The central fragment of Reelin, generated by proteolytic processing in vivo, is critical to its function during cortical plate development

The Inflammation-Induced Dysregulation of Reelin Homeostasis Hypothesis of Alzheimer's Disease

Alzheimer's disease (AD) accounts for most dementia cases, but we lack a complete understanding of the mechanisms responsible for the core pathology associated with the disease (e.g., amyloid plaque and neurofibrillary tangles). Inflammation has been identified as a key contributor of AD pathology, with recent evidence pointing towards Reelin dysregulation as being associated with inflammation. Here we describe Reelin signaling and outline existing research involving Reelin signaling in AD and inflammation. Research is described pertaining to the inflammatory and immunological functions of Reelin before we propose a mechanism through which inflammation renders Reelin susceptible to dysregulation resulting in the induction and exacerbation of AD pathology. Based on this hypothesis, it is predicted that disorders of both inflammation (including peripheral inflammation and neuroinflammation) and Reelin dysregulation (including disorders associated with upregulated Reelin expression and disorders of Reelin downregulation) have elevated risk of developing AD. We conclude with a description of AD risk in various disorders involving Reelin dysregulation and inflammation.

Extracellular molecular signals shaping dendrite architecture during brain development

Proper growth and branching of dendrites are crucial for adequate central nervous system (CNS) functioning. The neuronal dendritic geometry determines the mode and quality of information processing. Any defects in dendrite development will disrupt neuronal circuit formation, affecting brain function. Besides cell-intrinsic programmes, extrinsic factors regulate various aspects of dendritic development. Among these extrinsic factors are extracellular molecular signals which can shape the dendrite architecture during early development. This review will focus on extrinsic factors regulating dendritic growth during early neuronal development, including neurotransmitters, neurotrophins, extracellular matrix proteins, contact-mediated ligands, and secreted and diffusible cues. How these extracellular molecular signals contribute to dendritic growth has been investigated in developing nervous systems using different species, different areas within the CNS, and different neuronal types. The response of the dendritic tree to these extracellular molecular signals can result in growth-promoting or growth-limiting effects, and it depends on the receptor subtype, receptor quantity, receptor efficiency, the animal model used, the developmental time windows, and finally, the targeted signal cascade. This article reviews our current understanding of the role of various extracellular signals in the establishment of the architecture of the dendrites.

Reelin Signaling in Neurodevelopmental Disorders and Neurodegenerative Diseases

Reelin is an extracellular matrix glycoprotein involved in neuronal migration during embryonic brain development and synaptic plasticity in the adult brain. The role of Reelin in the developing central nervous system has been extensively characterized. Indeed, a loss of Reelin or a disruption in its signaling cascade leads to neurodevelopmental defects and is associated with ataxia, intellectual disability, autism, and several psychiatric disorders. In the adult brain, Reelin is critically involved in neurogenesis and synaptic plasticity. Reelin's signaling potentiates glutamatergic and GABAergic neurotransmission, induces synaptic maturation, and increases AMPA and NMDA receptor subunits' expression and activity. As a result, there is a growing literature reporting that a loss of function and/or reduction of Reelin is implicated in numerous neurodegenerative diseases. The present review summarizes the current state of the literature regarding the implication of Reelin and Reelin-mediated signaling during aging and neurodegenerative disorders, highlighting Reelin as a possible target in the prevention or treatment of progressive neurodegeneration.

Reelin through the years: From brain development to inflammation

Reelin was originally identified as a regulator of neuronal migration and synaptic function, but its non-neuronal functions have received far less attention. Reelin participates in organ development and physiological functions in various tissues, but it is also dysregulated in some diseases. In the cardiovascular system, Reelin is abundant in the blood, where it contributes to platelet adhesion and coagulation, as well as vascular adhesion and permeability of leukocytes. It is a pro-inflammatory and pro-thrombotic factor with important implications for autoinflammatory and autoimmune diseases such as multiple sclerosis, Alzheimer's disease, arthritis, atherosclerosis, or cancer. Mechanistically, Reelin is a large secreted glycoprotein that binds to several membrane receptors, including ApoER2, VLDLR, integrins, and ephrins. Reelin signaling depends on the cell type but mostly involves phosphorylation of NF-κB, PI3K, AKT, or JAK/STAT. This review focuses on non-neuronal functions and the therapeutic potential of Reelin, while highlighting secretion, signaling, and functional similarities between cell types.

