Cell migration in the rat embryonic neocortex

Delayed maturation and migration of excitatory neurons in the juvenile mouse paralaminar amygdala

The human amygdala paralaminar nucleus (PL) contains many immature excitatory neurons that undergo prolonged maturation from birth to adulthood. We describe a previously unidentified homologous PL region in mice that contains immature excitatory neurons and has previously been considered part of the amygdala intercalated cell clusters or ventral endopiriform cortex. Mouse PL neurons are born embryonically, not from postnatal neurogenesis, despite a subset retaining immature molecular and morphological features in adults. During juvenile-adolescent ages (P21-P35), the majority of PL neurons undergo molecular, structural, and physiological maturation, and a subset of excitatory PL neurons migrate into the adjacent endopiriform cortex. Alongside these changes, PL neurons develop responses to aversive and appetitive olfactory stimuli. The presence of this homologous region in both humans and mice points to the significance of this conserved mechanism of neuronal maturation and migration during adolescence, a key time period for amygdala circuit maturation and related behavioral changes.

Distinct distribution of subplate neuron subtypes between the sensory cortices during the early postnatal period

Subplate neurons (SpNs) are a heterogeneous neuronal population actively involved in early cortical circuit formation. In rodents, many SpNs survive and form layer 6b. The molecular heterogeneity of SpNs raises the question of whether different subpopulations of SpNs survive through the early postnatal period similarly and whether such diverse SpN populations in the auditory cortex (ACtx) share a common distribution pattern with other sensory systems. To address that, we investigated the expression pattern of multiple specific SpN markers in the ACtx, as well as in the visual (VCtx) and somatosensory (SCtx) cortices as controls, using complexin 3 (Cplx3) antibodies and different SpN-specific Cre-driver mice, such as connective tissue growth factor (CTGF), dopamine receptor D1 (Drd1a), and neurexophilin 4 (Nxph4). We focused on two early time windows in auditory development: (1) during the second postnatal week (PNW) before ear-canal opening and (2) during the third PNW after ear-canal opening. We compared the expression pattern of different SpN markers in ACtx with VCtx and SCtx. At both examined timepoints, Cplx3 and Nxph4 expressing SpNs form the largest and smallest population in the ACtx, respectively. Similar distribution patterns are observable in the VCtx and SCtx during the second PNW but not during the third PNW, for a higher proportion of Drd1a expressing SpNs is detected in the VCtx and CTGF expressing SpNs in the SCtx. This study suggests that different populations of SpNs might contribute differently to the development of individual sensory circuits.

Development of Auditory Cortex Circuits

The ability to process and perceive sensory stimuli is an essential function for animals. Among the sensory modalities, audition is crucial for communication, pleasure, care for the young, and perceiving threats. The auditory cortex (ACtx) is a key sound processing region that combines ascending signals from the auditory periphery and inputs from other sensory and non-sensory regions. The development of ACtx is a protracted process starting prenatally and requires the complex interplay of molecular programs, spontaneous activity, and sensory experience. Here, we review the development of thalamic and cortical auditory circuits during pre- and early post-natal periods.

Temporally Distinct Roles for the Zinc Finger Transcription Factor Sp8 in the Generation and Migration of Dorsal Lateral Ganglionic Eminence (dLGE)-Derived Neuronal Subtypes in the Mouse

Progenitors in the dorsal lateral ganglionic eminence (dLGE) are known to give rise to olfactory bulb (OB) interneurons and intercalated cells (ITCs) of the amygdala. The dLGE enriched transcription factor Sp8 is required for the normal generation of ITCs as well as OB interneurons, particularly the calretinin (CR)-expressing subtype. In this study, we used a genetic gain-of-function approach in mice to examine the roles Sp8 plays in controlling the development of dLGE-derived neuronal subtypes. Misexpression of Sp8 throughout the ventral telencephalic subventricular zone (SVZ) from early embryonic stages, led to an increased generation of ITCs which was dependent on Tshz1 gene dosage. Additionally, Sp8 misexpression impaired rostral migration of OB interneurons with clusters of CR interneurons seen in the SVZ along with decreased differentiation of calbindin OB interneurons. Sp8 misexpression throughout the ventral telencephalon also reduced ventral LGE neuronal subtypes including striatal projection neurons. Delaying Sp8 misexpression until E14-15 rescued the striatal and amygdala phenotypes but only partially rescued OB interneuron reductions, consistent with an early window of striatal and amygdala neurogenesis and ongoing OB interneuron generation at this late stage. Our results demonstrate critical roles for the timing and neuronal cell-type specificity of Sp8 expression in mouse LGE neurogenesis.

Tissue-Type Plasminogen Activator Controlled Corticogenesis Through a Mechanism Dependent of NMDA Receptors Expressed on Radial Glial Cells

Modifications of neuronal migration during development, including processes that control cortical lamination are associated with functional deficits at adult stage. Here, we report for the first time that the lack of the serine protease tissue-type Plasminogen Activator (tPA), previously characterized as a neuromodulator and a gliotransmitter, leads to an altered cortical lamination in adult. This results in a neuronal migration defect of tPA deficient neurons which are stopped in the intermediate zone at E16. This phenotype is rescued by re-expressing a wild-type tPA in cortical neurons at E14 but not by a tPA that cannot interact with NMDAR. We thus hypothetized that the tPA produced by cortical neuronal progenitors can control their own radial migration through a mechanism dependent of NMDAR expressed at the surface of radial glial cells (RGC). Accordingly, conditional deletion of tPA in neuronal progenitors at E14 or overexpression of a dominant-negative NMDAR that cannot bind tPA in RGC also delayed neuronal migration. Moreover, the lack of tPA lead to an impaired maturation and orientation of RGC. These data provide the first demonstration that the neuronal serine protease tPA is an actor of a proper corticogenesis by its ability to control NMDAR signaling in RGC.

The Laminar Organization of Piriform Cortex Follows a Selective Developmental and Migratory Program Established by Cell Lineage

Piriform cortex (PC) is a 3-layer paleocortex receiving primary afferent input from the olfactory bulb. The past decade has seen significant progress in understanding the synaptic, cellular and functional organization of PC, but PC embryogenesis continues to be enigmatic. Here, using birthdating strategies and clonal analyses, we probed the early development and laminar specificity of neurogenesis/gliogenesis as it relates to the organization of the PC. Our data demonstrate a temporal sequence of laminar-specific neurogenesis following the canonical "inside-out" pattern, with the notable exception of PC Layer II which exhibited an inverse "outside-in" temporal neurogenic pattern. Of interest, we found no evidence of a neurogenic gradient along the anterior to posterior axis, although the timing of neuronal migration and laminar development was delayed rostrally by approximately 24 h. To begin probing if lineage affected cell fate in the PC, we labeled PC neuroblasts using a multicolor technique and analyzed their laminar organization. Our results suggested that PC progenitors were phenotypically committed to reach specific layers early in the development. Collectively, these studies shed new light on the determinants of the laminar specificity of neuronal/glial organization in PC and the likely role of subpopulations of committed progenitors in regulating PC embryogenesis.

