Characterization of the various forms of the Reelin protein in the cerebrospinal fluid of normal subjects and in neurological diseases

Central repeat fragment of reelin leads to active reelin intracellular signaling and rescues cognitive deficits in a mouse model of reelin deficiency

Reelin and its receptor, ApoER2, play important roles in prenatal brain development and postnatally in synaptic plasticity, learning, and memory. Previous reports suggest that reelin's central fragment binds to ApoER2 and receptor clustering is involved in subsequent intracellular signaling. However, limitations of currently available assays have not established cellular evidence of ApoER2 clustering upon binding of the central reelin fragment. In the present study, we developed a novel, cell-based assay of ApoER2 dimerization using a "split-luciferase" approach. Specifically, cells were co-transfected with one recombinant ApoER2 receptor fused to the N-terminus of luciferase and one ApoER2 receptor fused to the C-terminus of luciferase. Using this assay, we directly observed basal ApoER2 dimerization/clustering in transfected HEK293T cells and, significantly, an increase in ApoER2 clustering in response to that central fragment of reelin. Furthermore, the central fragment of reelin activated intracellular signal transduction of ApoER2, indicated by increased levels of phosphorylation of Dab1, ERK1/2, and Akt in primary cortical neurons. Functionally, we were able to demonstrate that injection of the central fragment of reelin rescued phenotypic deficits observed in the heterozygous reeler mouse. These data are the first to test the hypothesis that the central fragment of reelin contributes to facilitating the reelin intracellular signaling pathway through receptor clustering.

The role of reelin in the pathological mechanism of depression from clinical to rodents

Major depressive disorder (MDD) is a devastating mental illness and the leading cause of disability worldwide. Previous studies have suggested that synaptic plasticity in the hippocampus plays an important role in depression pathogenesis. Reelin is expressed mainly in the frontal lobe and hippocampus, and is closely associated with neurodevelopment and synaptic plasticity. However, few studies have investigated its role in MDD combining clinical trials and animal experiments. We show that in a clinical trial, plasma reelin levels decreased in patients with first-episode drug-naïve MDD and increased after treatment; further, plasma reelin levels allowed to distinguish drug-naïve patients with first-episode MDD from healthy individuals. In rats, chronic mild and unpredictable stress led to a decrease in both reelin mRNA and protein levels in the hippocampus, which could be reversed by vortioxetine. Subsequent experiments confirmed that the reelin-ApoER2-NR2A /NR2B pathway regulates hippocampal synaptic plasticity and may be involved in depression or antidepressant responses. Our work contributes to a deeper understanding of MDD pathogenesis and provides new evidence that reelin should be considered a potential therapeutic target for MDD.

Functional Diversity of Neuronal Cell Adhesion and Recognition Molecule L1CAM through Proteolytic Cleavage

The neuronal cell adhesion and recognition molecule L1 does not only 'keep cells together' by way of homophilic and heterophilic interactions, but can also promote cell motility when cleaved into fragments by several proteases. It has largely been thought that such fragments are signs of degradation. Now, it is clear that proteolysis contributes to the pronounced functional diversity of L1, which we have reviewed in this work. L1 fragments generated at the plasma membrane are released into the extracellular space, whereas other membrane-bound fragments are internalised and enter the nucleus, thus conveying extracellular signals to the cell interior. Post-translational modifications on L1 determine the sequence of cleavage by proteases and the subcellular localisation of the generated fragments. Inside the neuronal cells, L1 fragments interact with various binding partners to facilitate morphogenic events, as well as regenerative processes. The stimulation of L1 proteolysis via injection of L1 peptides or proteases active on L1 or L1 mimetics is a promising tool for therapy of injured nervous systems. The collective findings gathered over the years not only shed light on the great functional diversity of L1 and its fragments, but also provide novel mechanistic insights into the adhesion molecule proteolysis that is active in the developing and diseased nervous system.

Reelin central fragment supplementation improves cognitive deficits in a mouse model of Fragile X Syndrome

Fragile X Syndrome (FXS) is the most common form of inherited intellectual disability and is characterized by autistic behaviors, childhood seizures, and deficits in learning and memory. FXS has a loss of function of the FMR1 gene that leads to a lack of Fragile X Mental Retardation Protein (FMRP) expression. FMRP is critical for synaptic plasticity, spatial learning, and memory. Reelin is a large extracellular glycoprotein essential for synaptic plasticity and numerous neurodevelopmental processes. Reduction in Reelin signaling is implicated as a contributing factor in disease etiology in several neurological disorders, including schizophrenia, and autism. However, the role of Reelin in FXS is poorly understood. We demonstrate a reduction in Reelin in Fmr1 knock-out (KO) mice, suggesting that a loss of Reelin activity may contribute to FXS. We demonstrate here that Reelin signaling enhancement via a single intracerebroventricular injection of the Reelin central fragment into Fmr1 KO mice can profoundly rescue cognitive deficits in hidden platform water maze and fear conditioning, as well as hyperactivity during the open field. Improvements in behavior were associated with rescued levels of post synaptic marker in Fmr1 KO mice when compared to controls. These data suggest that increasing Reelin signaling in FXS could offer a novel therapeutic for improving cognition in FXS.

