The Reelin signaling pathway promotes dendritic spine development in hippocampal neurons

Dab2IP GTPase activating protein regulates dendrite development and synapse number in cerebellum

DOC-2/DAB-2 interacting protein (Dab2IP) is a GTPase activating protein that binds to Disabled-1, a cytosolic adapter protein involved in Reelin signaling and brain development. Dab2IP regulates PI3K-AKT signaling and is associated with metastatic prostate cancer, abdominal aortic aneurysms and coronary heart disease. To date, the physiological function of Dab2IP in the nervous system, where it is highly expressed, is relatively unknown. In this study, we generated a mouse model with a targeted disruption of Dab2IP using a retrovirus gene trap strategy. Unlike reeler mice, Dab2IP knock-down mice did not exhibit severe ataxia or cerebellar hypoplasia. However, Dab2IP deficiency produced a number of cerebellar abnormalities such as a delay in the development of Purkinje cell (PC) dendrites, a decrease in the parallel fiber synaptic marker VGluT1, and an increase in the climbing fiber synaptic marker VGluT2. These findings demonstrate for the first time that Dab2IP plays an important role in dendrite development and regulates the number of synapses in the cerebellum.

Altered GABAergic and glutamatergic activity within the rat hippocampus and amygdala in rats subjected to repeated corticosterone administration but not restraint stress

We investigated the effect of two well characterized preclinical animal models of depression - repeated injections of corticosterone (CORT) and repeated restraint stress - on markers of GABAergic and glutamatergic activity in the hippocampus and amygdala. Stress is an identified risk factor for the onset of major depression, but the neurobiological mechanisms by which stress may produce depressogenic effects are not clear. Rats received one of the following four treatments for 21 consecutive days: daily single CORT injections (40mg/kg), daily single vehicle injections, daily 6h of restraint stress, or daily handling. After the 21-day stress period, all rats were sacrificed and hippocampal and amygdalar tissue was collected and prepared for Western blot analyses. We examined the effect of CORT and restraint stress on glutamate decarboxylase (GAD)-65 and GAD67, as well as the α1, α2, α3, and β2-3 GABA(A) receptor subunits, and the vesicular glutamate transporter (VGLUT)-2. We found that CORT significantly decreased GAD65 and the α2 receptor subunit and increased VGLUT2 within the hippocampus. We also found that CORT decreased GAD67 and the α2 receptor subunit in the amygdala. However, restraint stress had no significant effect on protein expression in either the hippocampus or the amygdala. These findings parallel our previous results showing that repeated CORT injections, but not restraint stress, increase depression-like behavior in rats, and suggest that the depressogenic effects of CORT may be related to alterations in GABAergic and glutamatergic neurotransmission in stress-sensitive regions of the brain.

Extrusion of misfolded and aggregated proteins--a protective strategy of aging neurons?

Cellular senescence is the consequence of repetitive exposures to oxidative stress, perturbed energy homeostasis, accumulation of damaged proteins and lesions in their nucleic acids. Whereas mitotic cells are equipped with efficient cell replacement strategies; postmitotic neurons have--with a few exceptions--no mechanism to substitute dysfunctional cells within a complex neuronal network. Here we propose a potential strategy by which aging neurons contend against abnormal accumulation of damaged/misfolded proteins. The suggested mechanism involves the formation of 'budding-like' extrusions and their subsequent clearance by glia. This hypothesis emerged from our previous investigations of the aged hippocampus revealing layer-specific accumulations of Reelin, a glycoprotein with fundamental roles during brain development and adult synaptic plasticity. We showed that Reelin deposits constitute a conserved neuropathological feature of aging, which is significantly accelerated in adult wild-type mice prenatally exposed to a viral-like infection. Here, we employed two- and three-dimensional immunoelectron microscopy to elucidate their morphological properties, localization and origin in immune challenged vs. control mice. In controls, Reelin-positive deposits were dispersed in the neuropil, some being engulfed by glia. In immune challenged mice, however, significantly more Reelin-immunoreactive deposits were associated with neuritic swellings containing mitochondria, vacuoles and cellular debris, pointing to their intracellular origin and suggesting that 'budding-like' neuronal extrusions of misfolded proteins and glial clearance may represent a protective strategy to counteract aging-associated impairments in proteosomal/lysosomal degradation. Neurons exposed to chronic neuroinflammation with increased levels of misfolded/damaged proteins, however, may fail to combat intraneuronal protein accumulations, a process probably underlying neuronal dysfunction and degeneration during aging.

Reelin demarcates a subset of pre-Bötzinger complex neurons in adult rat

Identification of two markers of neurons in the pre-Bötzinger complex (pre-BötC), the neurokinin 1 receptor (NK1R) and somatostatin (Sst) peptide, has been of great utility in understanding the essential role of the pre-BötC in breathing. Recently, the transcription factor dbx1 was identified as a critical, but transient, determinant of glutamatergic pre-BötC neurons. Here, to identify additional markers, we constructed and screened a single-cell subtractive cDNA library from pre-BötC inspiratory neurons. We identified the glycoprotein reelin as a potentially useful marker, because it is expressed in distinct populations of pre-BötC and inspiratory bulbospinal ventral respiratory group (ibsVRG) neurons. Reelin ibsVRG neurons were larger (27.1 ± 3.8 μm in diameter) and located more caudally (>12.8 mm caudal to Bregma) than reelin pre-BötC neurons (15.5 ± 2.4 μm in diameter, <12.8 mm rostral to Bregma). Pre-BötC reelin neurons coexpress NK1R and Sst. Reelin neurons were also found in the parahypoglossal and dorsal parafacial regions, pontine respiratory group, and ventromedial medulla. Reelin-deficient (Reeler) mice exhibited impaired respones to hypoxia compared with littermate controls. We suggest that reelin is a useful molecular marker for pre-BötC neurons in adult rodents and may play a functional role in pre-BötC microcircuits.

Extracellular matrix molecules, their receptors, and secreted proteases in synaptic plasticity

Neural cells secrete diverse molecules, which accumulate in the extracellular space and form the extracellular matrix (ECM). Interactions between cells and the ECM are well recognized to play the crucial role in cell migration and guidance of growing axons, whereas formation of mature neural ECM in the form of perineuronal nets is believed to restrict certain forms of developmental plasticity. On the other hand, major components of perineuronal nets and other ECM molecules support induction of functional plasticity, the most studied form of which is long-term potentiation. Here, we review the underlying mechanisms by which ECM molecules, their receptors and remodeling proteases regulate the induction and maintenance of synaptic modifications. In particular, we highlight that activity-dependent secretion and activation of proteases leads to a local cleavage of the ECM and release of signaling proteolytic fragments. These molecules regulate transmitter receptor trafficking, actin cytoskeleton, growth of dendritic spines, and formation of dendritic filopodia.