Phospholipase D1 Signaling: Essential Roles in Neural Stem Cell Differentiation

Phospholipase D1 (PLD1) is generally accepted as playing an important role in the regulation of multiple cell functions, such as cell growth, survival, differentiation, membrane trafficking, and cytoskeletal organization. Recent findings suggest that PLD1 also plays an important role in the regulation of neuronal differentiation of neuronal cells. Moreover, PLD1-mediated signaling molecules dynamically regulate the neuronal differentiation of neural stem cells (NSCs). Rho family GTPases and Ca²⁺-dependent signaling, in particular, are closely involved in PLD1-mediated neuronal differentiation of NSCs. Moreover, PLD1 has a significant effect on the neurogenesis of NSCs via the regulation of SHP-1/STAT3 activation. Therefore, PLD1 has now attracted significant attention as an essential neuronal signaling molecule in the nervous system. In the current review, we summarize recent findings on the regulation of PLD1 in neuronal differentiation and discuss the potential role of PLD1 in the neurogenesis of NSCs.

Novel genetic tools facilitate the study of cortical neuron migration

Key facets of mammalian forebrain cortical development include the radial migration of projection neurons and subsequent cellular differentiation into layer-specific subtypes. Inappropriate regulation of these processes can lead to a number of congenital brain defects in both mouse and human, including lissencephaly and intellectual disability. The genes regulating these processes are still not all identified, suggesting genetic analyses will continue to be a powerful tool in mechanistically studying the development of the cerebral cortex. Reelin is a molecule which we have understood to be critical for proper cortical development for many years. The precise mechanism of Reelin, however, is not fully understood. To address both of these unresolved issues, we report here the creation of a novel conditional allele of the Reelin gene and showcase the use of an Etv1-GFP transgenic line highlighting a subpopulation of the cortex: layer V pyramidal neurons. Together, these represent genetic tools which may facilitate the study of cortical development in a number of different ways.

Pallial patterning and the origin of the isocortex

Together with a complex variety of behavioral, physiological, morphological, and neurobiological innovations, mammals are characterized by the development of an extensive isocortex (also called neocortex) that is both laminated and radially organized, as opposed to the brain of birds and reptiles. In this article, we will advance a developmental hypothesis in which the mechanisms of evolutionary brain growth remain partly conserved across amniotes (mammals, reptiles and birds), all based on Pax6 signaling or related morphogens. Despite this conservatism, only in mammals there is an additional upregulation of dorsal and anterior signaling centers (the cortical hem and the anterior forebrain, respectively) that promoted a laminar and a columnar structure into the neocortex. It is possible that independently, some birds also developed an upregulated dorsal pallium.

Prenatal genesis of layer II doublecortin expressing neurons in neonatal and young adult guinea pig cerebral cortex

Cells expressing doublecortin (DCX+) occur at cortical layer II, predominantly over the paleocortex in mice/rats, but also across the neocortex among larger mammals. Here, we explored the time of origin of these cells in neonatal and 2-month-old guinea pigs following prenatal BrdU pulse-chasing. In the neocortex, BrdU+ cells birth-dated at embryonic day 21 (E21), E28, and E35 laminated over the cortical plate with an inside-out order. In the piriform cortex, cells generated at E21 and E28 occurred with a greater density in layer II than III. Many cells were generated at later time points until birth, occurring in the cortex without a laminar preference. DCX+ cells in the neocortex and piriform cortex partially co-colocalized with BrdU (up to 7.5%) in the newborns after pulse-chasing from E21 to E49 and in the 2 month-old animals after pulse-chasing from E28 to E60/61, with higher rates seen among the E21-E35 groups. Together, layer II DCX+ cells in neonatal and young adult guinea pigs may be produced over a wide prenatal time window, but mainly during the early phases of corticogenesis. Our data also show an earlier establishment of the basic lamination in the piriform relative to neocortical areas in guinea pigs.

Chronic maternal hyperglycemia induced during mid-pregnancy in rats increases RAGE expression, augments hippocampal excitability, and alters behavior of the offspring

Maternal diabetes during pregnancy may increase the risk of neurodevelopmental disorders in the offspring by increasing inflammation. A major source of inflammatory signaling observed in diabetes is activation of the receptor for advanced glycation end-products (RAGE), and increased RAGE expression has been reported in psychiatric disorders. Thus, we sought to examine whether maternal diabetes creates a proinflammatory state, triggered largely by RAGE signaling, that alters normal brain development and behavior of the offspring. We tested this hypothesis in rats using the streptozotocin (STZ; 50mg/kg; i.p.) model of diabetes induced during mid-pregnancy. Following STZ treatment, we observed a significant increase in RAGE protein expression in the forebrain of the offspring (postnatal day 1). Data obtained from whole-cell patch clamping of hippocampal neurons in cultures from the offspring of STZ-treated dams revealed a striking increase in excitability. When tested in a battery of behavioral tasks in early adulthood, the offspring of STZ-treated dams had significantly lower prepulse inhibition, reduced anxiety-like behavior, and altered object-place preference when compared to control offspring. In an operant-based strategy set-shifting task, STZ offspring did not differ from controls on an initial visual discrimination or reversal learning but took significantly longer to shift to a new strategy (i.e., set-shift). Insulin replacement with an implantable pellet in the dams reversed the effects of maternal diabetes on RAGE expression, hippocampal excitability, prepulse inhibition and object-place memory, but not anxiety-like behavior or set-shifting. Taken together, these results suggest that chronic maternal hyperglycemia alters normal hippocampal development and behavior of the offspring, effects that may be mediated by increased RAGE signaling in the fetal brain.

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.

The rate of X-ray-induced DNA double-strand break repair in the embryonic mouse brain is unaffected by exposure to 50 Hz magnetic fields

Purpose: Following in utero exposure to low dose radiation (10–200 mGy), we recently observed a linear induction of DNA double-strand breaks (DSB) and activation of apoptosis in the embryonic neuronal stem/progenitor cell compartment. No significant induction of DSB or apoptosis was observed following exposure to magnetic fields (MF). In the present study, we exploited this in vivo system to examine whether exposure to MF before and after exposure to 100 mGy X-rays impacts upon DSB repair rates.

Materials and methods: 53BP1 foci were quantified following combined exposure to radiation and MF in the embryonic neuronal stem/progenitor cell compartment. Embryos were exposed in utero to 50 Hz MF at 300 μT for 3 h before and up to 9 h after exposure to 100 mGy X-rays. Controls included embryos exposed to MF or X-rays alone plus sham exposures.

Results: Exposure to MF before and after 100 mGy X-rays did not impact upon the rate of DSB repair in the embryonic neuronal stem cell compartment compared to repair rates following radiation exposure alone.

Conclusions: We conclude that in this sensitive system MF do not exert any significant level of DNA damage and do not impede the repair of X-ray induced damage.

