Myosin V regulates synaptopodin clustering and localization in the dendrites of hippocampal neurons.

The spine apparatus (SA) is an endoplasmic reticulum-related organelle that is present in a subset of dendritic spines in cortical and pyramidal neurons, and plays an important role in Ca2+ homeostasis and dendritic spine plasticity. The protein synaptopodin is essential for the formation of the SA and is widely used as a maker for this organelle. However, it is still unclear which factors contribute to its localization at selected synapses, and how it triggers local SA formation. In this study, we characterized development, localization and mobility of synaptopodin clusters in hippocampal primary neurons, as well as the molecular dynamics within these clusters. Interestingly, synaptopodin at the shaft-associated clusters is less dynamic than at spinous clusters. We identify the actin-based motor proteins myosin V (herein referring to both the myosin Va and Vb forms) and VI as novel interaction partners of synaptopodin, and demonstrate that myosin V is important for the formation and/or maintenance of the SA. We found no evidence of active microtubule-based transport of synaptopodin. Instead, new clusters emerge inside spines, which we interpret as the SA being assembled on-site.

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

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

Trajectory Analysis Unveils Reelin's Role in the Directed Migration of Granule Cells in the Dentate Gyrus.

Reelin controls neuronal migration and layer formation. Previous studies in reeler mice deficient in Reelin focused on the result of the developmental process in fixed tissue sections. It has remained unclear whether Reelin affects the migratory process, migration directionality, or migrating neurons guided by the radial glial scaffold. Moreover, Reelin has been regarded as an attractive signal because newly generated neurons migrate toward the Reelin-containing marginal zone. Conversely, Reelin might be a stop signal because migrating neurons in reeler, but not in wild-type mice, invade the marginal zone. Here, we monitored the migration of newly generated proopiomelanocortin-EGFP-expressing dentate granule cells in slice cultures from reeler, reeler-like mutants and wild-type mice of either sex using real-time microscopy. We discovered that not the actual migratory process and migratory speed, but migration directionality of the granule cells is controlled by Reelin. While wild-type granule cells migrated toward the marginal zone of the dentate gyrus, neurons in cultures from reeler and reeler-like mutants migrated randomly in all directions as revealed by vector analyses of migratory trajectories. Moreover, live imaging of granule cells in reeler slices cocultured to wild-type dentate gyrus showed that the reeler neurons changed their directions and migrated toward the Reelin-containing marginal zone of the wild-type culture, thus forming a compact granule cell layer. In contrast, directed migration was not observed when Reelin was ubiquitously present in the medium of reeler slices. These results indicate that topographically administered Reelin controls the formation of a granule cell layer.SIGNIFICANCE STATEMENT Neuronal migration and the various factors controlling its onset, speed, directionality, and arrest are poorly understood. Slice cultures offer a unique model to study the migration of individual neurons in an almost natural environment. In the present study, we took advantage of the expression of proopiomelanocortin-EGFP by newly generated, migrating granule cells to analyze their migratory trajectories in hippocampal slice cultures from wild-type mice and mutants deficient in Reelin signaling. We show that the compartmentalized presence of Reelin is essential for the directionality, but not the actual migratory process or speed, of migrating granule cells leading to their characteristic lamination in the dentate gyrus.

Proteomics insights into infantile neuronal ceroid lipofuscinosis (CLN1) point to the involvement of cilia pathology in the disease.

Mutations in the depalmitoylation enzyme, palmitoyl protein thioesterase (PPT1), result in the early onset neurodegenerative disease known as Infantile Neuronal Ceroid Lipofuscinosis. Here, we provide proteomic evidence suggesting that PPT1 deficiency could be considered as a ciliopathy. Analysis of membrane proteins from brain enriched for acylated proteins from neonate Ppt1 knock out and control mice revealed a list of 88 proteins with differential expression levels. Amongst them, we identified Rab3IP, which regulates ciliogenesis in concert with Rab8 and Rab11. Immunostaining analysis revealed that PPT1 is localized in the cilia. Indeed, an unbiased proteomics analysis on isolated cilia revealed 660 proteins, which differed in their abundance levels between wild type and Ppt1 knock out. We demonstrate here that Rab3IP, Rab8 and Rab11 are palmitoylated, and that palmitoylation of Rab11 is required for correct intracellular localization. Cells and brain preparations from Ppt1-/- mice exhibited fewer cells with cilia and abnormally longer cilia, with both acetylated tubulin and Rab3IP wrongly distributed along the length of cilia. Most importantly, the analysis revealed a difference in the distribution and levels of the modified proteins in cilia in the retina of mutant mice versus the wildtype, which may be important in the early neurodegenerative phenotype. Overall, our results suggest a novel link between palmitoylated proteins, cilial organization and the pathophysiology of Neuronal Ceroid Lipofuscinosis.