F-spondin gene transfer improves memory performance and reduces amyloid-β levels in mice

Alzheimer's disease (AD) is the most prevalent form of dementia affecting the elderly. Evidence has emerged signifying that stimulation of the reelin pathway should promote neural plasticity and suppress molecular changes associated with AD, suggesting a potential therapeutic application to the disease. This was explored through the use of lentiviral vector-mediated overexpression of the reelin homolog, F-spondin, which is an activator of the reelin pathway. Intrahippocampal gene transfer of F-spondin improved spatial learning/memory in the Morris Water Maze and increased exploration of the novel object in the Novel Object Recognition test in wild-type mice. F-spondin overexpression also suppressed endogenous levels of amyloid beta (Aβ(42)) in these mice and reduced Aβ plaque deposition while improving synaptophysin expression in transgenic mouse models of AD. These data demonstrate pathologic and cognitive improvements in mice through F-spondin overexpression.

LRP-1 and LRP-2 receptors function in the membrane neuron. Trafficking mechanisms and proteolytic processing in Alzheimer's disease

Low density lipoprotein receptor-related protein (LRP) belongs to the low-density lipoprotein receptor family, generally recognized as cell surface endocytic receptors, which bind and internalize extracellular ligands for degradation in lysosomes. Neurons require cholesterol to function and keep the membrane rafts stable. Cholesterol uptake into the neuron is carried out by ApoE via LRPs receptors on the cell surface. In neurons the most important are LRP-1 and LRP-2, even it is thought that a causal factor in Alzheimer's disease (AD) is the malfunction of this process which cause impairment intracellular signaling as well as storage and/or release of nutrients and toxic compounds. Both receptors are multifunctional cell surface receptors that are widely expressed in several tissues including neurons and astrocytes. LRPs are constituted by an intracellular (ICD) and extracellular domain (ECD). Through its ECD, LRPs bind at least 40 different ligands ranging from lipoprotein and protease inhibitor complex to growth factors and extracellular matrix proteins. These receptors has also been shown to interact with scaffolding and signaling proteins via its ICD in a phosphorylation-dependent manner and to function as a co-receptor partnering with other cell surface or integral membrane proteins. Thus, LRPs are implicated in two major physiological processes: endocytosis and regulation of signaling pathways, which are both involved in diverse biological roles including lipid metabolism, cell growth processes, degradation of proteases, and tissue invasion. Interestingly, LRPs were also localized in neurons in different stages, suggesting that both receptors could be implicated in signal transduction during embryonic development, neuronal outgrowth or in the pathogenesis of AD.

Corticosterone regulates the expression of neuropeptide Y and reelin in MLO-Y4 cells

Osteocytes that have a dendritic appearance are widely believed to form a complex cellular network system and play crucial roles in mechanotransduction as a principal bone mechanosensor, which is the basis of their neuronallike biology, as previously reported. Neuropeptide Y (NPY) and reelin mRNA, which are brain-specific neurogenic markers, have been identified in osteocytes. However, changes in the production of NPY and reelin in response to specific biochemical stimulation are unknown. In this study, we investigated the in vitro effect of corticosterone, one of the endogenous glucocorticoids, on the expression of NPY and reelin in the MLO-Y4 osteocyte cell line. Cells were treated with corticosterone at different concentrations (10(-9) M-10(-5) M) for 1, 3, 6, 12 and 24 h. As revealed, corticosterone reduced the MLO-Y4 cell viability and proliferation in a dose- and time-dependent manner based on an MTT assay and a Vi-CELL analyzer. The cells were then incubated with corticosterone (10(-6) μM), and the NPY and reelin expression levels were detected at 1, 3, 6, 12 and 24 h using real-time PCR and Western blot analysis. These results demonstrated that at the gene and the protein levels, corticosterone significantly upregulated the NPY and reelin expression in a time-dependent manner. The application of a glucocorticoid receptor antagonist, RU486, reversed the reduced cell viability and the increased expression of NPY and reelin that were caused by corticosterone. To the best of our knowledge, this is the first report to verify that corticosterone regulates the NPY and reelin expression in osteocytes.

mGlu1α-dependent recruitment of excitatory GABAergic input to neocortical Cajal-Retzius cells

Cajal-Retzius cells are thought to play an important role for cortical development, and receive primarily spontaneous GABAergic input mediated by GABA(A) receptors. However, neither the effects of synaptically-released GABA on their excitability nor the cellular source(s) of spontaneous GABAergic currents have been yet determined. By directly recording electrophysiological responses from identified Cajal-Retzius cells of the CXCR4-EGFP mouse, we show that GABAergic input can trigger supra-threshold responses, and that the pharmacological activation of mGlu1α receptors with the group I agonist DHPG powerfully increases the frequency of spontaneous GABAergic currents. These effects appeared mediated by a network mechanism, because responses to DHPG were completely prevented both by surgical disconnection of layer I from lower layers and by exposure of slices to TTX. We propose that the cellular source underlying the observed effect of DHPG are layer I-targeting Martinotti-like interneurons, which we show express functional group I mGluRs and respond to DHPG with supra-threshold depolarization already at early developmental stages. In conclusion, our work suggests that conditions of enhanced glutamate release may be critical at early developmental stages for the recruitment of an mGlu1α-dependent micro-circuit, which then leads to the activation of Cajal-Retzius cells.

Decreased reelin expression and organophosphate pesticide exposure alters mouse behaviour and brain morphology

Genetic and environmental factors are both likely to contribute to neurodevelopmental disorders, including ASDs (autism spectrum disorders). In this study, we examined the combinatorial effect of two factors thought to be involved in autism – reduction in the expression of the extracellular matrix protein reelin and prenatal exposure to an organophosphate pesticide, CPO (chlorpyrifos oxon). Mice with reduced reelin expression or prenatal exposure to CPO exhibited subtle changes in ultrasound vocalization, open field behaviour, social interaction and repetitive behaviour. Paradoxically, mice exposed to both variables often exhibited a mitigation of abnormal behaviours, rather than increased behavioural abnormalities as expected. We identified specific differences in males and females in response to both of these variables. In addition to behavioural abnormalities, we identified anatomical alterations in the olfactory bulb, piriform cortex, hippocampus and cerebellum. As with our behavioural studies, anatomical alterations appeared to be ameliorated in the presence of both variables. While these observations support an interaction between loss of reelin expression and CPO exposure, our results suggest a complexity to this interaction beyond an additive effect of individual phenotypes.

Reelin modulates cytoskeletal organization by regulating Rho GTPases

The correct positioning of postmitotic neurons in the developing neocortex and other laminated brain structures requires the activation of a Reelin-lipoprotein receptor-Dab1 signaling cascade. The large glycoprotein Reelin is secreted by Cajal-Retzius pioneer neurons and bound by the apolipoprotein E receptor family members Apoer2 and Vldl receptor on responsive neurons and radial glia. This leads to the tyrosine phosphorylation of the cytoplasmic protein Disabled-1 (Dab1) by non-receptor tyrosine kinases of the Src family. Various signaling pathways downstream of Dab1 connect Reelin to the actin and microtubule cytoskeleton. Despite this knowledge, a comprehensive view linking the different cell-biological and biochemical actions of Reelin to its diverse physiological roles not only during neurodevelopment but also in the maintenance and functioning of the adult brain is still lacking. In this review, we discuss our finding that Reelin activates Rho GTPases in neurons in the light of other recent studies, which demonstrate a role of Reelin in Golgi organization, and suggest additional roles of Cdc42 activation by Reelin in radial glial cells of the developing cortex.