Specification of excitatory neurons in the developing cerebral cortex: progenitor diversity and environmental influences

The mature cerebral cortex harbors a heterogeneous population of glutamatergic neurons, organized into a highly intricate histological architecture. Classically, this mixed population of neurons was thought to be generated sequentially from a seemingly homogenous group of progenitors under the influence of external cues. This view, however, has been challenged in the last decade by evidences pointing to the existence of fate-restricted neuronal progenitors in the developing neocortex. Here, we review classical studies using cell transplantation, retroviral labeling and cell culture, as well as new data from genetic fate-mapping analysis, to discuss the lineage relationships between neocortical progenitors and subclasses of excitatory neurons. We also propose a temporal model to conciliate the existence of fate-restricted progenitors alongside multipotent progenitors in the neocortex. Finally, we discuss evidences for a critical period of plasticity among post mitotic excitatory cortical neurons when environmental influences could change neuronal cell fate.

Purification and culture of corticospinal motor neurons

Corticospinal motor neurons (CSMNs) residing in cortical layer V of the mammalian brain project their axons to the spinal cord, where they connect with spinal motor neurons (SMNs) located in the ventral horn of the spinal cord. CSMNs and SMNs control voluntary movements, and their importance becomes obvious in situations where this network breaks down (i.e., in amyotrophic lateral sclerosis [ALS] and after spinal cord injury). Here we provide an overview of recent progress in the anatomical, morphological, and genetic characterization of developing CSMNs, as well as their survival requirements. We also describe model systems used to study CSMNs and introduce an immunopanning procedure for the purification and culture of CSMNs. Although these procedures have so far been used to purify only rodent CSMNs, in principle they should work to purify CSMNs from any vertebrate species, as well any type of central nervous system (CNS) or peripheral nervous system (PNS) neuron that can be retrograde labeled.

Serotonin 2A receptor regulates microtubule assembly and induces dynamics of dendritic growth cones in rat cortical neurons in vitro

Serotonin (5-HT) regulates the development of cerebral cortex, but 5-HT receptors mediating the effects are poorly understood. We investigated roles of 5-HT2A receptor in dendritic growth cones using dissociation culture of rat cerebral cortex. Neurons at embryonic day 16 were cultured for 4 days and treated with 5-HT2A/2C receptor agonist (DOI) for 4h. DOI increased the size of growth cone periphery which was actin-rich and microtubule-associated protein 2-negative at the dendritic tip. The length increase of the growth cone periphery may be mediated by 5-HT2A receptor, because the 5-HT2A receptor antagonist reversed the effects of DOI. Moreover, the time-lapse analysis demonstrated the increase of morphological dynamics in dendritic growth cones by DOI. Next, to elucidate the mechanisms underlying the actions of 5-HT2A receptor in dendritic growth cones, we examined the cytoskeletal proteins, tyrosinated α-tubulin (Tyr-T; dynamic tubulin) and acetylated α-tubulin (Ace-T; stable tubulin). DOI increased the fluorescence intensity of Tyr-T, while decreased that of Ace-T in the dendritic growth cone periphery. These effects were reversed by the 5-HT2A receptor antagonist, suggesting that 5-HT2A receptor promotes microtubule dynamics. In summary, it was suggested that 5-HT2A receptor induces morphological changes and dynamics of dendritic growth cones through regulation of microtubule assembly.

DNA methylation program in developing hippocampus and its alteration by alcohol

During hippocampal development, the Cornus Ammonis (CA) and the dentate gyrus (DG) undergo waves of neurogenesis and neuronal migration and maturation independently. This stage is widely known to be vulnerable to environmental stresses, but its underlying mechanism is unclear. Alcohol exposure has been shown to alter the expression of genes that regulate the fate, survival, migration and differentiation of pyramidal and granule cells. Undermining this process might compromise hippocampal development underlying the learning and memory deficits known in Fetal Alcohol Spectrum Disorders (FASD). We have previously demonstrated that DNA methylation was programmed along with neural tube development. Here, we demonstrated that DNA methylation program (DMP) proceeded along with hippocampal neuronal differentiation and maturation, and how this DMP was affected by fetal alcohol exposure. C57BL/6 mice were treated with 4% v/v ethanol through a liquid diet along with pair-fed and chow-fed controls from gestation day (E) 7 to E16. We found that a characteristic DMP, including 5-methylcytidine (5mC), 5-hydroxylmethylcytidine (5hmC) and their binding proteins, led the hippocampal neuronal differentiation and maturation spatiotemporally as indicated by their phenotypic marks in the CA and DG pre- and post-natally. Alcohol hindered the acquisition and progression of methylation marks, and altered the chromatin translocation of these marks in the nucleus, which was correlated with developmental retardation.

Expression of transient receptor potential channels in the ependymal cells of the developing rat brain

Cerebrospinal fluid (CSF) plays an important role in providing brain tissue with a stable internal environment as well as in absorbing mechanical and thermal stresses. From its initial composition, derived from the amniotic fluid trapped by the closure of neuropores, CSF is modified by developing and differentiating ependymal cells lining the ventricular surface or forming the choroid plexus. Its osmolarity and ionic composition brings about a change through the action of many channels expressed on the ependymal cells. Some newly discovered transient receptor potential (TRP) channels are known to be expressed in the choroid plexus ependyma. To detect additional TRP channel expression, immunohistochemical screening was performed at the choroid plexus of 13-, 15-, 17-, and 19-day embryos, using antibodies against TRPV1, TRPV3, and TRPA1, and the expression was compared with those in the adult TRP channels. The level of TRP channel expression was higher in the choroid plexus which suggests more active functioning of TRP channels in the developing choroid plexus than the ventricular lining ependyma in the 15- and 17-day embryos. All the expression of TRP channels decreased at the 19th day of gestation. TRPA1 was expressed at a higher level than TRPV1 and TRPV3 in almost all stages in both the choroid plexus and ventricular lining epithelium. The highest level of TRPV1 and TRPV3 expression was observed in association with the glycogen deposits in the cytoplasm of the choroid plexus ependymal cells of the 15- and 17-day embryos.

Alterations in nitric oxide synthase-expressing neurons in the forebrain regions of rats after developmental exposure to organophosphates

Several mechanisms have been addressed as contributors to the long lasting behavioral deficits after developmental exposure to organophosphate (OP) compounds. Here, the effects of developmental exposure to two common OP insecticides, chlorpyrifos (CPF) and diazinon (DZN), on nitric oxide synthase (NOS)-expressing neurons in the rat forebrain are reported. A daily dose of 1mg/kg of either CPF or DZN was administered to rats during gestational days 15-18 or postnatal days (PND) 1-4. We then assessed NADPH-diaphorase and neuronal NOS (nNOS) immunohistochemistry in forebrain sections on different postnatal days. Prenatal exposure to CPF and DZN induced a transient reduction of NADPH-d(+)/nNOS-immunoreactive (IR) neurons in most cortical regions on PND 4 but exceptionally increased them in the entorhinal/piriform cortex. On PND 15, NADPH-d(+)/nNOS-IR neurons showed morphological abnormalities within entorhinal/piriform cortex of the rats that gestationally exposed to CPF. Postnatal exposure to CPF and DZN did not induce widespread effects on the number of NADPH-d(+)/nNOS-IR neurons on PNDs 7 and 15 but significantly reduced them in most cortical regions and hippocampal subfields on PND 60. The OPs affected NADPH-d(+)/nNOS-IR neurons in a sex independent manner and apparently spared them in the striatum. While the NADPH-d reactivity of microvessels was normally diminished by age, OP treated rats evidently preserved the NADPH-d reactivity of microvessels in the cerebral cortex and hippocampus. The effects of OPs on NADPH-d(+)/nNOS-IR neurons may contribute to the long-lasting behavioral outcomes and expand the neurotransmitter system that need to be considered in OP neurotoxicity evaluations.