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

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

Altered Connectivity and Synapse Maturation of the Hippocampal Mossy Fiber Pathway in a Mouse Model of the Fragile X Syndrome.

The Fragile X syndrome (FXS) as the most common monogenetic cause of cognitive impairment and autism indicates how tightly the dysregulation of synapse development is linked to cognitive deficits. Symptoms of FXS include excessive adherence to patterns that point to compromised hippocampal network formation. Surprisingly, one of the most complex hippocampal synapses connecting the dentate gyrus (DG) to CA3 pyramidal neurons has not been analyzed in FXS yet. Intriguingly, we found altered synaptic function between DG and CA3 in a mouse model of FXS (fmr1 knockout [KO]) demonstrated by increased mossy fiber-dependent miniature excitatory postsynaptic current (mEPSC) frequency at CA3 pyramidal neurons together with increased connectivity between granule cells and CA3 neurons. This phenotype is accompanied by increased activity of fmr1 KO animals in the marble burying task, detecting repetitive and obsessive compulsive behavior. Spine apparatus development and insertion of AMPA receptors is enhanced at postsynaptic thorny excrescences (TEs) in fmr1 KO mice. We report age-dependent alterations in TE morphology and in the underlying actin dynamics possibly linked to a dysregulation in profilin1 expression. TEs form detonator synapses guiding CA3 network activity. Thus, alterations described here are likely to contribute substantially to the impairment in hippocampal function and therefore to the pathogenesis of FXS.

The lipoprotein receptor LRP1 modulates sphingosine-1-phosphate signaling and is essential for vascular development.

Low density lipoprotein receptor-related protein 1 (LRP1) is indispensable for embryonic development. Comparing different genetically engineered mouse models, we found that expression of Lrp1 is essential in the embryo proper. Loss of LRP1 leads to lethal vascular defects with lack of proper investment with mural cells of both large and small vessels. We further demonstrate that LRP1 modulates Gi-dependent sphingosine-1-phosphate (S1P) signaling and integrates S1P and PDGF-BB signaling pathways, which are both crucial for mural cell recruitment, via its intracellular domain. Loss of LRP1 leads to a lack of S1P-dependent inhibition of RAC1 and loss of constraint of PDGF-BB-induced cell migration. Our studies thus identify LRP1 as a novel player in angiogenesis and in the recruitment and maintenance of mural cells. Moreover, they reveal an unexpected link between lipoprotein receptor and sphingolipid signaling that, in addition to angiogenesis during embryonic development, is of potential importance for other targets of these pathways, such as tumor angiogenesis and inflammatory processes.

Reelin Induces Branching of Neurons and Radial Glial Cells during Corticogenesis.

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

Synaptopodin regulates spine plasticity: mediation by calcium stores.

The role of synaptopodin (SP), an actin-binding protein residing in dendritic spines, in synaptic plasticity was studied in dissociated cultures of hippocampus taken from control and SP knock-out (SPKO) mice. Unlike controls, SPKO cultures were unable to express changes in network activity or morphological plasticity after intense activation of their NMDA receptors. SPKO neurons were transfected with SP-GFP, such that the only SP resident in these neurons is the fluorescent species. The localization and intensity of the transfected SP were similar to that of the native one. Because less than half of the spines in the transfected neurons contained SP, comparisons were made between SP-containing (SP(+)) and SP lacking (SP(-)) spines in the same dendritic segments. Synaptic plasticity was induced either in the entire network by facilitation of the activation of the NMDA receptor, or specifically by local flash photolysis of caged glutamate. After activation, spines that were endowed with SP puncta were much more likely to expand than SP(-) spines. The spine expansion was suppressed by thapsigargin, which disables calcium stores. The mechanism through which SP may promote plasticity is indicated by the observations that STIM-1, the sensor of calcium concentration in stores, and Orai-1, the calcium-induced calcium entry channel, are colocalized with SP, in the same dendritic spines. The structural basis of SP is likely to be the spine apparatus, found in control but not in SPKO cells. These results indicate that SP has an essential, calcium store-related role in regulating synaptic plasticity in cultured hippocampal neurons.