Influence of matrix metalloproteinase MMP-9 on dendritic spine morphology

An increasing body of data has shown that matrix metalloproteinase-9 (MMP-9), an extracellularly acting, Zn(2+)-dependent endopeptidase, is important not only for pathologies of the central nervous system but also for neuronal plasticity. Here, we use three independent experimental models to show that enzymatic activity of MMP-9 causes elongation and thinning of dendritic spines in the hippocampal neurons. These models are: a recently developed transgenic rat overexpressing autoactivating MMP-9, dissociated neuronal cultures, and organotypic neuronal cultures treated with recombinant autoactivating MMP-9. This dendritic effect is mediated by integrin β1 signalling. MMP-9 treatment also produces a change in the decay time of miniature synaptic currents; however, it does not change the abundance and localization of synaptic markers in dendritic protrusions. Our results, considered together with several recent studies, strongly imply that MMP-9 is functionally involved in synaptic remodelling.

Interference with reelin signaling in the lateral entorhinal cortex impairs spatial memory

Entorhinal neurons receive extensive intracortical projections, and form the primary input to the hippocampus via the perforant pathway. The glutamatergic cells of origin for the perforant pathway are distinguished by their expression of reelin, a glycoprotein involved in learning and synaptic plasticity. The functional significance of reelin signaling within the entorhinal cortex, however, remains unexplored. To determine whether interrupting entorhinal reelin signaling might have consequences for learning and memory, we administered recombinant receptor-associated protein (RAP) into the lateral entorhinal cortex (LEC) of young Long-Evans rats. RAP prevents reelin from binding to its receptors, and we verified the knockdown of reelin signaling by quantifying the phosphorylation state of reelin's intracellular signaling target, disabled-1 (DAB1). Effective knockdown of reelin signaling was associated with impaired performance in the hippocampus-dependent version of the water maze. Moreover, inhibition of reelin signaling induced a localized loss of synaptic marker expression in the LEC. These observations support a role for entorhinal reelin signaling in spatial learning, and suggest that an intact reelin signaling pathway is essential for synaptic integrity in the adult entorhinal cortex.

Rare de novo variants associated with autism implicate a large functional network of genes involved in formation and function of synapses

Identification of complex molecular networks underlying common human phenotypes is a major challenge of modern genetics. In this study, we develop a method for network-based analysis of genetic associations (NETBAG). We use NETBAG to identify a large biological network of genes affected by rare de novo CNVs in autism. The genes forming the network are primarily related to synapse development, axon targeting, and neuron motility. The identified network is strongly related to genes previously implicated in autism and intellectual disability phenotypes. Our results are also consistent with the hypothesis that significantly stronger functional perturbations are required to trigger the autistic phenotype in females compared to males. Overall, the presented analysis of de novo variants supports the hypothesis that perturbed synaptogenesis is at the heart of autism. More generally, our study provides proof of the principle that networks underlying complex human phenotypes can be identified by a network-based functional analysis of rare genetic variants.

Reelin supplementation enhances cognitive ability, synaptic plasticity, and dendritic spine density

Apolipoprotein receptors belong to an evolutionarily conserved surface receptor family that has intimate roles in the modulation of synaptic plasticity and is necessary for proper hippocampal-dependent memory formation. The known lipoprotein receptor ligand Reelin is important for normal synaptic plasticity, dendritic morphology, and cognitive function; however, the in vivo effect of enhanced Reelin signaling on cognitive function and synaptic plasticity in wild-type mice is unknown. The present studies test the hypothesis that in vivo enhancement of Reelin signaling can alter synaptic plasticity and ultimately influence processes of learning and memory. Purified recombinant Reelin was injected bilaterally into the ventricles of wild-type mice. We demonstrate that a single in vivo injection of Reelin increased activation of adaptor protein Disabled-1 and cAMP-response element binding protein after 15 min. These changes correlated with increased dendritic spine density, increased hippocampal CA1 long-term potentiation (LTP), and enhanced performance in associative and spatial learning and memory. The present study suggests that an acute elevation of in vivo Reelin can have long-term effects on synaptic function and cognitive ability in wild-type mice.

Development of patient-specific neurons in schizophrenia using induced pluripotent stem cells

Induced pluripotent stem cell (iPSC) technology has the potential to transform regenerative medicine. It also offers a powerful tool for establishing in vitro models of disease, in particular, for neuropsychiatric disorders where live human neurons are essentially impossible to procure. Using iPSCs derived from three schizophrenia (SZ) patients, one of whom has 22q11.2del (velocardiofacial syndrome; VCFS), the authors developed a culture system to study SZ on a molecular and cellular level. SZ iPSCs were differentiated into functional, primarily glutamatergic neurons that were able to fire action potentials after ∼8 weeks in culture. Early differentiating neurons expressed a number of transcription factors/chromatin remodeling proteins and synaptic proteins relevant to SZ pathogenesis, including ZNF804A, RELN, CNTNAP2, CTNNA2, SMARCA2, and NRXN1. Although a small number of lines were developed in this preliminary study, the SZ line containing 22q11.2del showed a significant delay in the reduction of endogenous OCT4 and NANOG expression that normally occurs during differentiation. Constitutive expression of OCT4 has been observed in Dgcr8-deficient mouse embryonic stem cells (mESCs); DGCR8 maps to the 22q11.2-deleted region. These findings demonstrate that the method of inducing neural differentiation employed is useful for disease modeling in SZ and that the transition of iPSCs with 22q11.2 deletions towards a differentiated state may be marked by subtle changes in expression of pluripotency-associated genes.

The diffusional properties of dendrites depend on the density of dendritic spines

We combined computational modeling and experimental measurements to determine the influence of dendritic structure on the diffusion of intracellular chemical signals in mouse cerebellar Purkinje cells and hippocamal CA1 pyramidal cells. Modeling predicts that molecular trapping by dendritic spines causes diffusion along spiny dendrites to be anomalous and that the value of the anomalous exponent (d(w) ) is proportional to spine density in both cell types. To test these predictions we combined the local photorelease of an inert dye, rhodamine dextran, with two-photon fluorescence imaging to track diffusion along dendrites. Our results show that anomalous diffusion is present in spiny dendrites of both cell types. Further, the anomalous exponent is linearly related to the density of spines in pyramidal cells and d(w) in Purkinje cells is consistent with such a relationship. We conclude that anomalous diffusion occurs in the dendrites of multiple types of neurons. Because spine density is dynamic and depends on neuronal activity, the degree of anomalous diffusion induced by spines can dynamically regulate the movement of molecules along dendrites.