Scratch2 modulates neurogenesis and cell migration through antagonism of bHLH proteins in the developing neocortex

Scratch genes (Scrt) are neural-specific zinc-finger transcription factors (TFs) with an unknown function in the developing brain. Here, we show that, in addition to the reported expression of mammalian Scrt2 in postmitotic differentiating and mature neurons in the developing and early postnatal brain, Scrt2 is also localized in subsets of mitotic and neurogenic radial glial (RGP) and intermediate (IP) progenitors, as well as in their descendants-postmitotic IPs and differentiating neurons at the border subventricular/intermediate zone. Conditional activation of transgenic Scrt2 in cortical progenitors in mice promotes neuronal differentiation by favoring the direct mode of neurogenesis of RGPs at the onset of neurogenesis, at the expense of IP generation. Neuronal amplification via indirect IP neurogenesis is thereby extenuated, leading to a mild postnatal reduction of cortical thickness. Forced in vivo overexpression of Scrt2 suppressed the generation of IPs from RGPs and caused a delay in the radial migration of upper layer neurons toward the cortical plate. Mechanistically, our results indicate that Scrt2 negatively regulates the transcriptional activation of the basic helix loop helix TFs Ngn2/NeuroD1 on E-box containing common target genes, including Rnd2, a well-known major effector for migrational defects in developing cortex. Altogether, these findings reveal a modulatory role of Scrt2 protein in cortical neurogenesis and neuronal migration.

Programming and reprogramming neuronal subtypes in the central nervous system

Recent discoveries in nuclear reprogramming have challenged the dogma that the identity of terminally differentiated cells cannot be changed. The identification of molecular mechanisms that reprogram differentiated cells to a new identity carries profound implications for regenerative medicine across organ systems. The central nervous system (CNS) has historically been considered to be largely immutable. However, recent studies indicate that even the adult CNS is imparted with the potential to change under the appropriate stimuli. Here, we review current knowledge regarding the capability of distinct cells within the CNS to reprogram their identity and consider the role of developmental signals in directing these cell fate decisions. Finally, we discuss the progress and current challenges of using developmental signals to precisely direct the generation of individual neuronal subtypes in the postnatal CNS and in the dish.

Hierarchical clustering of gene expression patterns in the Eomes + lineage of excitatory neurons during early neocortical development

BACKGROUND

Cortical neurons display dynamic patterns of gene expression during the coincident processes of differentiation and migration through the developing cerebrum. To identify genes selectively expressed by the Eomes + (Tbr2) lineage of excitatory cortical neurons, GFP-expressing cells from Tg(Eomes::eGFP) Gsat embryos were isolated to > 99% purity and profiled.

RESULTS

We report the identification, validation and spatial grouping of genes selectively expressed within the Eomes + cortical excitatory neuron lineage during early cortical development. In these neurons 475 genes were expressed ≥ 3-fold, and 534 genes ≤ 3-fold, compared to the reference population of neuronal precursors. Of the up-regulated genes, 328 were represented at the Genepaint in situ hybridization database and 317 (97%) were validated as having spatial expression patterns consistent with the lineage of differentiating excitatory neurons. A novel approach for quantifying in situ hybridization patterns (QISP) across the cerebral wall was developed that allowed the hierarchical clustering of genes into putative co-regulated groups. Forty four candidate genes were identified that show spatial expression with Intermediate Precursor Cells, 49 candidate genes show spatial expression with Multipolar Neurons, while the remaining 224 genes achieved peak expression in the developing cortical plate.

CONCLUSIONS

This analysis of differentiating excitatory neurons revealed the expression patterns of 37 transcription factors, many chemotropic signaling molecules (including the Semaphorin, Netrin and Slit signaling pathways), and unexpected evidence for non-canonical neurotransmitter signaling and changes in mechanisms of glucose metabolism. Over half of the 317 identified genes are associated with neuronal disease making these findings a valuable resource for studies of neurological development and disease.

Tangentially migrating transient glutamatergic neurons control neurogenesis and maintenance of cerebral cortical progenitor pools

The relative contribution of intrinsic and extrinsic cues in the regulation of cortical neurogenesis remains a crucial challenge in developmental neurobiology. We previously reported that a transient population of glutamatergic neurons, the cortical plate (CP) transient neurons, migrates from the ventral pallium (VP) over long distances and participate in neocortical development. Here, we show that the genetic ablation of this population leads to a reduction in the number of cortical neurons especially fated to superficial layers. These defects result from precocious neurogenesis followed by a depletion of the progenitor pools. Notably, these changes progress from caudolateral to rostrodorsal pallial territories between E12.5 and E14.5 along the expected trajectory of the ablated cells. Conversely, we describe enhanced proliferation resulting in an increase in the number of cortical neurons in the Gsx2 mutants which present an expansion of the VP and a higher number of CP transient neurons migrating into the pallium. Our findings indicate that these neurons act to maintain the proliferative state of neocortical progenitors and delay differentiation during their migration from extraneocortical regions and, thus, participate in the extrinsic control of cortical neuronal numbers.

Nestin reporter transgene labels multiple central nervous system precursor cells

Embryonic neuroepithelia and adult subventricular zone (SVZ) stem and progenitor cells express nestin. We characterized a transgenic line that expresses enhanced green fluorescent protein (eGFP) specified to neural tissue by the second intronic enhancer of the nestin promoter that had several novel features. During embryogenesis, the dorsal telencephalon contained many and the ventral telencephalon few eGFP+ cells. eGFP+ cells were found in postnatal and adult neurogenic regions. eGFP+ cells in the SVZ expressed multiple phenotype markers, glial fibrillary acidic protein, Dlx, and neuroblast-specific molecules suggesting the transgene is expressed through the lineage. eGFP+ cell numbers increased in the SVZ after cortical injury, suggesting this line will be useful in probing postinjury neurogenesis. In non-neurogenic regions, eGFP was strongly expressed in oligodendrocyte progenitors, but not in astrocytes, even when they were reactive. This eGFP+ mouse will facilitate studies of proliferative neuroepithelia and adult neurogenesis, as well as of parenchymal oligodendrocytes.