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.

In silico analysis of glycosylation pattern in 5th-6th repeat sequence of reelin glycoprotein

Reelin is an extracellular matrix glycoprotein that plays a key role in cortical development, maturation, synaptic plasticity, and memory formation in the adult mammalian brain. Glycosylation is a significant post- and co-translational modification of proteins. Although glycosylation contributes to the characteristic of proteins from their production to molecular interactions, the knowledge about the glycosylation pattern of reelin is very limited. In this study, we aimed to predict the potential glycosylation pattern of the 5^th-6^th repeat of central reelin fragment that responsible for their signaling, by using in silico methods. We found that the predicted glycosylation pattern of the 5^th-6^th repeat of human reelin was highly conserved between vertebrate species. However, this conservation was not observed in analyzed invertebrates. For the first time, we described the sites of glycosylation at a three-dimensional protein structure in human reelin. Because the sites were very closed to EGF-like repeats and receptor binding sites, they could contribute the interaction with a partner of reelin in addition to the effect of thermostability to protein. Many of the residues related glycosylation were also conserved in analyzed species. These findings may guide biochemical, genetic, and glycobiology base on further experiments about reelin glycosylation. The understanding of reelin glycosylation might change the point of view of treatment for many pathological conditions in neurodegenerative diseases such as Alzheimer's disease. Communicated by Ramaswamy H. Sarma.

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.

Tetrodotoxin prevents heat-shock induced granule cell dispersion in hippocampal slice cultures

Granule cell dispersion (GCD) has been associated as a pathological feature of temporal lobe epilepsy (TLE). Early-life epileptiform activity such as febrile seizures has been proposed to have a causal link to developing chronic TLE. During postnatal development, the hippocampus may be particularly vulnerable to hyperexcitability-induced insults since neuronal migration and differentiation are still ongoing in the hippocampus. Further, the extracellular matrix (ECM), here in particular the protein reelin, has been implicated in the pathophysiology of GCD. Thus, loss of reelin-expressing cells, Cajal-Retzius cells and subsets of interneurons, may be related to GCD. To study the possible role of febrile seizures, we previously induced GCD in vitro by subjecting hippocampal slice cultures to a transient heat-shock, which was not accompanied by loss of Cajal-Retzius cells. In order to examine the mechanisms involved in heat-shock induced GCD, the present study aimed to determine whether such dispersion could be prevented by blocking cellular electrical activity. Here we show that the extent of heat-shock induced GCD could be significantly reduced by treatment with the sodium channel blocker tetrodotoxin (TTX), suggesting that electrical activity is an important factor involved in heat-shock induced GCD.

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.

Considering the Role of Extracellular Matrix Molecules, in Particular Reelin, in Granule Cell Dispersion Related to Temporal Lobe Epilepsy

The extracellular matrix (ECM) of the nervous system can be considered as a dynamically adaptable compartment between neuronal cells, in particular neurons and glial cells, that participates in physiological functions of the nervous system. It is mainly composed of carbohydrates and proteins that are secreted by the different kinds of cell types found in the nervous system, in particular neurons and glial cells, but also other cell types, such as pericytes of capillaries, ependymocytes and meningeal cells. ECM molecules participate in developmental processes, synaptic plasticity, neurodegeneration and regenerative processes. As an example, the ECM of the hippocampal formation is involved in degenerative and adaptive processes related to epilepsy. The role of various components of the ECM has been explored extensively. In particular, the ECM protein reelin, well known for orchestrating the formation of neuronal layer formation in the cerebral cortex, is also considered as a player involved in the occurrence of postnatal granule cell dispersion (GCD), a morphologically peculiar feature frequently observed in hippocampal tissue from epileptic patients. Possible causes and consequences of GCD have been studied in various in vivo and in vitro models. The present review discusses different interpretations of GCD and different views on the role of ECM protein reelin in the formation of this morphological peculiarity.