Gender-specific effect of Mthfr genotype and neonatal vigabatrin interaction on synaptic proteins in mouse cortex

The enzyme methylenetetrahydrofolate reductase (MTHFR) is a part of the homocysteine and folate metabolic pathways, affecting the methylations of DNA, RNA, and proteins. Mthfr deficiency was reported as a risk factor for neurodevelopmental disorders such as autism spectrum disorder and schizophrenia. Neonatal disruption of the GABAergic system is also associated with behavioral outcomes. The interaction between the epigenetic influence of Mthfr deficiency and neonatal exposure to the GABA potentiating drug vigabatrin (GVG) in mice has been shown to have gender-dependent effects on mice anxiety and to have memory impairment effects in a gender-independent manner. Here we show that Mthfr deficiency interacts with neonatal GABA potentiation to alter social behavior in female, but not male, mice. This impairment was associated with a gender-dependent enhancement of proteins implicated in excitatory synapse plasticity in the female cortex. Reelin and fragile X mental retardation 1 protein (FMRP) levels and membrane GluR1/GluR2 ratios were elevated in wild-type mice treated neonatally with GVG and in Mthfr+/- mice treated with saline, but not in Mthfr+/- mice treated with GVG, compared with control groups (wild type treated with saline). A minor influence on the levels of these proteins was observed in male mice cortices, possibly due to high basal protein levels. Interaction between gender, genotype, and treatment was also observed in the GABA pathway. In female mice, GABA Aα2/gephyrin ratios were suppressed in all test groups; in male mice, a genotype-specific enhancement of GABA Aα2/gephyrin was observed. The lack of an effect on either reln or Fmr1 transcription suggests post-transcriptional regulation of these genes. Taken together, these findings suggest that Mthfr deficiency may interact with neonatal GABA potentiation in a gender-dependent manner to interrupt synaptic function. This may illustrate a possible mechanism for the epigenetic involvement of Mthfr deficiency in neurodevelopmental disorders.

Protein interactome reveals converging molecular pathways among autism disorders

To uncover shared pathogenic mechanisms among the highly heterogeneous autism spectrum disorders (ASDs), we developed a protein interaction network that identified hundreds of new interactions among proteins encoded by ASD-associated genes. We discovered unexpectedly high connectivity between SHANK and TSC1, previously implicated in syndromic autism, suggesting that common molecular pathways underlie autistic phenotypes in distinct syndromes. ASD patients were more likely to harbor copy number variations that encompass network genes than were control subjects. We also identified, in patients with idiopathic ASD, three de novo lesions (deletions in 16q23.3 and 15q22 and one duplication in Xq28) that involve three network genes (NECAB2, PKM2, and FLNA). The protein interaction network thus provides a framework for identifying causes of idiopathic autism and for understanding molecular pathways that underpin both syndromic and idiopathic ASDs.

Reelin deficiency causes specific defects in the molecular composition of the synapses in the adult brain

The extracellular protein Reelin regulates radial neuronal migration in the embryonic brain, promotes dendrite outgrowth in the developing postnatal forebrain, and strengthens synaptic transmission in the adult brain. Heterozygous reeler mice expressing reduced levels of Reelin are grossly normal but exhibit behavioral and physiological abnormalities. We previously demonstrated that dendritic spine density is reduced in the developing hippocampus of these mice. In this study, we investigated the consequence of Reelin deficiency on synapse formation in adult heterozygous reeler mice using imaging and biochemical approaches. Using a reeler colony that expresses yellow fluorescent protein in selected neurons, we analyzed spine density in hippocampal area CA1 by confocal microscopy and found modest abnormalities in heterozygous reeler mice. However, biochemical analysis of synaptic composition revealed specific postsynaptic defects in scaffolding proteins, neurotransmitter receptors, and signaling proteins. Using whole brain homogenates and purified pre- and postsynaptic fractions, we found that the defects were localized to the postsynaptic compartment of heterozygous reeler synapses. Decreased levels of postsynaptic density-95 (PSD-95), the N-methyl d-aspartate (NMDA) receptor subunits NR2A and NR2B, and the phosphatase PTEN were found specifically in the postsynaptic density fraction obtained from these mice. Furthermore, we found that PSD-95, NR2A, and PTEN interact with each other at the synapse. Finally, we show that levels of NR2A are reduced in conditional Pten knock out mice, demonstrating that the PTEN phosphatase regulates NMDA receptor expression at the synapse in vivo. These studies may provide insights into the etiology of cognitive disorders associated with deficiencies in Reelin signaling and PTEN dysfunction.

Cellular and laminar expression of Dab-1 during the postnatal critical period in cat visual cortex and the effects of dark rearing

This study describes postnatal critical period changes in cellular and laminar expression of Dab-1, a gene shown to play a role in controlling neuronal positioning during embryonic brain development, in cat visual cortex and the effects of dark rearing (DR). At 1week, there is dense cellular staining which is uniform across cortical layers and very light neuropil staining. At the peak of the critical period (5weeks), dense cell staining is largely restricted to large pyramidal cells of deep layer III and layer V, there is faint cell body staining throughout all cortical layers, neuropil staining is markedly increased and uniform in layers III to VI. This dramatic change in laminar and cellular labeling is independent of visual input, since immunostaining is similar in 5-week DR cats. By 10weeks, the mature laminar and cellular staining pattern is established and the major subsequent change is a further reduction in the density of cellular staining in all cortical layers. Neuropil staining is pronounced and uniform across cortical layers. These developmental changes are altered by DR. Quantification by cell counts indicated that age and DR interact such that differences in cellular expression are opposite in direction between 5- and 20-week-old cats. This bidirectional regulation of cellular expression is the same in all cortical laminae. The bidirectional regulation of cellular expression matches the effects of age and DR on physiological plasticity during the critical period as assessed by ocular dominance shifts in response to monocular deprivation.

Role for Reelin in neurotransmitter release

The extracellular matrix molecule Reelin is known to control neuronal migration during development. Recent evidence suggests that it also plays a role in the maturation of postsynaptic dendrites and spines as well as in synaptic plasticity. Here, we aimed to address the question whether Reelin plays a role in presynaptic structural organization and function. Quantitative electron microscopic analysis of the number of presynaptic boutons in the stratum radiatum of hippocampal region CA1 did not reveal differences between wild-type animals and Reelin-deficient reeler mutant mice. However, additional detailed analysis showed that the number of presynaptic vesicles was significantly increased in CA1 synapses of reeler mutants. To test the hypothesis that vesicle fusion is altered in reeler, we studied proteins known to control transmitter release. SNAP25, a protein of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex, was found to be significantly reduced in reeler mutants, whereas other SNARE complex proteins remained unaltered. Addition of recombinant Reelin to organotypic slice cultures of reeler hippocampi substantially rescued not only SNAP25 protein expression levels but also the number of vesicles per bouton area indicating a role for Reelin in presynaptic functions. Next, we analyzed paired-pulse facilitation, a presynaptic mechanism associated with transmitter release, and observed a significant decrease at CA1 synapses of reeler mutants when compared with wild-type animals. Together, these novel findings suggest a role for Reelin in modulating presynaptic release mechanisms.