Time course of postnatal distribution of doublecortin immunoreactive developing/maturing neurons in the somatosensory cortex and hippocampal CA1 region of C57BL/6 mice

In this study, we observed neuroblast differentiation in the somatosensory cortex (SSC) and hippocampal CA1 region (CA1), which is vulnerable to oxidative stress, of the mouse at various early postnatal days (P) 1, 7, 14, and 21 using doublecortin (DCX, a marker for neuroblasts). Cresyl violet and NeuN (Neuronal Nuclei) staining showed development of layers as well as neurons in the SSC and CA1. At P1, DCX-positive neuroblasts expressed strong DCX immunoreactivity in both the SSC and CA1. Thereafter, DCX immunoreactivity was decreased with time. At P7, many DCX-immunoreactive neuroblasts were well detected in the SSC and CA1. At P14, some DCX-positive neuroblasts were found in the SSC and CA1: The immunoreactivity was weak. At P21, DCX immunoreactivity was hardly found in cells in the SSC and CA1. These results suggest that DCX-positive neuroblasts were significantly decreased in the mouse SSC and CA1 from P14.

Morphological development of thick-tufted layer v pyramidal cells in the rat somatosensory cortex

The thick-tufted layer V pyramidal (TTL5) neuron is a key neuron providing output from the neocortex. Although it has been extensively studied, principles governing its dendritic and axonal arborization during development are still not fully quantified. Using 3-D model neurons reconstructed from biocytin-labeled cells in the rat somatosensory cortex, this study provides a detailed morphological analysis of TTL5 cells at postnatal day (P) 7, 14, 21, 36, and 60. Three developmental periods were revealed, which were characterized by distinct growing rates and properties of alterations in different compartments. From P7 to P14, almost all compartments grew fast, and filopodia-like segments along apical dendrite disappeared; From P14 to P21, the growth was localized on specified segments of each compartment, and the densities of spines and boutons were significantly increased; From P21 to P60, the number of basal dendritic segments was significantly increased at specified branch orders, and some basal and oblique dendritic segments were lengthened or thickened. Development changes were therefore seen in two modes: the fast overall growth during the first period and the slow localized growth (thickening mainly on intermediates or lengthening mainly on terminals) at the subsequent stages. The lengthening may be accompanied by the retraction on different segments. These results reveal a differential regulation in the arborization of neuronal compartments during development, supporting the notion of functional compartmental development. This quantification provides new insight into the potential value of the TTL5 morphology for information processing, and for other purposes as well.

Spred1, a negative regulator of Ras-MAPK-ERK, is enriched in CNS germinal zones, dampens NSC proliferation, and maintains ventricular zone structure

Neural stem cells (NSCs) have great potential for self-renewal, which must be tightly regulated to generate appropriate cell numbers during development and to prevent tumor formation. The Ras-MAPK-ERK pathway affects mitogen-stimulated proliferation, and negative regulators are likely to be important for keeping self-renewal in check. Sprouty-related protein with an EVH1 domain (Spred1) is a recently discovered negative Ras-MAPK-ERK regulator linked to a neurofibromatosis 1 (NF-1)-like human syndrome; however, its role in CNS development has not been explored. We show that Spred1 is highly enriched in CNS germinal zones during neurogenesis. Spred1 knockdown increases NSC self-renewal and progenitor proliferation cell-autonomously, and overexpression causes premature differentiation. Surprisingly, Spred1 knockdown in vivo in the embryonic mouse forebrain frequently resulted in periventricular heterotopia, developmental abnormalities often associated with mutations in genes in the vesicular trafficking pathway that cause disruption of germinal zones and impair cell migration. In cortical progenitor cells, Spred1 localizes within distinct vesicles, indicating a potential role in transport. Spred1 knockdown gradually leads to disruption of the apical ventricular zone and loss of radial glia alignment. This impairs late neuronal migration, resulting in the formation of periventricular masses. Thus, Spred1 is critical for normal cortical development, as it modulates progenitor self-renewal/proliferation and helps maintain the integrity and organization of germinal zones.

Early exposure to nicotine during critical periods of brain development: Mechanisms and consequences

Tobacco use during pregnancy continues to be a major problem with more than 16% of pregnant women in the United States continuing to smoke during pregnancy. Tobacco smoke is known to contain more than 4,000 different chemicals, and while many of these compounds have the potential to interfere with proper neurodevelopment, there is direct evidence that nicotine, the major psychoactive substance present in tobacco, acts as a neuroteratogen. Nicotine activates, and subsequently desensitizes, neuronal nicotinic acetylcholine receptor subtypes (AChRs), which are expressed in the developing central nervous system (CNS) prior to the in-growth of cholinergic neurons. Nicotinic AChRs are present by the first trimester of development in both humans and rodents, and activation of these receptors by acetylcholine is thought to play a critical role in CNS development. The purpose of the current review is to provide an overview of the role that nicotinic AChRs play in the developing CNS and to describe the effects of nicotine exposure during early development on neuronal cell biology, nicotinic AChR expression and neurotransmitter system (e.g., dopamine, norepinephrine, serotonin) function. In particular, differences that occur as a result of the timing and duration of nicotine exposure will be discussed. Emphasis will be placed on preclinical studies examining particular periods of time which correspond to periods of prenatal development in humans (i.e., first, second and third trimesters). Finally, the effects of early nicotine exposure on neurobehavioral development as it pertains to specific disorders, i.e., attention deficit hyperactivity disorder (ADHD), depression and addiction, will be discussed.

Selective cortical layering abnormalities and behavioral deficits in cortex-specific Pax6 knock-out mice

The transcription factor Pax6 has been implicated in neocortical neurogenesis in vertebrates, including humans. Analyses of the role of Pax6 in layer formation and cognitive abilities have been hampered by perinatal lethality of Pax6 mutants. Here, we generated viable mutants exhibiting timed, restricted inactivation of Pax6 during early and late cortical neurogenesis using Emx1-Cre and hGFAP-Cre lines, respectively. The disruption of Pax6 at the onset of neurogenesis using Emx1-Cre line resulted in premature cell cycle exit of early progenitors, increase of early born neuronal subsets located in the marginal zone and lower layers, and a nearly complete absence of upper layer neurons, especially in the rostral cortex. Furthermore, progenitors, which accumulated in the enlarged germinal neuroepithelium at the pallial/subpallial border in the Pax6 mutants, produced an excess of oligodendrocytes. The inactivation of Pax6 after generation of the lower neuronal layers using hGFAP-Cre line did not affect specification or numbers of late-born neurons, indicating that the severe reduction of upper layer neurons in Pax6 deficiency is mostly attributable to a depletion of the progenitor pool, available for late neurogenesis. We further show that Pax6(fl/fl);Emx1-Cre mutants exhibited deficiencies in sensorimotor information integration, and both hippocampus-dependent short-term and neocortex-dependent long-term memory recall. Because a majority of the morphological and behavior disabilities of the Pax6 mutant mice parallel abnormalities reported for aniridia patients, a condition caused by PAX6 haploinsufficiency, the Pax6 conditional mutant mice generated here represent a valuable genetic tool to understand how the developmental cortical disruption can lead to a human behavior abnormality.