Altered Balance of Reelin Proteolytic Fragments in the Cerebrospinal Fluid of Alzheimer's Disease Patients

Reelin binds to the apolipoprotein E receptor apoER2 to activate an intracellular signaling cascade. The proteolytic cleavage of reelin follows receptor binding but can also occur independently of its binding to receptors. This study assesses whether reelin proteolytic fragments are differentially affected in the cerebrospinal fluid (CSF) of Alzheimer's disease (AD) subjects. CSF reelin species were analyzed by Western blotting, employing antibodies against the N- and C-terminal domains. In AD patients, we found a decrease in the 420 kDa full-length reelin compared with controls. In these patients, we also found an increase in the N-terminal 310 kDa fragment resulting from the cleavage at the so-called C-t site, whereas the 180 kDa fragment originated from the N-t site remained unchanged. Regarding the C-terminal proteolytic fragments, the 100 kDa fragment resulting from the cleavage at the C-t site also displayed increased levels, whilst the one resulting from the N-t site, the 250 kDa fragment, decreased. We also detected the presence of an aberrant reelin species with a molecular mass of around 500 kDa present in AD samples (34 of 43 cases), while it was absent in the 14 control cases analyzed. These 500 kDa species were only immunoreactive to N-terminal antibodies. We validated the occurrence of these aberrant reelin species in an Aβ42-treated reelin-overexpressing cell model. When we compared the AD samples from APOE genotype subgroups, we only found minor differences in the levels of reelin fragments associated to the APOE genotype, but interestingly, the levels of fragments of apoER2 were lower in APOE ε4 carriers with regards to APOE ε3/ε3. The altered proportion of reelin/apoER2 fragments and the occurrence of reelin aberrant species suggest a complex regulation of the reelin signaling pathway, which results impaired in AD subjects.

SCO-spondin, a giant matricellular protein that regulates cerebrospinal fluid activity

Cerebrospinal fluid is a clear fluid that occupies the ventricular and subarachnoid spaces within and around the brain and spinal cord. Cerebrospinal fluid is a dynamic signaling milieu that transports nutrients, waste materials and neuroactive substances that are crucial for the development, homeostasis and functionality of the central nervous system. The mechanisms that enable cerebrospinal fluid to simultaneously exert these homeostatic/dynamic functions are not fully understood. SCO-spondin is a large glycoprotein secreted since the early stages of development into the cerebrospinal fluid. Its domain architecture resembles a combination of a matricellular protein and the ligand-binding region of LDL receptor family. The matricellular proteins are a group of extracellular proteins with the capacity to interact with different molecules, such as growth factors, cytokines and cellular receptors; enabling the integration of information to modulate various physiological and pathological processes. In the same way, the LDL receptor family interacts with many ligands, including β-amyloid peptide and different growth factors. The domains similarity suggests that SCO-spondin is a matricellular protein enabled to bind, modulate, and transport different cerebrospinal fluid molecules. SCO-spondin can be found soluble or polymerized into a dynamic threadlike structure called the Reissner fiber, which extends from the diencephalon to the caudal tip of the spinal cord. Reissner fiber continuously moves caudally as new SCO-spondin molecules are added at the cephalic end and are disaggregated at the caudal end. This movement, like a conveyor belt, allows the transport of the bound molecules, thereby increasing their lifespan and action radius. The binding of SCO-spondin to some relevant molecules has already been reported; however, in this review we suggest more than 30 possible binding partners, including peptide β-amyloid and several growth factors. This new perspective characterizes SCO-spondin as a regulator of cerebrospinal fluid activity, explaining its high evolutionary conservation, its apparent multifunctionality, and the lethality or severe malformations, such as hydrocephalus and curved body axis, of knockout embryos. Understanding the regulation and identifying binding partners of SCO-spondin are crucial for better comprehension of cerebrospinal fluid physiology.

