN migrate. Ectopic expression of Reelin in the pathway of SPN through electroporation of full-length Reelin DNA stopped SPN migration toward their destination. The spatiotemporal pattern of Reelin expression, along with the inhibition of SPN migration by exogenous Reelin, suggests that Reelin functions as a barrier to SPN migration during normal development of the spinal cord.

Reelin regulates cadherin function via Dab1/Rap1 to control neuronal migration and lamination in the neocortex

Neuronal migration is critical for establishing neocortical cell layers and migration defects can cause neurological and psychiatric diseases. Recent studies show that radially migrating neocortical neurons use glia-dependent and glia-independent modes of migration, but the signaling pathways that control different migration modes and the transitions between them are poorly defined. Here, we show that Dab1, an essential component of the reelin pathway, is required in radially migrating neurons for glia-independent somal translocation, but not for glia-guided locomotion. During migration, Dab1 acts in translocating neurons to stabilize their leading processes in a Rap1-dependent manner. Rap1, in turn, controls cadherin function to regulate somal translocation. Furthermore, cell-autonomous neuronal deficits in somal translocation are sufficient to cause severe neocortical lamination defects. Thus, we define the cellular mechanism of reelin function during radial migration, elucidate the molecular pathway downstream of Dab1 during somal translocation, and establish the importance of glia-independent motility in neocortical development.

Building a human cortex: the evolutionary differentiation of Cajal-Retzius cells and the cortical hem

Cajal-Retzius (CR) cells are the most significant source of reelin, an extracellular matrix glycoprotein essential for cortical development. Strategically located in the marginal zone, CR cells control radial migration and laminar positioning of pyramidal neurons of the cortical plate. They degenerate and undergo cell death when cortical migration is completed. In human cortex development, reelin-expressing CR cells are already present in the early preplate, and continue to increase in number after the appearance of the cortical plate. In the course of the first half of gestation, the reelin signal in the marginal zone undergoes a huge amplification in parallel with the growth of the cortical plate and the expansion of the cortical surface. A significant source of CR cells is the cortical hem, a putative signalling centre at the interface of the prospective hippocampus and the choroid plexus. Hem-derived CR cells co-express reelin and p73, a transcription factor of the p53-family. They form the predominant CR cell population of the human neocortex. Characteristically, CR cells express the anti-apoptotic isoform DeltaNp73 which may be responsible for the protracted lifespan of human CR cells and the morphological differentiation of their axonal plexus. This dense fibre plexus, absent in lower mammals, amplifies the reelin-signal and establishes a physical boundary between the cortical plate and the marginal zone. In this review, we analyze the multiple sources of reelin/p73 positive CR cells at the interface of various telencephalic centres and the choroid plexus of the lateral ventricles. Additional populations of CR cells may derive from the thalamic eminence in the ventral thalamus and from the strionuclear neuroepithelium, or 'amygdalar hem'. Comparative studies in a variety of species indicate that the cortical hem is the main origin of CR cells destined for the neocortex, and is most highly developed in the human brain. The close association between cortical hem and choroid plexus suggests a concerted role in the evolutionary increase of CR cells, amplification of the reelin signal in the marginal zone, and cortical expansion.

Extracellular matrix: functions in the nervous system

An astonishing number of extracellular matrix glycoproteins are expressed in dynamic patterns in the developing and adult nervous system. Neural stem cells, neurons, and glia express receptors that mediate interactions with specific extracellular matrix molecules. Functional studies in vitro and genetic studies in mice have provided evidence that the extracellular matrix affects virtually all aspects of nervous system development and function. Here we will summarize recent findings that have shed light on the specific functions of defined extracellular matrix molecules on such diverse processes as neural stem cell differentiation, neuronal migration, the formation of axonal tracts, and the maturation and function of synapses in the peripheral and central nervous system.