Retardation in somatosensory cortex development induced by postnatal BrdU treatment in mice

Cerebral dysgeneses are in the background of several neurological and mental disturbances. The aim of the present study was to investigate structural and activity changes following disturbed postnatal neuronal development in mice. Newborn C57Bl6 mice were exposed to 5-bromo-2'-deoxyuridine (BrdU: daily 50 microg/g body weight) during a period between postnatal days P0-P5 or P0-P11, respectively, and neuronal malformation and malfunctioning of somatosensory (barrel field) cortex was analyzed in adolescent animals. Alterations in histological architecture of interneuronal and glial elements were studied and correlated with electrophysiological modifications. Between P30 and P35 days litters underwent ex vivo electrophysiological experiments to examine the changes in basic excitability and in synaptic efficacy. Parallel immunohistochemistry was performed to detect BrdU, GABA and GFAP. There were no BrdU immunopositive cell nuclei in control animals, but marked staining was observed in both BrdU treated groups. Lessening in the number of GABAergic neurons was observed in the treated groups. GFAP immunohistochemical analysis has shown an increased number of activated astroglial cells in treated animals. Reduction of the number of GABAergic neurons was observed in the treated groups. Electrophysiological recordings on cortical slices showed increased excitability in the treated groups.

Evidences for tangential migrations in Xenopus telencephalon: developmental patterns and cell tracking experiments

Extensive tangential cell migrations have been described in the developing mammalian, avian, and reptilian forebrain, and they are viewed as a powerful developmental mechanism to increase neuronal complexity in a given brain structure. Here, we report for the first time anatomical and cell tracking evidence for the presence of important migratory processes in the developing forebrain of the anamniote Xenopus laevis. Combining developmental gene expression patterns (Pax6, Nkx2.1, Isl1, Lhx5, Lhx9, and Dll3), neurotransmitter identity (GABA, NOS, ChAT), and connectivity information, several types of putative migratory cell populations and migration routes originating in the ventral pallium and the subpallium are proposed. By means of in vivo cell tracking experiments, pallio-subpallial and subpallio-pallial migrating neurons are visualized. Among them, populations of Nkx2.1(+) striatal interneurons and pallial GABAergic interneurons, which also express the migratory marker doublecortin, are identified. Finally, we find that these tangentially migrating pallial interneurons travel through an "isl1-free channel" that may guide their course through the subpallium. Our findings strongly suggest that the developing Xenopus telencephalon shares many similarities with amniotes in terms of neuronal specification and migrations. However, some differences are discussed, particularly with regard to the evolution of the pallium.

Constitutive EGFR signaling in oligodendrocyte progenitors leads to diffuse hyperplasia in postnatal white matter

Gliogenesis requires the careful orchestration of migration, differentiation, and proliferation of progenitors. Signaling through the epidermal growth factor receptor (EGFR) has been implicated in regulating these processes in a variety of cell types, including neural progenitors. By retroviral infection, we constitutively expressed an EGFR-GFP fusion protein in white matter glial progenitors at postnatal day 3 of the rat forebrain in vivo and analyzed the development of these cells over the subsequent 15 weeks. EGFR-GFP+ cells remained proliferative and migratory, gradually populating the brains ipsilateral and contralateral to the side of viral infection, but never differentiated into mature glia. The accumulation of these cells doubled the total cell density in white matter and led to a 10-fold increase in the abundance of glial progenitors, giving rise to a progenitor "hyperplasia." The marker profile of infected cells, NG2+, olig2+, PDGFR-alpha+, nestin+, GFAP-, and CC1-, indicated a close resemblance to oligodendrocyte progenitors. Positive immunostaining for phosphorylated EGFR colocalized with punctate accumulation of EGFR-GFP, indicating that a subset of receptors was engaged in active signaling. Furthermore, EGF was required to observe phospho-tyrosine EGFR immunostaining of glial progenitors in culture. These observations suggest that constitutive EGFR expression can inhibit glial differentiation, but requires ligand as well.

A critical function for beta-amyloid precursor protein in neuronal migration revealed by in utero RNA interference

Physiological processing of the beta-amyloid precursor protein (APP) generates amyloid beta-protein, which can assemble into oligomers that mediate synaptic failure in Alzheimer's disease. Two decades of research have led to human trials of compounds that chronically target this processing, and yet the normal function of APP in vivo remains unclear. We used the method of in utero electroporation of shRNA constructs into the developing cortex to acutely knock down APP in rodents. This approach revealed that neuronal precursor cells in embryonic cortex require APP to migrate correctly into the nascent cortical plate. cDNAs encoding human APP or its homologues, amyloid precursor-like protein 1 (APLP1) or APLP2, fully rescued the shRNA-mediated migration defect. Analysis of an array of mutations and deletions in APP revealed that both the extracellular and cytoplasmic domains of APP are required for efficient rescue. Whereas knock-down of APP inhibited cortical plate entry, overexpression of APP caused accelerated migration of cells past the cortical plate boundary, confirming that normal APP levels are required for correct neuronal migration. In addition, we found that Disabled-1 (Dab1), an adaptor protein with a well established role in cortical cell migration, acts downstream of APP for this function in cortical plate entry. We conclude that full-length APP functions as an important factor for proper migration of neuronal precursors into the cortical plate during the development of the mammalian brain.

The role of DCX and LIS1 in migration through the lateral cortical stream of developing forebrain

During forebrain development the lateral cortical stream (LCS) supplies neurons to structures in the ventral telencephalon including the amygdala and piriform cortex. In the current study, we used spatially directed in utero electroporation and RNAi to investigate mechanisms of migration to the ventral telencephalon. Cells labeled by in utero electroporation of the lateral ventricular zone migrated into the LCS, and entered the lateral neocortex, piriform cortex and amygdala, where they differentiated primarily as pyramidal neurons. RNAi of DCX or LIS1 disrupted migration into amygdala and piriform cortex and caused many neurons to accumulate in the external and amygdalar capsules. RNAi of LIS1 and DCX had similar as well as distinguishable effects on the pattern of altered migration. Combinatorial RNAi of LIS1 and DCX further suggested interaction in the functions of LIS1 and DCX on the morphology and migration of migrating neurons in the LCS. Together, these results confirm that the LCS contributes pyramidal neurons to ventral forebrain structures and reveals that DCX and LIS1 have important functions in this major migratory pathway in the developing forebrain.

A rapid screening method for population-specific neuronal motogens, substrates and associated signaling pathways

We developed and characterized an assay that allows for rapid examination of migration of specific neuronal populations within a mixed population using the Boyden chamber principle. Migration of cerebellar interneurons and granule cells was examined using mice expressing enhanced green fluorescent protein (eGFP) under the glutamate decarboxylase (GAD(65)) and growth-associated protein-43 (GAP43) promoters, respectively. Brain-derived neurotrophic factor (BDNF) was used as the prototypic motogen for both populations. Fluorescent light-blocking inserts (FluoroBlok) with different pore sizes and densities were compared in a two-compartment assay. Immunodetection of polarity markers and nuclear staining indicated that dendrites and somata are preferentially extended through the pores in response to BDNF. Inserts coated with extracellular matrix (ECM) proteins were used to examine interactions between BDNF and the ECM during migration. ECM proteins alone stimulated migration when the lower side of the insert was coated, however coating of both sides of the insert slowed migration when compared to poly-D-lysine. Addition of a PI 3-kinase inhibitor to the lower compartment blocked BDNF-stimulated migration of both populations while a Src inhibitor reduced laminin-stimulated migration of interneurons, but not granule cells. We also examined use of neurons cultured from GAD(65)-eGFP mice as a reporter system for promoter activity. GAD(65)-eGFP mice may also be useful as a model for promoter regulation and the potential confounding effects of eGFP induction by the stimuli are also addressed. This assay allows for rapid analysis of motogens, substrates and signaling pathways that regulate migration of selected neuronal populations.