Determination of terminal glycan and total monosaccharide profiles of reelin glycoprotein in SH-SY5Y neuroblastoma cell line by lectin blotting and capillary liquid chromatography electrospray ionization-ion trap tandem mass spectrometry system

Reelin (400 kDa) is an extracellular matrix glycoprotein that is a key regulator of the many significant biological processes including the brain formation, cell aggregation, and dendrite formation. The glycosylation contributes to the nature of the protein through folding, localization and trafficking, solubility, antigenicity, biological activity, and half-life. Although reelin is to be known as a glycoprotein, the knowledge of its glycosylation is very limited. In this study, we aimed to characterize the terminal glycan profile of reelin by lectin blotting and monosaccharide analysis of glycan chains by capillary liquid chromatography electrospray ionization ion trap tandem mass spectrometry (CapLC-ESI-MS/MS) in SH-SY5Y neuroblastoma cell line. According to our results, reelin was detected in different protein fragments (310, 250, and 85 kDa) in addition to full-length form (400 kDa) in the cell line. The reelin glycoprotein was found to carry the β-N-Acetylglucosamine, α-Mannose, β-Galactose, and α-2,3 and α2,6 linked sialic acids by lectin blotting. Nevertheless, these terminal monosaccharides were found in different intensity according to reelin fragments. Besides, we purified a reelin fragment (250 kDa), and we analyzed it for their monosaccharide by CapLC-ESI-MS/MS. We found that reelin contained five types of monosaccharides, which were consisted of N-Acetylgalactosamine, N-Acetylglucosamine, Galactose, Glucose, Mannose and Sialic acid, from high to low abundance respectively. The present results provide a valuable guide for biochemical, genetic, and glycobiology based further experiments about reelin glycosylation in cancer perspective.

Sex-dimorphic effects of biogenesis of lysosome-related organelles complex-1 deficiency on mouse perinatal brain development

The function(s) of the Biogenesis of Lysosome-related Organelles Complex-1 (BLOC-1) during brain development is to date largely unknown. Here, we investigated how its absence alters the trajectory of postnatal brain development using as model the pallid mouse. Most of the defects observed early postnatally in the mutant mice were more prominent in males than in females and in the hippocampus. Male mutant mice, but not females, had smaller brains as compared to sex-matching wild types at postnatal day 1 (P1), this deficit was largely recovered by P14 and P45. An abnormal cytoarchitecture of the pyramidal cell layer of the hippocampus was observed in P1 pallid male, but not female, or juvenile mice (P45), along with severely decreased expression levels of the radial glial marker Glutamate-Aspartate Transporter. Transcriptomic analyses showed that the overall response to the lack of functional BLOC-1 was more pronounced in hippocampi at P1 than at P45 or in the cerebral cortex. These observations suggest that absence of BLOC-1 renders males more susceptible to perinatal brain maldevelopment and although most abnormalities appear to have been resolved in juvenile animals, still permanent defects may be present, resulting in faulty neuronal circuits, and contribute to previously reported cognitive and behavioral phenotypes in adult BLOC-1-deficient mice.

Disease-Specific Changes in Reelin Protein and mRNA in Neurodegenerative Diseases

Reelin is an extracellular glycoprotein that modulates neuronal function and synaptic plasticity in the adult brain. Decreased levels of Reelin activity have been postulated as a key factor during neurodegeneration in Alzheimer’s disease (AD) and in aging. Thus, changes in levels of full-length Reelin and Reelin fragments have been revealed in cerebrospinal fluid (CSF) and in post-mortem brains samples of AD patients with respect to non-AD patients. However, conflicting studies have reported decreased or unchanged levels of full-length Reelin in AD patients compared to control (nND) cases in post-mortem brains and CSF samples. In addition, a compelling analysis of Reelin levels in neurodegenerative diseases other than AD is missing. In this study, we analyzed brain levels of RELN mRNA and Reelin protein in post-mortem frontal cortex samples from different sporadic AD stages, Parkinson’s disease with dementia (PDD), and Creutzfeldt-Jakob disease (sCJD), obtained from five different Biobanks. In addition, we measured Reelin protein levels in CSF samples of patients with mild cognitive impairment (MCI), dementia, or sCJD diagnosis and a group of neurologically healthy cases. The results indicate an increase in RELN mRNA in the frontal cortex of advanced stages of AD and in sCJD(I) compared to controls. This was not observed in PDD and early AD stages. However, Reelin protein levels in frontal cortex samples were unchanged between nND and advanced AD stages and PDD. Nevertheless, they decreased in the CSF of patients with dementia in comparison to those not suffering with dementia and patients with MCI. With respect to sCJD, there was a tendency to increase in brain samples in comparison to nND and to decrease in the CSF with respect to nND. In conclusion, Reelin levels in CSF cannot be considered as a diagnostic biomarker for AD or PDD. However, we feel that the CSF Reelin changes observed between MCI, patients with dementia, and sCJD might be helpful in generating a biomarker signature in prodromal studies of unidentified dementia and sCJD.