Characterization and distribution of Reelin-positive interneuron subtypes in the rat barrel cortex.

GABAergic inhibitory interneurons (IN) represent a heterogeneous population with different electrophysiological, morphological, and molecular properties. The correct balance between interneuronal subtypes is important for brain function and is impaired in several neurological and psychiatric disorders. Here we show the data of 123 molecularly and electrophysiologically characterized neurons of juvenile rat barrel cortex acute slices, 48 of which expressed Reelin (Reln). Reln mRNA was exclusively detected in Gad65/67-positive cells but was found in interneuronal subtypes in different proportions: all cells of the adapting-Somatostatin (SST) cluster expressed Reln, whereas 63% of the adapting-neuropeptide Y (NPY, 50% of the fast-spiking Parvalbumin (PVALB), and 27% of the adapting/bursting-Vasoactive Intestinal Peptide (VIP) cluster were Reln-positive. Silhouette analysis revealed a high impact of the parameter Reln on cluster quality. By analyzing the co-localization of RELN immunoreactivity with those of different IN-markers, we found that RELN is produced layer-independently in SST-, NPY-, and NOS1-expressing INs, whereas co-localization of RELN and VIP was mostly absent. Of note, RELN co-localized with PVALB, predominantly in INs of layers IV/V (>30%). Our findings emphasize RELN's role as an important IN-marker protein and provide a basis for the functional characterization of Reln-expressing INs and its role in the regulation of inhibitory IN networks.

Epilepsy-induced motility of differentiated neurons.

Neuronal ectopia, such as granule cell dispersion (GCD) in temporal lobe epilepsy (TLE), has been assumed to result from a migration defect during development. Indeed, recent studies reported that aberrant migration of neonatal-generated dentate granule cells (GCs) increased the risk to develop epilepsy later in life. On the contrary, in the present study, we show that fully differentiated GCs become motile following the induction of epileptiform activity, resulting in GCD. Hippocampal slice cultures from transgenic mice expressing green fluorescent protein in differentiated, but not in newly generated GCs, were incubated with the glutamate receptor agonist kainate (KA), which induced GC burst activity and GCD. Using real-time microscopy, we observed that KA-exposed, differentiated GCs translocated their cell bodies and changed their dendritic organization. As found in human TLE, KA application was associated with decreased expression of the extracellular matrix protein Reelin, particularly in hilar interneurons. Together these findings suggest that KA-induced motility of differentiated GCs contributes to the development of GCD and establish slice cultures as a model to study neuronal changes induced by epileptiform activity.

Secretory COPII coat component Sec23a is essential for craniofacial chondrocyte maturation.

An increasing number of human disorders have been linked to mutations in genes of the secretory pathway. The chemically induced zebrafish crusher variant results in malformed craniofacial skeleton, kinked pectoral fins and a short body length. By positional cloning, we identified a nonsense mutation converting leucine to a stop codon (L402X) in the sec23a gene, an integral component of the COPII complex, which is critical for anterograde protein trafficking between endoplasmic reticulum and Golgi apparatus. Zebrafish crusher mutants develop normally until the onset of craniofacial chondrogenesis. crusher chondrocytes accumulate proteins in a distended endoplasmic reticulum, resulting in severe reduction of cartilage extracellular matrix (ECM) deposits, including type II collagen. We demonstrate that the paralogous gene sec23b is also an essential component of the ECM secretory pathway in chondrocytes. In contrast, knockdown of the COPI complex does not hinder craniofacial morphogenesis. As SEC23A lesions cause the cranio-lenticulo-sutural dysplasia syndrome, crusher provides the first vertebrate model system that links the biology of endoplasmic reticulum to Golgi trafficking with a clinically relevant dysmorphology.