ApoE receptor 2 regulates synapse and dendritic spine formation

BACKGROUND

Apolipoprotein E receptor 2 (ApoEr2) is a postsynaptic protein involved in long-term potentiation (LTP), learning, and memory through unknown mechanisms. We examined the biological effects of ApoEr2 on synapse and dendritic spine formation-processes critical for learning and memory.

METHODOLOGY/PRINCIPAL FINDINGS

In a heterologous co-culture synapse assay, overexpression of ApoEr2 in COS7 cells significantly increased colocalization with synaptophysin in primary hippocampal neurons, suggesting that ApoEr2 promotes interaction with presynaptic structures. In primary neuronal cultures, overexpression of ApoEr2 increased dendritic spine density. Consistent with our in vitro findings, ApoEr2 knockout mice had decreased dendritic spine density in cortical layers II/III at 1 month of age. We also tested whether the interaction between ApoEr2 and its cytoplasmic adaptor proteins, specifically X11α and PSD-95, affected synapse and dendritic spine formation. X11α decreased cell surface levels of ApoEr2 along with synapse and dendritic spine density. In contrast, PSD-95 increased cell surface levels of ApoEr2 as well as synapse and dendritic spine density.

CONCLUSIONS/SIGNIFICANCE

These results suggest that ApoEr2 plays important roles in structure and function of CNS synapses and dendritic spines, and that these roles are modulated by cytoplasmic adaptor proteins X11α and PSD-95.

Lipoprotein receptors--an evolutionarily ancient multifunctional receptor family

The evolutionarily ancient low-density lipoprotein (LDL) receptor gene family represents a class of widely expressed cell surface receptors. Since the dawn of the first primitive multicellular organisms, several structurally and functionally distinct families of lipoprotein receptors have evolved. In accordance with the now obsolete 'one-gene-one-function' hypothesis, these cell surface receptors were originally perceived as mere transporters of lipoproteins, lipids, and nutrients or as scavenger receptors, which remove other kinds of macromolecules, such as proteases and protease inhibitors from the extracellular environment and the cell surface. This picture has since undergone a fundamental change. Experimental evidence has replaced the perception that these receptors serve merely as cargo transporters. Instead it is now clear that the transport of macromolecules is inseparably intertwined with the molecular machinery by which cells communicate with each other. Lipoprotein receptors are essentially sensors of the extracellular environment that participate in a wide range of physiological processes by physically interacting and coevolving with primary signal transducers as co-regulators. Furthermore, lipoprotein receptors modulate cellular trafficking and localization of the amyloid precursor protein (APP) and the β-amyloid peptide (Aβ), suggesting a role in the pathogenesis of Alzheimer's disease. Moreover, compelling evidence shows that LDL receptor family members are involved in tumor development and progression.

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.

Molecular and electrophysiological characterization of GFP-expressing CA1 interneurons in GAD65-GFP mice

The use of transgenic mice in which subtypes of neurons are labeled with a fluorescent protein has greatly facilitated modern neuroscience research. GAD65-GFP mice, which have GABAergic interneurons labeled with GFP, are widely used in many research laboratories, although the properties of the labeled cells have not been studied in detail. Here we investigate these cells in the hippocampal area CA1 and show that they constitute ∼20% of interneurons in this area. The majority of them expresses either reelin (70±2%) or vasoactive intestinal peptide (VIP; 15±2%), while expression of parvalbumin and somatostatin is virtually absent. This strongly suggests they originate from the caudal, and not the medial, ganglionic eminence. GFP-labeled interneurons can be subdivided according to the (partially overlapping) expression of neuropeptide Y (42±3%), cholecystokinin (25±3%), calbindin (20±2%) or calretinin (20±2%). Most of these subtypes (with the exception of calretinin-expressing interneurons) target the dendrites of CA1 pyramidal cells. GFP-labeled interneurons mostly show delayed onset of firing around threshold, and regular firing with moderate frequency adaptation at more depolarized potentials.

Reelin signals through apolipoprotein E receptor 2 and Cdc42 to increase growth cone motility and filopodia formation

Lipoprotein receptor signaling regulates the positioning and differentiation of postmitotic neurons during development and modulates neuronal plasticity in the mature brain. Depending on the contextual situation, the lipoprotein receptor ligand Reelin can have opposing effects on cortical neurons. We show that Reelin increases growth cone motility and filopodia formation, and identify the underlying signaling cascade. Reelin activates the Rho GTPase Cdc42, known for its role in neuronal morphogenesis and directed migration, in an apolipoprotein E receptor 2-, Disabled-1-, and phosphatidylinositol 3-kinase-dependent manner. We demonstrate that neuronal vesicle trafficking, a Cdc42-controlled process, is increased after Reelin treatment and further provide evidence that the peptidergic VIP/PACAP38 system and Reelin can functionally interact to promote axonal branching. In conclusion, Reelin-induced activation of Cdc42 contributes to the regulation of the cytoskeleton of individual responsive neurons and converges with other signaling cascades to orchestrate Rho GTPase activity and promote neuronal development. Our data link the observation that defects in Rho GTPases and Reelin signaling are responsible for developmental defects leading to neurological and psychiatric disorders.

Early life stress stimulates hippocampal reelin gene expression in a sex-specific manner: evidence for corticosterone-mediated action

Early life stress predisposes to the development of psychiatric disorders. In this context the hippocampal formation is of particular interest, because it is affected by stress on the structural and cognitive level. Since little is known how early life stress is translated on the molecular level, we mimicked early life stress in mouse models and analyzed the expression of the glycoprotein Reelin, a master molecule for development and differentiation of the hippocampus. From postnatal day 1 (P1) to P14, mouse pups were subjected to one of the following treatments: nonhandling (NH), handling (H), maternal separation (MS), and early deprivation (ED) followed by immediate (P15) or delayed (P70) real time RT-PCR analysis of reelin mRNA expression. We show that at P15, reelin mRNA levels were significantly increased in male H and ED groups when compared with the NH group. In contrast, no stress-induced alterations of reelin mRNA expression were found in female animals. This sex difference in stress-mediated stimulation of reelin expression was maintained into adulthood, since at P70 intergroup differences were still found in male, but not in female mice. On the cellular level, however, we did not find any significant differences in cell densities of Reelin-immunolabeled neurons between treatment groups or sexes, but an overall reduction of Reelin-expressing neurons in the adult hippocampus when compared to P15. To address the question whether corticosterone mediates the stress-induced up-regulation of reelin gene expression, we used age-matched hippocampal slice cultures derived from male and female mouse pups. Quantitative determination of mRNA levels revealed that corticosterone treatment significantly up-regulated reelin mRNA expression in male, but not in female hippocampi. Taken together, these results show a sex-specific regulation of reelin gene expression by early life experience, most likely mediated by corticosterone.