Reelin Immunoreactivity in the Adult Spinal Cord: A Comparative Study in Rodents, Carnivores, and Non-human Primates

Reelin is a large extracellular matrix (ECM) glycoprotein secreted by several neuronal populations in a specific manner in both the developing and the adult central nervous system. The extent of Reelin protein distribution and its functional role in the adult neocortex is well documented in different mammal models. However, its role in the adult spinal cord has not been well characterized and its distribution in the rodent spinal cord is fragmentary and has not been investigated in carnivores or primates as of yet. To gain insight into which neuronal populations and specific circuits may be influenced by Reelin in the adult spinal cord, we have conducted light and confocal microscopy study analysis of Reelin-immunoreactive cell types in the adult spinal cord. Here, we describe and compare Reelin immunoreactive cell type and distribution in the spinal cord of adult non-human primate (macaque monkeys, Macaca mulatta), carnivore (ferret, Mustela putorius) and rodent (rat, Rattus norvegicus). Our results show that in all three species studied, Reelin-immunoreactive neurons are present in the intermediate gray matter, ventricular zone and superficial dorsal horn and intermedio-lateral nucleus, while positive cells in the Clarke nucleus are only found in rats and primates. In addition, Reelin intermediolateral neurons colocalize with choline acetyltransferase (ChAT) only in macaque whilst motor neurons also colocalize Reelin and ChAT in macaque, ferret and rat spinal cord. The different expression patterns might reflect a differential role for Reelin in the pathways involved in the coordination of locomotor activity in the fore- and hind limbs.

CSF-ApoER2 fragments as a read-out of reelin signaling: Distinct patterns in sporadic and autosomal-dominant Alzheimer disease

Reelin is a glycoprotein associated with synaptic plasticity and neurotransmission. The malfunctioning of reelin signaling in the brain is likely to contribute to the pathogenesis of Alzheimer's disease (AD). Reelin binding to Apolipoprotein E receptor 2 (ApoER2) activates downstream signaling and induces the proteolytic cleavage of ApoER2, resulting in the generation of soluble fragments. To evaluate the efficiency of reelin signaling in AD, we have quantified the levels of reelin and soluble ectodomain fragments of ApoER2 (ectoApoER2) in the cerebrospinal fluid (CSF). CSF from sporadic AD patients (sAD; n = 14, age 54-83 years) had lower levels of ecto-ApoER2 (~31% reduction; p = .005) compared to those in the age-matched controls (n = 10, age 61-80), and a higher reelin/ecto-ApoER2 ratio. In contrast, autosomal dominant AD patients, carriers of PSEN1 mutations (ADAD; n = 7, age 31-49 years) had higher ecto-ApoER2 levels (~109% increment; p = .001) and a lower reelin/ecto-ApoER2 ratio than the non-mutation carriers from the same families (n = 7, age 25-47 years). Our data suggest that the levels of ecto-ApoER2 in CSF could be a suitable read-out of an impaired reelin signaling in AD, but also indicate differences between sAD and ADAD.

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

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

SIGNIFICANCE STATEMENT

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

Reelin Signaling Inactivates Cofilin to Stabilize the Cytoskeleton of Migrating Cortical Neurons

Neurons are highly polarized cells. They give rise to several dendrites but only one axon. In addition, many neurons show a preferred orientation. For example, pyramidal neurons of the cerebral cortex extend their apical dendrites toward the cortical surface while their axons run in opposite direction toward the white matter. This characteristic orientation reflects the migratory trajectory of a pyramidal cell during cortical development: the leading process (the future apical dendrite) extends toward the marginal zone (MZ) and the trailing process (the future axon) toward the intermediate zone (IZ) while the cells migrate radially to reach their destination in the cortical plate (CP). In this review article, we summarize the function of Reelin, an extracellular matrix protein synthesized by Cajal-Retzius cells in the MZ, in the development of the characteristic orientation of the leading processes running perpendicular to the cortical surface. Reelin promotes migration toward the cortical surface since late-generated cortical neurons in the reeler mutant are unable to reach upper cortical layers. Likewise, Reelin is important for the orientation and maintenance of the leading processes of migrating neurons since they are misoriented in the developing reeler cortex, as are the apical dendrites of pyramidal cells in the mature mutant. Reelin-induced phosphorylation of cofilin, an actin-associated protein, is crucial since pyramidal neurons transfected by in utero electroporation (IUE) with a non-phosphorylatable form of cofilin (cofilin^S3A) show severe migration defects reminiscent of those in the reeler mutant. Remarkably, migration of neurons in the cortex of reeler mice was partially rescued by transfecting them with LIM kinase 1 (LIMK1), the kinase that induces phosphorylation of cofilin at serine3, or with a pseudo-phosphorylated cofilin mutant (cofilin^S3E). Together these results indicate that Reelin-induced phosphorylation of cofilin is an important component in the orientation and directed migration of cortical neurons and in their correct lamination.