Reelin Is Required for Maintenance of Granule Cell Lamination in the Healthy and Epileptic Hippocampus

One characteristic feature of mesial temporal lobe epilepsy is granule cell dispersion (GCD), a pathological widening of the granule cell layer in the dentate gyrus. The loss of the extracellular matrix protein Reelin, an important positional cue for neurons, correlates with GCD formation in MTLE patients and in rodent epilepsy models. Here, we used organotypic hippocampal slice cultures (OHSC) from transgenic mice expressing enhanced green fluorescent protein (eGFP) in differentiated granule cells (GCs) to monitor GCD formation dynamically by live cell video microscopy and to investigate the role of Reelin in this process. We present evidence that following treatment with the glutamate receptor agonist kainate (KA), eGFP-positive GCs migrated mainly toward the hilar region. In the hilus, Reelin-producing neurons were rapidly lost following KA treatment as shown in a detailed time series. Addition of recombinant Reelin fragments to the medium effectively prevented the KA-triggered movement of eGFP-positive GCs. Placement of Reelin-coated beads into the hilus of KA-treated cultures stopped the migration of GCs in a distance-dependent manner. In addition, quantitative Western blot analysis revealed that KA treatment affects the Reelin signal transduction pathway by increasing intracellular adaptor protein Disabled-1 synthesis and reducing the phosphorylation of cofilin, a downstream target of the Reelin pathway. Both events were normalized by addition of recombinant Reelin fragments. Finally, following neutralization of Reelin in healthy OHSC by incubation with the function-blocking CR-50 Reelin antibody, GCs started to migrate without any direction preference. Together, our findings demonstrate that normotopic position of Reelin is essential for the maintenance of GC lamination in the dentate gyrus and that GCD is the result of a local Reelin deficiency.

Reelin restricts dendritic growth of interneurons in the neocortex

Reelin is a large secreted glycoprotein that regulates neuronal migration, lamination and establishment of dendritic architecture in the embryonic brain. Reelin expression switches postnatally from Cajal-Retzius cells to interneurons. However, reelin function in interneuron development is still poorly understood. Here, we have investigated the role of reelin in interneuron development in the postnatal neocortex. To preclude early cortical migration defects caused by reelin deficiency, we employed a conditional reelin knockout (RelncKO) mouse to induce postnatal reelin deficiency. Induced reelin deficiency caused dendritic hypertrophy in distal dendritic segments of neuropeptide Y-positive (NPY+) and calretinin-positive (Calr+) interneurons, and in proximal dendritic segments of parvalbumin-positive (Parv+) interneurons. Chronic recombinant Reelin treatment rescued dendritic hypertrophy in Relncko interneurons. Moreover, we provide evidence that RelncKO interneuron hypertrophy is due to presynaptic GABABR dysfunction. Thus, GABABRs in RelncKO interneurons were unable to block N-type (Cav2.2) Ca2+ channels that control neurotransmitter release. Consequently, the excessive Ca2+ influx through AMPA receptors, but not NMDA receptors, caused interneuron dendritic hypertrophy. These findings suggest that reelin acts as a 'stop-growth-signal' for postnatal interneuron maturation.

Regulation of Reelin functions by specific proteolytic processing in the brain

The secreted glycoprotein Reelin plays important roles in both brain development and function. During development, Reelin regulates neuronal migration and dendrite development. In the mature brain, the glycoprotein is involved in synaptogenesis and synaptic plasticity. It has been suggested that Reelin loss or decreased function contributes to the onset and/or deterioration of neuropsychiatric diseases, including schizophrenia and Alzheimer's disease. While the molecular mechanisms underpinning Reelin function remain unclear, recent studies have suggested that the specific proteolytic cleavage of Reelin may play central roles in the embryonic and postnatal brain. In this review, we focus on Reelin proteolytic processing and review its potential physiological roles.