Retardation of neurobehavioral development and reelin down-regulation regulated by further DNA methylation in the hippocampus of the rat pups are associated with maternal deprivation

It is known that early life stress has profound effects in early developing hippocampus. Reelin is a large protein that regulates neuronal migration during embryonic development. The expression of reelin persists in brain, but its function is little known. The aim of the present study was to investigate the effects of maternal deprivation (MD) on early neurobehavioral development of rats, and the role of reelin and the potential mechanism underlying regulation of its expression in hippocampus. Rat pups were removed from mothers during the postnatal day (PND) 2-15 for 3h a day. Reflex developments including grasping, gait, righting, cliff avoidance, auditory startle, hot-plate test and negative geotaxis, were tested during the first 3 weeks. The level of reelin mRNA and reelin gene methylation in the hippocampal formation were determined using real-time PCR analysis. As expected, some differences appeared in the measure of neurobehavior and expression of reelin in rat pups. Several significant deficiencies were observed in bodyweight, auditory startle and grasping reflex while a great enhancement in hot-plate test in rat pups suffering from MD. On PND 22, the expression of reelin mRNA reduced in the hippocampus followed by MD. Meanwhile, the changes of DNA methylation showed an opposite trend compared with the reelin expression. The results suggest that MD in early life has harmful effects on neurobehavioral development, and causes the down-regulation of reelin mRNA by further DNA methylation in postnatal hippocampus.

Emerging topics in Reelin function

Reelin signalling in the early developing cortex regulates radial migration of cortical neurons. Later in development, Reelin promotes maturation of dendrites and dendritic spines. Finally, in the mature brain, it is involved in modulating synaptic function. In recent years, efforts to identify downstream signalling events induced by binding of Reelin to lipoprotein receptors led to the characterization of novel components of the Reelin signalling cascade. In the present review, we first address distinct functions of the Reelin receptors Apoer2 and Vldlr in cortical layer formation, followed by a discussion on the recently identified downstream effector molecule n-cofilin, involved in regulating actin cytoskeletal dynamics required for coordinated neuronal migration. Next, we discuss possible functions of the recently identified Reelin-Notch signalling crosstalk, and new aspects of the role of Reelin in the formation of the dentate radial glial scaffold. Finally, progress in characterizing the function of Reelin in modulating synaptic function in the adult brain is summarized. The present review has been inspired by a session entitled 'Functions of Reelin in the developing and adult hippocampus', held at the Spring Hippocampal Research Conference in Verona/Italy, June 2009.

Building and remodeling synapses

Synaptic junctions are generated by adhesion proteins that bridge the synaptic cleft to firmly anchor pre- and postsynaptic membranes. Several cell adhesion molecule (CAM) families localize to synapses, but it is not yet completely understood how each synaptic CAM family contributes to synapse formation and/or structure, and whether or how smaller groups of CAMs serve as minimal, functionally cooperative adhesive units upon which structure is based. Synapse structure and function evolve over the course of development, and in mature animals, synapses are composed of a greater number of proteins, surrounded by a stabilizing extracellular matrix, and often contacted by astrocytic processes. Thus, in mature networks undergoing plasticity, persistent changes in synapse strength, morphology, or number must be accompanied by selective and regulated remodeling of the neuropil. Recent work indicates that regulated, extracellular proteolysis may be essential for this, and rather than simply acting permissively to enable synapse plasticity, is more likely playing a proactive role in driving coordinated synaptic structural and functional modifications that underlie persistent changes in network activity.

Neural stem cells reduce hippocampal tau and reelin accumulation in aged Ts65Dn Down syndrome mice

Tau accumulation, in the form of neurofibrillary tangles (NFT), is an early neuropathological characteristic of Alzheimer's disease (AD) and early onset AD frequently seen in Down syndrome (DS). We investigated the presence of tau accumulation in the brains of aging DS mice using the Ts65Dn mouse model. All aged mice appeared to have substantial clusters of extracellular granules that were positive for tau and reelin, but not for amyloid-β or APP. These clusters were found primarily in CA1 of the hippocampus. In addition, the aged trisomic DS mice had a significantly greater accumulation of extracellular tau/reelin granular deposits compared to disomic littermates. These granules were similar to those described by others who also found extracellular proteinous granules in the brains of non-DS mice engineered to model aging and/or AD. When neural stem cells (NSC) were implanted unilaterally into the hippocampus of the Ts65Dn mice, the tau/reelin-positive granules were significantly reduced in both trisomic and disomic mice. Our findings indicate that changes in tau/reelin-positive granules could be used as an index for neuropathological assessment in aging DS and AD. Furthermore, changes in granule density could be used to test the efficacy of novel treatments, such as NSC implantation. Lastly, it is speculated that the unique abilities of NSC to migrate and express growth factors might be a contributing factor to reducing tau/reelin accumulation in aging DS and AD.

The coexpression of reelin and neuronal nitric oxide synthase in a subpopulation of dentate gyrus neurons is downregulated in heterozygous reeler mice

Reelin is an extracellular matrix protein expressed in several interneuron subtypes in the hippocampus and dentate gyrus. Neuronal nitric oxide synthase (nNOS) is also expressed by interneurons in these areas. We investigated whether reelin and nNOS are co-localized in the same population of hippocampal interneurons, and whether this colocalization is altered in the heterozygous reeler mouse. We found colocalization of nNOS in reelin-positive cells in the CA1 stratum radiatum and lacunosum moleculare, the CA3 stratum radiatum, and the dentate gyrus subgranular zone, molecular layer, and hilus. In heterozygous reeler mice, the colocalization of nNOS in reelin-positive cells was significantly decreased only in the subgranular zone and molecular layer. The coexpression of reelin and nNOS in several hippocampal regions suggests that reelin and nNOS may work synergistically to promote glutamatergic function, and the loss of this coexpression in heterozygous reeler mice may underlie some of the behavioral deficits observed in these animals.

Reelin Signaling, Hippocampal Neurogenesis, and Efficacy of Aspirin Intake & Stem Cell Transplantation in Aging and Alzheimer's disease

Comprehending the mechanisms underlying the pathophysiology of aging and Alzheimer's disease has immense value for developing strategies that promote successful aging and prevent or cure Alzheimer's disease. The first issue of the new journal, "Aging & Disease" comprises articles that discuss the current knowledge pertaining to changes in reelin signaling in normal & pathological forms of aging, memory and neurogenesis in Aging & Alzheimer's disease, the efficacy of a non-steroidal anti-inflammatory drug aspirin in combination with docosahexaenoic acid for reducing the risk for Alzheimer's disease, and the usefulness of stem cell transplantation for improving memory in aging and Alzheimer's disease. The highlights and the importance of the above issues to Aging and Alzheimer's disease are discussed in this commentary.

Imbalanced pattern completion vs. separation in cognitive disease: network simulations of synaptic pathologies predict a personalized therapeutics strategy

Background

Diverse Mouse genetic models of neurodevelopmental, neuropsychiatric, and neurodegenerative causes of impaired cognition exhibit at least four convergent points of synaptic malfunction: 1) Strength of long-term potentiation (LTP), 2) Strength of long-term depression (LTD), 3) Relative inhibition levels (Inhibition), and 4) Excitatory connectivity levels (Connectivity).