How does Reelin signaling regulate the neuronal cytoskeleton during migration?

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

Increased Blood-Reelin-Levels in First Episode Schizophrenia

Background

Reelin is an extracellular glycoprotein involved in several functions of brain development, synaptogenesis and dendritic proliferation. Numerous studies found perturbation in the reelin system and altered serum reelin levels in neuropsychiatric patients using the western blot procedure. In the international literature, this is the first study that made use of an enzyme-linked immunosorbent assay to analyze serum reelin protein concentration quantitatively.

Rationale

In order to study possible alterations in reelin blood levels in schizophrenia, we analyzed this signal in schizophrenic patients with a first episode hallucinatory and paranoid syndrome and control subjects in a pilot study design.

Results

We found increased blood reelin protein concentration in schizophrenic patients compared to healthy controls.

Discussion

Our findings point to a relevant role of reelin metabolism in the pathogenesis of schizophrenia.Reelin could be a biomarker for the course of disease or psychopharmacological treatment.

Conclusion

We conclude that the reelin protein blood concentration might be a relevant signal with respect to the pathophysiology of schizophrenia.

Reelin immunoreactivity in neuritic varicosities in the human hippocampal formation of non-demented subjects and Alzheimer's disease patients

BACKGROUND

Reelin and its downstream signaling members are important modulators of actin and microtubule cytoskeleton dynamics, a fundamental prerequisite for proper neurodevelopment and adult neuronal functions. Reductions in Reelin levels have been suggested to contribute to Alzheimer's disease (AD) pathophysiology. We have previously reported an age-related reduction in Reelin levels and its accumulation in neuritic varicosities along the olfactory-limbic tracts, which correlated with cognitive impairments in aged mice. Here, we aimed to investigate whether a similar Reelin-associated neuropathology is observed in the aged human hippocampus and whether it correlated with dementia status.

RESULTS

Our immunohistochemical stainings revealed the presence of N- and C-terminus-containing Reelin fragments in corpora amylacea (CAm), aging-associated spherical deposits. The density of these deposits was increased in the molecular layer of the subiculum of AD compared to non-demented individuals. Despite the limitation of a small sample size, our evaluation of several neuronal and glial markers indicates that the presence of Reelin in CAm might be related to aging-associated impairments in neuronal transport leading to accumulation of organelles and protein metabolites in neuritic varicosities, as previously suggested by the findings and discussions in rodents and primates.

CONCLUSIONS

Our results indicate that aging- and disease-associated changes in Reelin levels and proteolytic processing might play a role in the formation of CAm by altering cytoskeletal dynamics. However, its presence may also be an indicator of a degenerative state of neuritic compartments.

The involvement of Reelin in neurodevelopmental disorders

Reelin is a glycoprotein that serves important roles both during development (regulation of neuronal migration and brain lamination) and in adulthood (maintenance of synaptic function). A number of neuropsychiatric disorders including autism, schizophrenia, bipolar disorder, major depression, Alzheimer's disease and lissencephaly share a common feature of abnormal Reelin expression in the brain. Altered Reelin expression has been hypothesized to impair neuronal connectivity and synaptic plasticity, leading ultimately to the cognitive deficits present in these disorders. The mechanisms for abnormal Reelin expression in some of these disorders are currently unknown although possible explanations include early developmental insults, mutations, hypermethylation of the promoter for the Reelin gene (RELN), miRNA silencing of Reelin mRNA, FMRP underexpression and Reelin processing abnormalities. Increasing Reelin expression through pharmacological therapies may help ameliorate symptoms resulting from Reelin deficits. This article is part of the Special Issue entitled 'Neurodevelopmental Disorders'.