Differential GABAB-receptor-mediated effects in perisomatic- and dendrite-targeting parvalbumin interneurons.

Inhibitory parvalbumin-containing interneurons (PVIs) control neuronal discharge and support the generation of theta- and gamma-frequency oscillations in cortical networks. Fast GABAergic input onto PVIs is crucial for their synchronization and oscillatory entrainment, but the role of metabotropic GABA(B) receptors (GABA(B)Rs) in mediating slow presynaptic and postsynaptic inhibition remains unknown. In this study, we have combined high-resolution immunoelectron microscopy, whole-cell patch-clamp recording, and computational modeling to investigate the subcellular distribution and effects of GABA(B)Rs and their postsynaptic effector Kir3 channels in rat hippocampal PVIs. Pre-embedding immunogold labeling revealed that the receptors and channels localize at high levels to the extrasynaptic membrane of parvalbumin-immunoreactive dendrites. Immunoreactivity for GABA(B)Rs was also present at lower levels on PVI axon terminals. Whole-cell recordings further showed that synaptically released GABA in response to extracellular stimulation evokes large GABA(B)R-mediated slow IPSCs in perisomatic-targeting (PT) PVIs, but only small or no currents in dendrite-targeting (DT) PVIs. In contrast, paired recordings demonstrated that GABA(B)R activation results in presynaptic inhibition at the output synapses of both PT and DT PVIs, but more strongly in the latter. Finally, computational analysis indicated that GABA(B) IPSCs can phasically modulate the discharge of PT interneurons at theta frequencies. In summary, our results show that GABA(B)Rs differentially mediate slow presynaptic and postsynaptic inhibition in PVIs and can contribute to the dynamic modulation of their activity during oscillations. Furthermore, these data provide evidence for a compartment-specific molecular divergence of hippocampal PVI subtypes, suggesting that activation of GABA(B)Rs may shift the balance between perisomatic and dendritic inhibition.

Low density lipoprotein receptor-related protein 1 (LRP1) modulates N-methyl-D-aspartate (NMDA) receptor-dependent intracellular signaling and NMDA-induced regulation of postsynaptic protein complexes.

The lipoprotein receptor LRP1 is essential in neurons of the central nervous system, as was revealed by the analysis of conditional Lrp1-deficient mouse models. The molecular basis of its neuronal functions, however, is still incompletely understood. Here we show by immunocytochemistry, electron microscopy, and postsynaptic density preparation that LRP1 is located postsynaptically. Basal and NMDA-induced phosphorylation of the transcription factor cAMP-response element-binding protein (CREB) as well as NMDA target gene transcription are reduced in LRP1-deficient neurons. In control neurons, NMDA promotes γ-secretase-dependent release of the LRP1 intracellular domain (LRP1-ICD). However, pull-down and chromatin immunoprecipitation (ChIP) assays showed no direct interaction between the LRP1-ICD and either CREB or target gene promoters. On the other hand, NMDA-induced degradation of the postsynaptic scaffold protein PSD-95 was impaired in the absence of LRP1, whereas its ubiquitination was increased, indicating that LRP1 influences the composition of postsynaptic protein complexes. Accordingly, NMDA-induced internalization of the AMPA receptor subunit GluA1 was impaired in LRP1-deficient neurons. These results show a role of LRP1 in the regulation and turnover of synaptic proteins, which may contribute to the reduced dendritic branching and to the neurological phenotype observed in the absence of LRP1.

Tau's role in the developing brain: implications for intellectual disability.

Microdeletions encompassing the MAPT (Tau) locus resulting in intellectual disability raised the hypothesis that Tau may regulate early functions in the developing brain. Our results indicate that neuronal migration was inhibited in mouse brains following Tau reduction. In addition, the leading edge of radially migrating neurons was aberrant in spite of normal morphology of radial glia. Furthermore, intracellular mitochondrial transport and morphology were affected. In early postnatal brains, a portion of Tau knocked down neurons reached the cortical plate. Nevertheless, they exhibited far less developed dendrites and a striking reduction in connectivity evident by the size of boutons. Our novel results strongly implicate MAPT as a dosage-sensitive gene in this locus involved in intellectual disability. Furthermore, our results are likely to impact our understanding of other diseases involving Tau.