Results

To test the hypothesis that pathological increases or decreases in these synaptic properties could underlie imbalances at the level of basic neural network function, we explored each type of malfunction in a simulation of autoassociative memory. These network simulations revealed that one impact of impairments or excesses in each of these synaptic properties is to shift the trade-off between pattern separation and pattern completion performance during memory storage and recall. Each type of synaptic pathology either pushed the network balance towards intolerable error in pattern separation or intolerable error in pattern completion. Imbalances caused by pathological impairments or excesses in LTP, LTD, inhibition, or connectivity, could all be exacerbated, or rescued, by the simultaneous modulation of any of the other three synaptic properties.

Conclusions

Because appropriate modulation of any of the synaptic properties could help re-balance network function, regardless of the origins of the imbalance, we propose a new strategy of personalized cognitive therapeutics guided by assay of pattern completion vs. pattern separation function. Simulated examples and testable predictions of this theorized approach to cognitive therapeutics are presented.

Nova2 regulates neuronal migration through an RNA switch in disabled-1 signaling

Neuronal migration leads to a highly organized laminar structure in the mammalian brain, and its misregulation causes lissencephaly and behavioral and cognitive defects. Reelin signaling, which is mediated in part by a key adaptor, disabled-1 (Dab1), plays a critical but incompletely understood role in this process. We found that the neuron-specific RNA-binding protein Nova2 regulates neuronal migration in late-generated cortical and Purkinje neurons. An unbiased HITS-CLIP and exon junction array search for Nova-dependent reelin-pathway RNAs at E14.5 revealed only one candidate-an alternatively spliced isoform of Dab1 (Dab1.7bc). In utero electroporation demonstrated that Dab1.7bc was sufficient to induce neuronal migration defects in wild-type mice and exacerbate defects when Dab1 levels were reduced, whereas Dab1 overexpression mitigates defects in Nova2 null mice. Thus, Nova2 regulates an RNA switch controlling the ability of Dab1 to mediate neuronal responsiveness to reelin signaling and neuronal migration, suggesting new links between splicing regulation, brain disease, and development.

Reelin-mediated Signaling during Normal and Pathological Forms of Aging

Reelin is a large extracellular matrix protein essential for mediating proper neuronal positioning during development. Employing the same lipoprotein-mediated signaling cascade, Reelin regulates NMDA receptor homeostasis and modulates synaptic function and plasticity in adult synapses. In line, aging-related reduction in Reelin expression has been shown to contribute to cognitive impairments during normal aging. Although recent experimental evidence suggests an involvement of dysfunctional Reelin in pathological forms of aging, such as late-onset Alzheimer's disease (AD), the molecular mechanisms by which this conserved extracellular glycoprotein contributes to the pathogenesis of AD remains still largely unknown. In the present review, we briefly summarize the role of Reelin in the developing and adult brain and discuss the implication of loss- or gain-of-functions of developmental programs in the adult brain as putative inducing factors of pathological forms of aging. Finally, we will propose some new concepts on the role of inflammatory cytokines in interfering with Reelin-mediated signaling during neurodevelopment and adult synaptic function, and discuss how this could be translated into a novel non-transgenic mouse model of late-onset AD. Thus, the findings presented in this review are aimed to highlight the important role of Reelin-mediated signaling in maintaining a crucial developmental program in the adult brain that is required to prevent the shift from normal to pathological aging.

Behavioral and neurobiological consequences of prolonged glucocorticoid exposure in rats: relevance to depression

Stress is a critical environmental trigger for the development of clinical depression, yet little is known about the specific neurobiological mechanisms by which stress influences the development of depressive symptomatology. Animal models provide an efficient way to study the etiology of human disorders such as depression, and a number of preclinical models have been developed to assess the link between stress, glucocorticoids, and depressive behavior. These mode ls typically make use of repeated exposure to physical or psychological stressors in rodents or other small laboratory animals. This review focuses primarily on a recently developed preclinical model of depression that uses exogenous administration of the stress hormone corticosterone (CORT) in rodents instead of exposure to physical or psychological stressors. Repeated CORT administration in rats or mice produces reliable behavioral and neurobiological alterations that parallel many of the core symptoms and neurobiological changes associated with human depression. This provides an opportunity to study behavior and neurobiology in the same animal, so that the neurobiological factors that underlie specific symptoms can be identified. Taken together, these findings suggest that exogenous CORT administration is a useful method for studying the relationship between stress, glucocorticoids, and depression. Further study with this model may provide important new data regarding the neurobiological bases of depression.

Distinctive properties of CXC chemokine receptor 4-expressing Cajal-Retzius cells versus GABAergic interneurons of the postnatal hippocampus

The CXC chemokine receptor 4 (CXCR4) for the chemokine (C-X-C motif) ligand 12/stromal cell-derived factor-1 alpha (CXCL12/SDF-1 alpha) is highly expressed in the postnatal CA1 stratum lacunosum-moleculare. However, both the network events triggered by SDF-1 alpha in this microcircuit and the cellular targets of this chemokine remain virtually unexplored. Here, we have studied SDF-1 alpha-mediated neuromodulation of the stratum lacunosum-moleculare by directly comparing the properties of CXCR4-expressing Cajal-Retzius cells vs. CXCR4-non-expressing interneurons, and by recording the electrophysiological effects caused by application of SDF-1 alpha on either cell type. We demonstrate that SDF-1 alpha dramatically reduces spontaneous firing in Cajal-Retzius cells via hyerpolarization, and that cessation of firing is prevented by the CXCR4-specific antagonist AMD3100. In contrast, no effects on the excitability of interneurons of the same layer were observed following exposure to the chemokine. We also provide evidence that, despite the expression of functional glutamate receptors, Cajal-Retzius cells are integrated in the synaptic network of the stratum lacunosum-moleculare via excitatory GABAergic input. Furthermore, we show that the axons of Cajal-Retzius cells target specifically the stratum lacunosum-moleculare and the dentate gyrus, but lack postsynaptic specializations opposite to their axonal varicosities. These results, taken together with our observation that SDF-1 alpha reduces evoked field responses at the entorhinal cortex-CA1 synapse, suggest that Cajal-Retzius cells produce a diffuse output that may impact information processing of stratum lacunosum-moleculare. We propose that pathological alterations of local levels of SDF-1 alpha or CXCR4 expression may affect the functions of an important hippocampal microcircuit.