Decisive role of Reelin signaling during early stages of Alzheimer's disease

Alzheimer's disease (AD) is one of the largest unmet medical concerns of our society. Around 25 million patients worldwide together with their families are still waiting for an effective treatment. We have recently initiated a re-evaluation of our knowledge of the molecular and cellular mechanisms underlying sporadic AD. Based on the existing literature, we have proposed a mechanistic explanation of how the late-onset form of the disease may evolve on the cellular level. Here, we expand this hypothesis by addressing the pathophysiological changes underlying the early and almost invariant appearance of the neurofibrillary tangles, the only reliable correlate of the cognitive status, in distinct brain areas and their consistent "spread" along interconnected neurons as the disease advances. In this review we present and discuss novel evidence that the extracellular signaling protein Reelin, expressed along the olfactory and limbic pathways in the adult brain, might hold a key to understand the earliest steps of the disease, highlighting the olfactory pathway as the brain's Achilles heel involved in the initiation of the pathophysiological characteristic of late-onset AD.

TIMP-1 inhibits the proteolytic processing of Reelin in experimental epilepsy

Temporal lobe epilepsy is frequently associated with granule cell dispersion (GCD), an abnormal widening of the granule cell layer in the dentate gyrus. There is increasing evidence that a loss and the functional inactivation of the positional signal Reelin is involved in GCD formation. Reelin is synthesized and released by Cajal-Retzius cells and interneurons, and its function depends on proteolytic cleavage after secretion. Epileptic conditions impair Reelin processing by inhibition of matrix metalloprotease (MMP) activity and cause the extracellular accumulation of unprocessed Reelin. Here we investigated how epileptic conditions inhibit MMP activity. We used kainate (KA) treatment of organotypic hippocampal slice cultures as an epilepsy model and found a significant increase of tissue inhibitor of metalloproteases 1 (TIMP-1) levels and strongly enhanced TIMP-1 immunolabeling in hippocampal neurons. Functional inhibition of TIMP-1 prevented the KA-induced impairment of Reelin cleavage indicating that TIMP-1 inhibits MMP activity. Moreover, application of recombinant TIMP-1 alone was sufficient to impair Reelin processing and to induce GCD, similar to that observed after KA treatment. In summary, we present evidence that epileptic conditions inhibit MMP activity by up-regulation of endogenous TIMP-1, which in turn leads to extracellular accumulation of uncleaved and inactive Reelin and thereby to GCD.

Reelin sets the pace of neocortical neurogenesis

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

Early effects of lipopolysaccharide-induced inflammation on foetal brain development in rat

Studies in humans and animal models link maternal infection and imbalanced levels of inflammatory mediators in the foetal brain to the aetiology of neuropsychiatric disorders. In a number of animal models, it was shown that exposure to viral or bacterial agents during a period that corresponds to the second trimester in human gestation triggers brain and behavioural abnormalities in the offspring. However, little is known about the early cellular and molecular events elicited by inflammation in the foetal brain shortly after maternal infection has occurred. In this study, maternal infection was mimicked by two consecutive intraperitoneal injections of 200 μg of LPS (lipopolysaccharide)/kg to timed-pregnant rats at GD15 (gestational day 15) and GD16. Increased thickness of the CP (cortical plate) and hippocampus together with abnormal distribution of immature neuronal markers and decreased expression of markers for neural progenitors were observed in the LPS-exposed foetal forebrains at GD18. Such effects were accompanied by decreased levels of reelin and the radial glial marker GLAST (glial glutamate transporter), and elevated levels of pro-inflammatory cytokines in maternal serum and foetal forebrains. Foetal inflammation elicited by maternal injections of LPS has discrete detrimental effects on brain development. The early biochemical and morphological changes described in this work begin to explain the sequelae of early events that underlie the neurobehavioural deficits reported in humans and animals exposed to prenatal insults.

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.

Go or stop? Divergent roles of Reelin in radial neuronal migration

Neuronal migration is an essential step of brain development and is controlled by a variety of cellular proteins and extracellular matrix molecules. Reelin, an extracellular matrix protein, is required for neuronal migration. Over the past 10 years, the Reelin signaling cascade has been studied intensively. However, the role of Reelin in neuronal migration has remained unclear. Different Reelin fragments and different Reelin receptors suggest multiple functions of Reelin. In this review, the authors focus on Reelin effects on the actin cytoskeleton of migrating neurons.

Developmental emergence of reelin deficits in the prefrontal cortex of Wistar rats reared in social isolation

As the pathophysiological mechanism(s) of many neuropsychiatric disorders relate to GABAergic interneuron structure and function, we employed isolation rearing of Wistar rats as a model to correlate developmental emergence of cognitive deficits with the expression of reelin-producing interneurons in the medial prefrontal cortex (PFC). Prepulse inhibition deficits emerged at postnatal day 60 and persisted into adulthood. Paralleling the emergence of these neurobehavioural deficits was an increase in reelin production and reelin-immunopositive cells in layer I of the PFC and this later became significantly reduced at postnatal day 80. Cells expressing reelin immunoreactivity in a horizontal orientation were mainly located to the upper regions of layer I whereas those with a vertical orientation, whose arbors extend into cortical layers II and III, were more numerous in the lower regions of layer I and became significantly dysregulated during postnatal development. No behavioural deficits or altered reelin expression was observed at postnatal days 30 or 40. Developmental emergence of neurobehavioural and reelin deficits in isolation reared animals is proposed to reflect maladaptive wiring within the medial prefrontal cortex during a critical maturation period of this circuitry.