Structural studies of reelin N-terminal region provides insights into a unique structural arrangement and functional multimerization

The large, secreted glycoprotein reelin regulates embryonic brain development as well as adult brain functions. Although reelin binds to its receptors via its central part, the N-terminal region directs multimer formation and is critical for efficient signal transduction. In fact, the inhibitory antibody CR-50 interacts with the N-terminal region and prevents higher-order multimerization and signalling. Reelin is a multidomain protein in which the central part is composed of eight characteristic repeats, named reelin repeats, each of which is further divided by insertion of a epidermal growth factor (EGF) module into two subrepeats. In contrast, the N-terminal region shows unique 'irregular' domain architecture since it comprises three consecutive subrepeats without the intervening EGF module. Here, we determined the crystal structure of the murine reelin fragment named RX-R1 including the irregular region and the first reelin repeat at 2.0-Å resolution. The overall structure of RX-R1 has a branched Y-shaped form. Interestingly, two incomplete subrepeats cooperatively form one entire subrepeat structure, though an additional subrepeat is inserted between them. We further reveal that Arg335 of RX-R1 is crucial for binding CR-50. A possible self-association mechanism via the N-terminal region is proposed based on our results.

The structure-function relationship of a signaling-competent, dimeric Reelin fragment

Reelin operates through canonical and non-canonical pathways that mediate several aspects of brain development and function. Reelin's dimeric central fragment (CF), generated through proteolytic cleavage, is required for the lipoprotein-receptor-dependent canonical pathway activation. Here, we analyze the signaling properties of a variety of Reelin fragments and measure the differential binding affinities of monomeric and dimeric CF fragments to lipoprotein receptors to investigate the mode of canonical signal activation. We also present the cryoelectron tomography-solved dimeric structure of Reelin CF and support it using several other biophysical techniques. Our findings suggest that Reelin CF forms a covalent parallel dimer with some degree of flexibility between the two protein chains. As a result of this conformation, Reelin binds to lipoprotein receptors in a manner inaccessible to its monomeric form and is capable of stimulating canonical pathway signaling.

Purification of a heterodimeric Reelin construct to investigate binding stoichiometry

Reelin is a secreted glycoprotein that is integral in neocortex development and synaptic function. Reelin exists as a homodimer with two chains linked by a disulfide bond at cysteine 2101, a feature that is vital to the protein's function. This is highlighted by the fact that only dimeric Reelin can elicit efficient, canonical signaling, even though a mutated (C2101A) monomeric construct of Reelin retains the capacity to bind to its receptors. Receptor clustering has been shown to be important in the signaling pathway, however direct evidence regarding the stoichiometry of Reelin-receptor binding interaction is lacking. Here we describe the construction and purification of a heterodimeric Reelin construct to investigate the stoichiometry of Reelin-receptor binding and how it affects Reelin pathway signaling. We have devised different strategies and have finalized a protocol to produce a heterodimer of Reelin's central fragment using differential tagging and tandem affinity chromatography, such that chain A is wild type in amino acid sequence whereas chain B includes a receptor binding site mutation (K2467A). We also validate that the heterodimer is capable of binding to the extracellular domain of one of Reelin's known receptors, calculating the KD of the interaction. This heterodimeric construct will enable us to understand in greater detail the mechanism by which Reelin interacts with its known receptors and initiates pathway signaling.

ApoER2: Functional Tuning Through Splicing

Alternative splicing occurs in over 95% of protein-coding genes and contributes to the diversity of the human proteome. Apolipoprotein E receptor 2 (apoER2) is a critical modulator of neuronal development and synaptic plasticity in the brain and is enriched in cassette exon splicing events, in which functional exons are excluded from the final transcript. These alternative splicing events affect apoER2 function, as individual apoER2 exons tend to encode distinct protein functional domains. Although several apoER2 splice variants have been characterized, much work remains to understand how apoER2 splicing events modulate distinct apoER2 activities, including ligand binding specificity, synapse formation and plasticity. Additionally, little is known about how apoER2 splicing events are regulated. Often, alternative splicing events are regulated through the combinatorial action of RNA-binding proteins and other epigenetic mechanisms, however, the regulatory pathways corresponding to each specific exon are unknown in most cases. In this mini-review, we describe the structure of apoER2, highlight the unique functions of known isoforms, discuss what is currently known about the regulation of apoER2 splicing by RNA-binding proteins and pose new questions that will further our understanding of apoER2 splicing complexity.