Cognitive decline is associated with reduced reelin expression in the entorhinal cortex of aged rats

Brain regions and neural circuits differ in their vulnerability to changes that occur during aging and in age-related neurodegenerative diseases. Among the areas that comprise the medial temporal lobe memory system, the layer II neurons of the entorhinal cortex, which form the perforant path input to the hippocampal formation, exhibit early alterations over the course of aging Reelin, a glycoprotein implicated in synaptic plasticity, is expressed by entorhinal cortical layer II neurons. Here, we report that an age-related reduction in reelin expression in the entorhinal cortex is associated with cognitive decline. Using immunohistochemistry and in situ hybridization, we observed decreases in the number of Reelin-immunoreactive cells and reelin messenger RNA expression in the lateral entorhinal cortex of aged rats that are cognitively impaired relative to young adults and aged rats with preserved cognitive abilities. The lateral entorhinal cortex of aged rats with cognitive impairment also exhibited changes in other molecular markers, including increased accumulation of phosphorylated tau and decreased synaptophysin immunoreactivity. Taken together, these findings suggest that reduced reelin expression, emanating from layer II entorhinal neurons, may contribute to network dysfunction that occurs during memory loss in aging.

Beta amyloid-independent role of amyloid precursor protein in generation and maintenance of dendritic spines

Synapse loss induced by amyloid beta (Abeta) is thought to be a primary contributor to cognitive decline in Alzheimer's disease. Abeta is generated by proteolysis of amyloid precursor protein (APP), a synaptic receptor whose physiological function remains unclear. In the present study, we investigated the role of APP in dendritic spine formation, which is known to be important for learning and memory. We found that overexpression of APP increased spine number, whereas knockdown of APP reduced spine density in cultured hippocampal neurons. This spine-promoting effect of APP required both the extracellular and intracellular domains of APP, and was accompanied by specific upregulation of the GluR2, but not the GluR1, subunit of AMPA receptors. In an in vivo experiment, we found that cortical layers II/III and hippocampal CA1 pyramidal neurons in 1 year-old APP-deficient mice had fewer and shorter dendritic spines than wild-type littermates. In contrast, transgenic mice overexpressing mutant APP exhibited increased spine density compared to control animals, though only at a young age prior to overaccumulation of soluble amyloid. Additionally, increased glutamate synthesis was observed in young APP transgenic brains, whereas glutamate levels were decreased and GABA levels were increased in APP-deficient mice. These results demonstrate that APP is important for promoting spine formation and is required for proper spine development.

Reelin regulates postnatal neurogenesis and enhances spine hypertrophy and long-term potentiation

Reelin, an extracellular protein essential for neural migration and lamination, is also expressed in the adult brain. To unravel the function of this protein in the adult forebrain, we generated transgenic mice that overexpress Reelin under the control of the CaMKIIalpha promoter. Overexpression of Reelin increased adult neurogenesis and impaired the migration and positioning of adult-generated neurons. In the hippocampus, the overexpression of Reelin resulted in an increase in synaptic contacts and hypertrophy of dendritic spines. Induction of long-term potentiation (LTP) in alert-behaving mice showed that Reelin overexpression evokes a dramatic increase in LTP responses. Hippocampal field EPSP during a classical conditioning paradigm was also increased in these mice. Our results indicate that Reelin levels in the adult brain regulate neurogenesis and migration, as well as the structural and functional properties of synapses. These observations suggest that Reelin controls developmental processes that remain active in the adult brain.

Serum response factor regulates hippocampal lamination and dendrite development and is connected with reelin signaling

During brain development, neurons and their nerve fibers are often segregated in specific layers. The hippocampus is a well-suited model system to study lamination in health and aberrant cell/fiber lamination associated with neurological disorders. SRF (serum response factor), a transcription factor, regulates synaptic-activity-induced immediate-early gene (IEG) induction and cytoskeleton-based neuronal motility. Using early postnatal conditional SRF ablation, we uncovered distorted hippocampal lamination, including malpositioning of granule cell neurons and disruption of layer-restricted termination of commissural-associational and mossy fiber axons. Besides axons, dendrite branching and spine morphogenesis in Srf mutants were impaired, offering a first morphological basis for SRF's reported role in learning and memory. Srf mutants resemble mice lacking components of the reelin signaling cascade, a fundamental signaling entity in brain lamination. Our data indicate that reelin signaling and SRF-mediated gene transcription might be connected: reelin induces IEG and cytoskeletal genes in an SRF-dependent manner. Further, reelin-induced neurite motility is blocked in Srf mutants and constitutively active SRF rescues impaired neurite extension in reeler mouse mutants in vitro. In sum, data provided in this report show that SRF contributes to hippocampal layer and nerve fiber organization and point at a link between Srf gene transcription and reelin signaling.

Reelin promotes neuronal orientation and dendritogenesis during preplate splitting

The secreted ligand Reelin is thought to regulate the translocation and positioning of prospective layer 6 (L6) neurons into the preplate, a plexus of neurons overlying the ventricular zone. We examined wild type and Reelin-deficient cortices and found that L6 neurons were equivalently positioned beneath the pia during the period of preplate splitting and initial cortical plate (CP) formation. The absence of detectable L6 ectopia in "reeler" cortices at this developmental time point indicates that Reelin-signaling might not regulate L6 neuron migration or gross positioning during preplate splitting. To explore the acute response of L6 neurons to Reelin, subpial injections of Reelin were made into Reelin-deficient explants. Reelin injection caused L6 neurons to orient their nuclei and polarize their Golgi toward the pia while initiating exuberant dendritic (MAP2+) outgrowth within 4 h. This rapid Reelin-dependent neuronal orientation and alignment created CP-like histology without any significant change in the mean position of the population of L6 neurons. Conversely, subplate cells and chondroitin sulfate proteoglycan immunoreactivity were found at significantly deeper positions from the pial surface after injection, suggesting that Reelin partially rescues preplate splitting within 4 h. Thus, Reelin has a direct role in promoting rapid morphological differentation and orientation of L6 neurons during preplate splitting.

Identification of gene transcripts expressed by postsynaptic neurons during synapse formation encoding cell surface proteins with presumptive synaptogenic activity

Synapse formation is a well-programmed developmental process involving a variety of cell-cell interactions carried out by distinct groups of molecules. Various molecules that contribute to the assembly of synaptic contacts have been characterized; however, the repertoire of identified proteins expressed by postsynaptic neurons capable of inducing presynaptic differentiation is quite limited. To identify gene transcripts encoding cell surface proteins expressed by postsynaptic cells with molecular features suggestive of synaptogenic activity, this study carried out a genome-wide expression analysis in the chick ciliary ganglion during the different phases of synapse formation. It was found that from the 21,493 gene-probes detected throughout development, 302 protein-coding transcripts were upregulated during the initiation of synapse formation. Analysis of this pool of transcripts showed that 51 of them encoded cell surface proteins (27 membrane-bound and 24 secreted) with protein-protein interacting domains. This includes twelve cell adhesion molecules, six ligand-receptors, six proteins with ligand-like domains, three membrane bound enzymes, eight components of the extracellular matrix, three neuropeptides, three cytokines and growth factors, five extracellular modulators of cell signaling, and five unrelated secreted proteins. Furthermore, the role of synaptic transmission during the initiation of synapse formation was evaluated by assessing the effect of synaptic activity blockade with d-tubocurarine on the expression levels of the pool of 51 transcripts encoding cell surface proteins. Treatment with d-tubocurarine reduced the expression levels of 22% of the selected genes, while the expression levels of 78% of the genes was not affected or was enhanced.