Processing of Reelin by embryonic neurons is important for function in tissue but not in dissociated cultured neurons

Reelin, the protein defective in reeler mutant mice, plays a key role during brain development. Reelin is processed proteolytically at two sites, and the central fragment mimics function in vitro. Here, we show that processing is functionally important in vivo, a question that could not be addressed in our previous study. New monoclonal antibodies directed against central Reelin block its binding to lipoprotein receptors and perturb cortical development in vitro, confirming the importance of the central fragment that is detected in tissue and body fluids. Processing occurs when Reelin is incubated with embryonic neurons in culture or with their supernatant, but inhibition of processing by a metalloproteinase blocker does not prevent Reelin signaling in neurons. Furthermore, neurons internalize similarly full-length or central Reelin. In contrast, inhibition of processing prevents signaling and perturbs cortical development in cultured embryonic brain slices. Moreover, in vivo, the concentration of central Reelin is dramatically and selectively increased in receptor-deficient tissue, suggesting its specific downregulation after binding to receptors and internalization. We propose that processing by end-migration neurons is required in tissue (where Reelin is likely anchored to the extracellular matrix) to release the central fragment that diffuses locally and signals to target cells, whereas, in vitro, all Reelin forms have indiscriminate access to cells, so that cleavage is not necessary for signaling.

Proteolytic cleavage of the neural cell adhesion molecule by ADAM17/TACE is involved in neurite outgrowth

The transmembrane and multidomain neural cell adhesion molecule (NCAM) plays important functional roles in the developing and adult nervous system. NCAM is proteolytically processed and appears in soluble forms in the cerebrospinal fluid and in serum under normal and pathological conditions. In this report, we present evidence that the metalloprotease a disintegrin and a metalloprotease (ADAM)17/tumour necrosis factor alpha converting enzyme (TACE) cleaves the polysialylated as well as the non-polysialylated transmembrane isoforms of NCAM, whereas the glycophosphatidylinositol-linked isoform of NCAM is not proteolytically cleaved. A truncated, enzymatically inactive mutant of TACE did not result in release of the NCAM110 cleavage product. Proteolytic cleavage was enhanced by a calmodulin-specific inhibitor and the actin-destabilizing agents cytochalasin D and latrunculin B. In contrast, the microtubule-stabilizing agent colchicine or microtubule-destabilizing agent paclitaxel did not affect the release of the 110-kDa fragment of NCAM. Neurite outgrowth from cerebellar microexplants was inhibited in the presence of the metalloprotease inhibitor GM 6001 on substrate-coated NCAM, but not on poly-l-lysine. Upon transfection of hippocampal neurones with an enzymatically inactive mutant of TACE, NCAM-stimulated neurite outgrowth was inhibited without affecting neurite outgrowth on poly-l-lysine, showing that proteolytic processing of NCAM by the metalloprotease TACE is involved in NCAM-mediated neurite outgrowth.

Extracellular matrix molecules and synaptic plasticity: immunomapping of intracellular and secreted Reelin in the adult rat brain

Reelin, a large extracellular matrix glycoprotein, is secreted by several neuron populations in the developing and adult rodent brain. Secreted Reelin triggers a complex signaling pathway by binding lipoprotein and integrin membrane receptors in target cells. Reelin signaling regulates migration and dendritic growth in developing neurons, while it can modulate synaptic plasticity in adult neurons. To identify which adult neural circuits can be modulated by Reelin-mediated signaling, we systematically mapped the distribution of Reelin in adult rat brain using sensitive immunolabeling techniques. Results show that the distribution of intracellular and secreted Reelin is both very widespread and specific. Some interneuron and projection neuron populations in the cerebral cortex contain Reelin. Numerous striatal neurons are weakly immunoreactive for Reelin and these cells are preferentially located in striosomes. Some thalamic nuclei contain Reelin-immunoreactive cells. Double-immunolabeling for GABA and Reelin reveals that the Reelin-immunoreactive cells in the visual thalamus are the intrinsic thalamic interneurons. High local concentrations of extracellular Reelin selectively outline several dendrite spine-rich neuropils. Together with previous mRNA data, our observations suggest abundant axoplasmic transport and secretion in pathways such as the retino-collicular tract, the entorhino-hippocampal ('perforant') path, the lateral olfactory tract or the parallel fiber system of the cerebellum. A preferential secretion of Reelin in these neuropils is consistent with reports of rapid, activity-induced structural changes in adult brain circuits.