Validating in utero electroporation for the rapid analysis of gene regulatory elements in the murine telencephalon

With the ultimate goal of understanding how genetic modules have evolved in the telencephalon, we set out to modernize the functional analysis of cross-species cis-regulatory elements in mouse. In utero electroporation is rapidly replacing transgenesis as the method of choice for gain- and loss-of-function studies in the murine telencephalon, but the application of this technique to the analysis of transcriptional regulation has yet to be fully explored and exploited. To empirically define the developmental stages required to target specific populations of neurons in the dorsal telencephalon, or pallium, which gives rise to the neocortex in mouse, we performed a temporal and spatial analysis of the migratory properties of electroporated versus birth-dated cells. Next, we compared the activities of two known Ngn2 enhancers via transgenesis and in utero electroporation, demonstrating that the latter technique more faithfully reports the endogenous telencephalic expression pattern observed in an Ngn2lacZ knock-in line. Finally, we used this approach to test the telencephalic activities of a series of deletion constructs comprised of the zebrafish ER81 upstream regulatory region, allowing us to identify a previously uncharacterized enhancer that displays cross-species activity in the murine piriform cortex and lateral neocortex, yet not in more medial domains of the forebrain. Taken together, our data supports the contention that in utero technology can be exploited to rapidly examine the architecture and evolution of pallial-specific cis-regulatory elements.

Differences in neurogenesis differentiate between core and shell regions of auditory nuclei in the turtle (Pelodiscus sinensis): evolutionary implications

There is a clear core-versus-shell distinction in cytoarchitecture, electrophysiological properties and neural connections in the mesencephalic and diencephalic auditory nuclei of amniotes. Determining whether the embryogenesis of auditory nuclei shows a similar organization is helpful for further understanding the constituent organization and evolution of auditory nuclei. Therefore in the present study, we injected [(3)H]-thymidine into turtle embryos (Pelodiscus sinensis) at various stages of development. Upon hatching, [(3)H]-thymidine labeling was examined in both the core and shell auditory regions in the midbrain, diencephalon and dorsal ventricular ridge. Met-enkephalin and substance P immunohistochemistry was used to distinguish the core and shell regions. In the mesencephalic auditory nucleus, the occurrence of heavily labeled neurons in the nucleus centralis of the torus semicircularis reached its peak at embryonic day 9, one day later than the surrounding shell. In the diencephalic auditory nucleus, the production of heavily labeled neurons in the central region of the reuniens (Re) was highest at embryonic day (E) 8, one day later than that in the shell region of reuniens. In the region of the dorsal ventricular ridge that received inputs from the central region of Re, the appearance of heavily labeled neurons also reached a peak one day later than that in the area receiving inputs from the shell region of reuniens. Thus, there is a core-versus-shell organization of neuronal generation in reptilian auditory areas.

Involvement of the myelin-associated inhibitor Nogo-A in early cortical development and neuronal maturation

Nogo-A is a myelin-associated protein expressed by neurons and myelinating mature oligodendrocytes in the central nervous system. Although most research has focused on the participation of Nogo-A in the prevention of axonal regeneration and plasticity in the adult, little attention has been paid to the putative functions of Nogo-A during embryonic development. Here we examined the general pattern and cell-specific distribution of Nogo-A in the prenatal mouse telencephalon. In addition, we studied the development of the major axon tracts and radial and tangential migration in Nogo-A/B/C knockout mice. The pattern of Nogo-A showed distinct distribution in radial glia and postmitotic neurons, in which it is particularly enriched in developing axons. Similarly, Nogo-A was enriched at the leading process of tangentially migrating interneurons but not detectable in radial migrating neurons. Although a low level of Nogo-A appears to be on the surface of many cortical neurons, most proteins have intracellular localization. In Nogo-deficient background, neurons displayed early polarization and increased branching in vitro, probably reflecting a cell-intrinsic role of Nogo proteins in branching reduction, and early tangential migration was delayed. On the basis of these observations, we propose that Nogo proteins, particularly Nogo-A, are involved in multiple processes during cortical development.

Embryonic and early postnatal abnormalities contributing to the development of hippocampal malformations in a rodent model of dysplasia

While there are many recent examples of single gene deletions that lead to defects in cortical development, most human cases of cortical disorganization can be attributed to a combination of environmental and genetic factors. Elucidating the cellular or developmental basis of teratogenic exposures in experimental animals is an important approach to understanding how environmental insults at particular developmental junctures can lead to complex brain malformations. Rats with prenatal exposure to methylazoxymethanol (MAM) reproduce many anatomical features seen in epilepsy patients. Previous studies have shown that heterotopic clusters of neocortically derived neurons exhibit hyperexcitable firing activity and may be a source of heightened seizure susceptibility; however, the events that lead to the formation of these abnormal cell clusters is unclear. Here we used a panel of molecular markers and birthdating studies to show that in MAM-exposed rats the abnormal cell clusters (heterotopia) first appear postnatally in the hippocampus (P1-2) and that their appearance is preceded by a distinct sequence of perturbations in neocortical development: 1) disruption of the radial glial scaffolding with premature astroglial differentiation, and 2) thickening of the marginal zone with redistribution of Cajal-Retzius neurons to deeper layers. These initial events are followed by disruption of the cortical plate and appearance of subventricular zone nodules. Finally, we observed the erosion of neocortical subventricular zone nodules into the hippocampus around parturition followed by migration of nodules to hippocampus. We conclude that prenatal MAM exposure disrupts critical developmental processes and prenatal neocortical structures, ultimately resulting in neocortical disorganization and hippocampal malformations.