Purification of an Intact Human Protein Overexpressed from Its Endogenous Locus via Direct Genome Engineering

The overproduction and purification of human proteins is a requisite of both basic and medical research. Although many recombinant human proteins have been purified, current protein production methods have several limitations; recombinant proteins are frequently truncated, fail to fold properly, and/or lack appropriate post-translational modifications. In addition, such methods require subcloning of the target gene into relevant plasmids, which can be difficult for long proteins with repeated domains. Here we devised a novel method for target protein production by introduction of a strong promoter for overexpression and an epitope tag for purification in front of the endogenous human gene, in a sense performing molecular cloning directly in the human genome, which does not require cloning of the target gene. As a proof of concept, we successfully purified intact human Reelin protein, which is lengthy (3460 amino acids) and contains repeating domains, and confirmed that it was biologically functional.

Reduced Reelin Expression in the Hippocampus after Traumatic Brain Injury

Traumatic brain injury (TBI) is a relatively common occurrence following accidents or violence, and often results in long-term cognitive or motor disability. Despite the high health cost associated with this type of injury, presently there are no effective treatments for many neurological symptoms resulting from TBI. This is due in part to our limited understanding of the mechanisms underlying brain dysfunction after injury. In this study, we used the mouse controlled cortical impact (CCI) model to investigate the effects of TBI, and focused on Reelin, an extracellular protein that critically regulates brain development and modulates synaptic activity in the adult brain. We found that Reelin expression decreases in forebrain regions after TBI, and that the number of Reelin-expressing cells decrease specifically in the hippocampus, an area of the brain that plays an important role in learning and memory. We also conducted in vitro experiments using mouse neuronal cultures and discovered that Reelin protects hippocampal neuronal cells from glutamate-induced neurotoxicity, a well-known secondary effect of TBI. Together our findings suggest that the loss of Reelin expression may contribute to neuronal death in the hippocampus after TBI, and raise the possibility that increasing Reelin levels or signaling activity may promote functional recovery.

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

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

Molecular mechanisms of cell polarity in a range of model systems and in migrating neurons

Cell polarity is defined as the asymmetric distribution of cellular components along an axis. Most cells, from the simplest single-cell organisms to highly specialized mammalian cells, are polarized and use similar mechanisms to generate and maintain polarity. Cell polarity is important for cells to migrate, form tissues, and coordinate activities. During development of the mammalian cerebral cortex, cell polarity is essential for neurogenesis and for the migration of newborn but as-yet undifferentiated neurons. These oriented migrations include both the radial migration of excitatory projection neurons and the tangential migration of inhibitory interneurons. In this review, I will first describe the development of the cerebral cortex, as revealed at the cellular level. I will then define the core molecular mechanisms - the Par/Crb/Scrib polarity complexes, small GTPases, the actin and microtubule cytoskeletons, and phosphoinositides/PI3K signaling - that are required for asymmetric cell division, apico-basal and front-rear polarity in model systems, including C elegans zygote, Drosophila embryos and cultured mammalian cells. As I go through each core mechanism I will explain what is known about its importance in radial and tangential migration in the developing mammalian cerebral cortex.

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.

Impact of Apolipoprotein E gene polymorphism during normal and pathological conditions of the brain across the lifespan

The central nervous system (CNS) is the cellular substrate for the integration of complex, dynamic, constant, and simultaneous interactions among endogenous and exogenous stimuli across the entire human lifespan. Numerous studies on aging-related brain diseases show that some genes identified as risk factors for some of the most common neurodegenerative diseases - such as the allele 4 of APOE gene (APOE4) for Alzheimer's disease (AD) - have a much earlier neuro-anatomical and neuro-physiological impact. The impact of APOE polymorphism appears in fact to start as early as youth and early-adult life. Intriguingly, though, those same genes associated with aging-related brain diseases seem to influence different aspects of the brain functioning much earlier actually, that is, even from the neonatal periods and earlier. The APOE4, an allele classically associated with later-life neurodegenerative disorders as AD, seems in fact to exert a series of very early effects on phenomena of neuroplasticity and synaptogenesis that begin from the earliest periods of life such as the fetal ones.We reviewed some of the findings supporting the hypothesis that APOE polymorphism is an early modifier of various neurobiological aspects across the entire human lifespan - from the in-utero to the centenarian life - during both normal and pathological conditions of the brain.