Reelin expression and glycosylation patterns are altered in Alzheimer's disease

Reelin is a glycoprotein that is essential for the correct cytoarchitectonic organization of the developing CNS. Its function in the adult brain is less understood, although it has been proposed that Reelin is involved in signaling pathways linked to neurodegeneration. Here we analyzed Reelin expression in brains and cerebrospinal fluid (CSF) from Alzheimer's disease (AD) patients and nondemented controls. We found a 40% increase in the Reelin protein levels in the cortex of AD patients compared with controls. Similar increases were detected at the Reelin mRNA transcriptional level. This expression correlates with parallel increases in CSF but not in plasma samples. Next, we examined whether CSF Reelin levels were also altered in neurological diseases, including frontotemporal dementia, progressive supranuclear palsy, and Parkinson's disease. The Reelin 180-kDa band increased in all of the neurodegenerative disorders analyzed. Moreover, the 180-kDa Reelin levels correlated positively with Tau protein in CSF. Finally, we studied the pattern of Reelin glycosylation by using several lectins and the anti-HNK-1 antibody. Glycosylation differed in plasma and CSF. Furthermore, the pattern of Reelin lectin binding differed between the CSF of controls and in AD. Our results show that Reelin is up-regulated in the brain and CSF in several neurodegenerative diseases and that CSF and plasma Reelin have distinct cellular origins, thereby supporting that Reelin is involved in the pathogenesis of a number of neurodegenerative diseases.

Recent progress in understanding the role of Reelin in radial neuronal migration, with specific emphasis on the dentate gyrus

Ten years following identification of Reelin as the product of the gene mutated in reeler mice, the signalling pathway activated by Reelin is being progressively unravelled with the identification of lipoprotein receptors as reelin receptors, of the Dab1 adapter and of some other proximal components in target cells. However, we are still a long way from understanding the action of this complex protein during brain development and maturation. The present review is organized in two parts. First, we summarize our present understanding of Reelin signalling. Then, we review critically some cell biological mechanisms for the action of Reelin based on recent studies on the development of the dentate gyrus, which has proved an extremely useful and tractable model system.

Reelin-immunoreactivity in the hippocampal formation of 9-month-old wildtype mouse: effects of APP/PS1 genotype and ovariectomy

Reelin, an extracellular matrix protein has an important role in the migration, correct positioning and maturation of neurons during development. Though it is generally down-regulated in the postnatal period, expression of this large glycoprotein continues in the adult brain in some cell populations. In the present study, we examined the distribution of reelin-immunoreactivity (-ir) in the hippocampal formation of 9-month-old wildtype mice (WT). Then, reelin-ir in normal mice was compared to that of transgenic mice (APP/PS1) carrying mutated human APP and PS1 genes, which are linked to the familial form of Alzheimer's disease (AD). The APP/PS1 mice were additionally burdened with a second risk factor for AD, namely depletion of circulating gonadal hormones by ovariectomy (APP/PS1 + OVX). The analyses revealed that in adult WT reelin-ir is expressed by Cajal-Retzius cells and a subgroup of interneurons throughout the hippocampal formation. In addition, layer II projection neurons in the lateral entorhinal subfields are reelin-ir. Interestingly, ovariectomy decreases the number of reelin-ir cells in the hilus in WT mice, whereas AD-related genotype alone induces only a non-significant reduction. Unexpectedly, additional stress, e.g., depletion of gonadal hormones, does not aggravate the slight reduction in the reelin cell number in the APP/PS1 mice. We propose that the changes in normal reelin-ir are linked to disturbances in repair mechanisms in which APP/PS1 and gonadal hormones are involved and which are perturbed in neurodegenerative conditions, namely AD.

Reelin glycoprotein: structure, biology and roles in health and disease

Reelin glycoprotein is a secretory serine protease with dual roles in mammalian brain: embryologically, it guides neurons and radial glial cells to their corrected positions in the developing brain; in adult brain, Reelin is involved in a signaling pathway which underlies neurotransmission, memory formation and synaptic plasticity. Disruption of Reelin signaling pathway by mutations and selective hypermethylation of the Reln gene promoter or following various pre- or postnatal insults may lead to cognitive deficits present in neuropsychiatric disorders like autism or schizophrenia.