Cellular correlate of assembly formation in oscillating hippocampal networks in vitro.

Neurons form transiently stable assemblies that may underlie cognitive functions, including memory formation. In most brain regions, coherent activity is organized by network oscillations that involve sparse firing within a well-defined minority of cells. Despite extensive work on the underlying cellular mechanisms, a fundamental question remains unsolved: how are participating neurons distinguished from the majority of nonparticipators? We used physiological and modeling techniques to analyze neuronal activity in mouse hippocampal slices during spontaneously occurring high-frequency network oscillations. Network-entrained action potentials were exclusively observed in a defined subset of pyramidal cells, yielding a strict distinction between participating and nonparticipating neurons. These spikes had unique properties, because they were generated in the axon without prior depolarization of the soma. GABA(A) receptors had a dual role in pyramidal cell recruitment. First, the sparse occurrence of entrained spikes was accomplished by intense perisomatic inhibition. Second, antidromic spike generation was facilitated by tonic effects of GABA in remote axonal compartments. Ectopic spike generation together with strong somatodendritic inhibition may provide a cellular mechanism for the definition of oscillating assemblies.

Role of the postnatal radial glial scaffold for the development of the dentate gyrus as revealed by Reelin signaling mutant mice.

During dentate gyrus development, the early embryonic radial glial scaffold is replaced by a secondary glial scaffold around birth. In contrast to neocortical and early dentate gyrus radial glial cells, these postnatal glial cells are severely altered with regard to position and morphology in reeler mice lacking the secreted protein Reelin. In this study, we focus on the functional impact of these defects. Most radial glial cells throughout the nervous system serve as scaffolds for migrating neurons and precursor cells for both neurogenesis and gliogenesis. Precursor cell function has been demonstrated for secondary radial glial cells but the exact function of these late glial cells in granule cell migration and positioning is not clear. No data exist concerning the interplay between granule neurons and late radial glial cells during dentate gyrus development. Herein, we show that despite the severe morphological defects in the reeler dentate gyrus, the precursor function of secondary radial glial cells is not impaired during development in reeler mice. In addition, selective ablation of Disabled-1, an intracellular adaptor protein essential for Reelin signaling, in neurons but not in glial cells allowed us to distinguish effects of Reelin signaling on radial glial cells from possible secondary effects based on defective granule cells positioning.

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.

Selective degeneration of septal and hippocampal GABAergic neurons in a mouse model of amyloidosis and tauopathy.

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by brain accumulation of amyloid-ß peptide and neurofibrillary tangles, which are believed to initiate a pathological cascade that results in progressive impairment of cognitive functions and eventual neuronal death. To obtain a mouse model displaying the typical AD histopathology of amyloidosis and tauopathy, we generated a triple-transgenic mouse line (TauPS2APP) by overexpressing human mutations of the amyloid precursor protein, presenilin2 and tau genes. Stereological analysis of TauPS2APP mice revealed significant neurodegeneration of GABAergic septo-hippocampal projection neurons as well as their target cells, the GABAergic hippocampal interneurons. In contrast, the cholinergic medial septum neurons remained unaffected. Moreover, the degeneration of hippocampal GABAergic interneurons was dependent on the hippocampal subfield and interneuronal subtype investigated, whereby the dentate gyrus and the NPY-positive interneurons, respectively, were most strongly affected. Neurodegeneration was also accompanied by a change in the mRNA expression of markers for inhibitory interneurons. In line with the loss of inhibitory neurons, we observed functional changes in TauPS2APP mice relative to WT mice, with strongly enhanced long-term potentiation in the medial-perforant pathway input to the dentate gyrus, and stereotypic hyperactivity. Our data indicate that inhibitory neurons are the targets of neurodegeneration in a mouse model of amyloidosis and tauopathy, thus pointing to a possible role of the inhibitory network in the pathophysiological and functional cascade of Alzheimer's disease.

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.