Physiological significance of proteolytic processing of Reelin revealed by cleavage-resistant Reelin knock-in mice

Reelin is a secreted protein that plays versatile roles in neuronal development and function. The strength of Reelin signaling is regulated by proteolytic processing, but its importance in vivo is not yet fully understood. Here, we generated Reelin knock-in (PA-DV KI) mice in which the key cleavage site of Reelin was abolished by mutation. As expected, the cleavage of Reelin was severely abrogated in the cerebral cortex and hippocampus of PA-DV KI mice. The amount of Dab1, whose degradation is induced by Reelin signaling, decreased in these tissues, indicating that the signaling strength of Reelin was augmented. The brains of PA-DV KI mice were largely structurally normal, but unexpectedly, the hippocampal layer was disturbed. This phenotype was ameliorated in hemizygote PA-DV KI mice, indicating that excess Reelin signaling is detrimental to hippocampal layer formation. The neuronal dendrites of PA-DV KI mice had more branches and were elongated compared to wild-type mice. These results present the first direct evidence of the physiological importance of Reelin cleavage.

Expression and Preparation of Recombinant Reelin and ADAMTS-3 Proteins

Reelin is a large secreted protein that is essential for the brain development and function. Reelin is negatively regulated by the specific cleavage by a disintegrin and metalloproteinase with thrombospondin type 1 motifs 3 (ADAMTS-3) which is also secreted from neurons. It is likely that there are other proteases that can cleave Reelin. This chapter describes the protocol for expression and handling of recombinant Reelin and ADAMTS-3 proteins to facilitate investigation of these proteins.

Dynamics, nanomechanics and signal transduction in reelin repeats

Reelin is a large glycoprotein controlling brain development and cell adhesion. It regulates the positioning of neurons, as well as neurotransmission and memory formation. Perturbations in reelin signaling are linked to psychiatric disorders. Reelin participates in signal transduction by binding to the lipoprotein receptors VLDLR and ApoER2 through its central region. This part is rich in repeating BNR-EGF-BNR modules. We used standard molecular dynamics, steered molecular dynamics, and perturbation response scanning computational methods to characterize unique dynamical properties of reelin modules involved in signaling. Each module has specific sensors and effectors arranged in a similar topology. In the modules studied, disulfide bridges play a protective role, probably making both selective binding and protease activity of reelin possible. Results of single reelin molecule stretching by atomic force microscopy provide the first data on the mechanical stability of individual reelin domains. The forces required for partial unfolding of the modules studied are below 60 pN.

N-cadherin-regulated FGFR ubiquitination and degradation control mammalian neocortical projection neuron migration

The functions of FGF receptors (FGFRs) in early development of the cerebral cortex are well established. Their functions in the migration of neocortical projection neurons, however, are unclear. We have found that FGFRs regulate multipolar neuron orientation and the morphological change into bipolar cells necessary to enter the cortical plate. Mechanistically, our results suggest that FGFRs are activated by N-Cadherin. N-Cadherin cell-autonomously binds FGFRs and inhibits FGFR K27- and K29-linked polyubiquitination and lysosomal degradation. Accordingly, FGFRs accumulate and stimulate prolonged Erk1/2 phosphorylation. Neurons inhibited for Erk1/2 are stalled in the multipolar zone. Moreover, Reelin, a secreted protein regulating neuronal positioning, prevents FGFR degradation through N-Cadherin, causing Erk1/2 phosphorylation. These findings reveal novel functions for FGFRs in cortical projection neuron migration, suggest a physiological role for FGFR and N-Cadherin interaction in vivo and identify Reelin as an extracellular upstream regulator and Erk1/2 as downstream effectors of FGFRs during neuron migration.