Decreased adult hippocampal neurogenesis in the PDAPP mouse model of Alzheimer's disease

Abnormal subgranular zone (SGZ) neurogenesis is proposed to contribute to Alzheimer's disease (AD)-related decreases in hippocampal function. Our goal was to examine hippocampal neurogenesis in the PDAPP mouse, a model of AD with age-dependent accumulation of amyloid-beta(42) (Abeta(42))-containing plaques that is well studied with regard to AD therapies. A secondary goal was to determine whether altered neurogenesis in the PDAPP mouse is associated with abnormal maturation or number of mature cells. A tertiary goal was to provide insight into why hippocampal neurogenesis appears to be increased in AD post-mortem tissue and decreased in most AD mouse models. We report an age-dependent decrease in SGZ proliferation in homozygous PDAPP mice. At 1 year of age, PDAPP mice also had new dentate gyrus granule neurons with abnormal maturation and fewer dying cells relative to control mice. In contrast to decreased SGZ cell birth, PDAPP mice had increased birth of immature neurons in the outer portion of the granule cell layer (oGCL), providing insight into why some studies link AD with increased neurogenesis. However, these ectopic oGCL cells were still rare compared with SGZ proliferating cells, emphasizing that the primary characteristic of PDAPP mice is decreased neurogenesis. The decrease in SGZ neurogenesis was not associated with an age-dependent loss of dentate granule neurons. The altered neurogenesis in the PDAPP mouse may contribute to the age-related cognitive deficits reported in this model of AD and may be a useful adjunct target for assessing the impact of AD therapies.

Massive loss of Cajal-Retzius cells does not disrupt neocortical layer order

Cajal-Retzius (CR) cells, the predominant source of reelin in developing neocortex, are thought to be essential for the inside out formation of neocortical layers. Fate mapping revealed that a large population of neocortical CR cells arises from the cortical hem. To investigate the function of CR cells, we therefore genetically ablated the hem. Neocortical CR cells were distributed beneath the pial surface in control mice, but were virtually absent in hem-ablated mice from embryonic day (E) 10.5 until birth. CR cells derived from other sources did not invade the neocortical primordium to compensate for hem loss. We predicted that neocortical layers would be inverted in hem-ablated animals, as in reeler mice, deficient in reelin signaling. Against expectation, layers showed the standard order. Low levels of reelin in the cortical primordium, or diffusion of reelin from other sites, may have allowed lamination to proceed. Our findings indicate, however, that the sheet of reelin-rich CR cells that covers the neocortical primordium is not required to direct layer order.

Selective requirement of Pax6, but not Emx2, in the specification and development of several nuclei of the amygdaloid complex

The amygdaloid complex is a group of nuclei that are thought to originate from multiple sites of the dorsal and ventral telencephalic neuroepithelium. The mechanisms that regulate their development are essentially unknown. We studied the role of Pax6 and Emx2, two transcription factors that regulate regional specification and growth of the telencephalon, in the morphogenesis of the amygdaloid complex. We used a set of specific marker genes that identify distinct amygdaloid nuclei to analyze Pax6/Small eye and Emx2 knock-out mutant mouse brains. We found that there is a selective requirement for Pax6, but not Emx2, in the formation a subset of nuclei within the amygdaloid complex. Specifically, structures that were not previously considered to be developmentally linked, the nucleus of the lateral olfactory tract and the lateral, basolateral, and basomedial nuclei, all appear to have a common requirement for Pax6. Together, our findings provide new insights into the origins and mechanisms underlying the development of the amygdaloid complex.

Neurogenesis and neuroprotection induced by peripheral immunomodulatory treatment of experimental autoimmune encephalomyelitis

Brain insults such as the autoimmune inflammatory process in multiple sclerosis (MS) and experimental autoimmune encephalomyelitis (EAE) induce a measure of neurogenesis, but its regenerative therapeutic consequence is limited, because it fails to regenerate functional neurons and compensate the damage. Here, we investigated whether peripheral immunomodulatory treatment for MS/EAE, glatiramer acetate (GA), can enhance neurogenesis and generate neuroprotection in the CNS of EAE-inflicted mice. EAE was induced by myelin oligodendrocyte glycoprotein peptide, either in yellow fluorescent protein (YFP) 2.2 transgenic mice, which selectively express YFP on their neuronal population, or in C57BL/6 mice. The in situ effect of GA was studied in various brain regions; neuroprotection and neurogeneration were evaluated and quantified by measuring the expression of different neuronal antigens and in vivo proliferation markers. The results demonstrated that in EAE-inflicted mice, neuroproliferation was initially elevated after disease appearance but subsequently declined below that of naive mice. In contrast, GA treatment in various stages of the disease led to sustained reduction in the neuronal/axonal damage typical to the neurodegenerative disease course. Moreover, three processes characteristic of neurogenesis, namely cell proliferation, migration, and differentiation, were augmented and extended by GA treatment in EAE mice compared with EAE-untreated mice and naive controls. The newborn neuroprogenitors manifested massive migration through exciting and dormant migration pathways, into injury sites in brain regions, which do not normally undergo neurogenesis, and differentiated to mature neuronal phenotype. This suggests a direct linkage between immunomodulation, neurogenesis, and an in situ therapeutic consequence in the CNS.

Embryogenesis of the rat telencephalon--a morphologic and stereologic analysis

Comparative researches of borderline between telencephalon neuroepithelium and its surrounding mesenchyme in successive early developing stages lack in literature. The aim of this investigation was to carry out systematic morphologic and stereologic analyses of rat telencephalon in early developmental stages. We analysed semithin (1 microm) serial sections of rat embryonic brain from the 12th (E12) to the 15th (E15) day of gestation. The surface densities (SV) of an external mesenchymal surface, an internal mesenchymal surface and a neuroepithelial (ventricular) surface were examined stereologically and compared. The surface density of the external mesenchymal surface was the biggest at E12 (4,05 mm-1) and the least at E15 (1,87 mm-1)-p<0,0005. The surface density of the internal mesenchymal surface was the biggest at E12 (4,02 mm-1) and the least at E15 (2,69 mm-1)-p<0,0005. The surface density of the internal neuroepithelial surface was the biggest at E12 (3,31 mm-1) and the least at E15 (2,01 mm-1)-p<<0,0005. Our stereological examines give objective numerical proof of significant morphogenetic changes in telencephalon shape described by morphologic analyses. The major advantage of stereological methods is the possibility to carry out the estimation procedures in specified, well-defined brain regions or layers.

Ontogenesis of the sulfhydryl oxidase QSOX expression in rat brain

The spatiotemporal pattern of distribution of the sulfhydryl oxidase QSOX throughout ontogeny was mapped in rat brain using immunohistochemistry. The enzyme was detected on embryonic day (E) 12 in the dawning mantle layer, but the adult-like pattern was acquired postnatally around day 30 (P30). Throughout ontogenesis, rQSOX was detected in immature and mature neurons, but not in glial cells. The rQSOX developmental pattern can be divided into four periods: on E12 the enzyme was detected in the brainstem, more precisely in motoneurons; later (E16), rQSOX-positive cells were also observed in the forebrain, in the caudoputamen, and the subventricular zone. During late embryogenesis (E18-20), the amount of rQSOX cells considerably increased throughout the brain; they initially appeared in the hippocampus, then in the isocortex. From birth onwards, complex modifications of the rQSOX distribution occurred leading to the adult pattern by P30. Although rQSOX exhibits an overall increasing spatiotemporal pattern of distribution, different expression strategies were distinguished depending on the cell type or brain area. By comparing the rQSOX ontogeny with data on neurogenesis and brain histogenesis, we hypothesize that the enzyme could play a role in guiding migrating cells, their settling, and neuronal maturation, e.g., during outgrowth and synaptogenesis.