Differential binding of anti-Reelin monoclonal antibodies reveals the characteristics of Reelin protein under various conditions

Reelin is a large secreted protein that is essential for the development and function of the central nervous system. Dimerization and/or oligomerization is required for its biological activity, but the underlying mechanism is not fully understood. There are several widely used anti-Reelin antibodies and we noticed that their reactivity to monomeric or dimeric Reelin protein is different. We also found that their reactivity to Reelin in the solution or in fixed brain tissues also differs. Our results provide the information regarding how the N-terminal region of Reelin folds and contributes to the formation of higher order structure. We also provide a caveat that appropriate use of anti-Reelin antibody is necessary for quantitative analyses.

Reelin is modulated by diet-induced obesity and has direct actions on arcuate proopiomelanocortin neurons

Objective

Reelin (RELN) is a large glycoprotein involved in synapse maturation and neuronal organization throughout development. Deficits in RELN signaling contribute to multiple psychological disorders, such as autism spectrum disorder, schizophrenia, and bipolar disorder. Nutritional stress alters RELN expression in brain regions associated with these disorders; however, the involvement of RELN in the neural circuits involved in energy metabolism is unknown. The RELN receptors apolipoprotein E receptor 2 (ApoER2) and very low-density lipoprotein receptor (VLDLR) are involved in lipid metabolism and expressed in the hypothalamus. Here we explored the involvement of RELN in hypothalamic signaling and the impact of diet-induced obesity (DIO) on this system.

Methods

Adult male mice were fed a chow diet or maintained on a high-fat diet (HFD) for 12–16 weeks. HFD-fed DIO mice exhibited decreased ApoER2 and VLDLR expression and increased RELN protein in the hypothalamus. Electrophysiology was used to determine the mechanism by which the central fragment of RELN (CF-RELN) acts on arcuate nucleus (ARH) satiety-promoting proopiomelanocortin (POMC) neurons and the impact of DIO on this circuitry.

Results

CF-RELN exhibited heterogeneous presynaptic actions on inhibitory inputs onto ARH-POMC-EGFP neurons and consistent postsynaptic actions. Additionally, central administration of CF-RELN caused a significant increase in ARH c-Fos expression and an acute decrease in food intake and body weight.

Conclusions

We conclude that RELN signaling is modulated by diet, that RELN is involved in synaptic signaling onto ARH-POMC neurons, and that altering central CF-RELN levels can impact food intake and body weight.

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Diet-induced obesity alters reelin protein levels and expression of ApoER2 and VLDLR.

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Reelin has direct, but divergent actions on GABAergic inputs onto POMC neurons.

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Central administration of reelin protein decreases food intake and body weight.

Differential Action of Reelin on Oligomerization of ApoER2 and VLDL Receptor in HEK293 Cells Assessed by Time-Resolved Anisotropy and Fluorescence Lifetime Imaging Microscopy

The canonical Reelin signaling cascade regulates correct neuronal layering during embryonic brain development. Details of this pathway are still not fully understood since the participating components are highly variable and create a complex mixture of interacting molecules. Reelin is proteolytically processed resulting in five different fragments some of which carrying the binding site for two different but highly homologous receptors, apolipoprotein E receptor 2 (ApoER2) and very low density lipoprotein receptor (VLDLR). The receptors are expressed in different variants in different areas of the developing brain. Binding of Reelin and its central fragment to the receptors results in phosphorylation of the intracellular adapter disabled-1 (Dab1) in neurons. Here, we studied the changes of the arrangement of the receptors upon Reelin binding and its central fragment at the molecular level in human embryonic kidney 293 (HEK293) cells by time-resolved anisotropy and fluorescence lifetime imaging microscopy (FLIM). In the off-state of the pathway ApoER2 and VLDLR form homo or hetero-di/oligomers. Upon binding of full length Reelin ApoER2 and VLDLR homo-oligomers are rearranged to higher order receptor clusters which leads to Dab1 phosphorylation. When the central fragment of Reelin binds to the receptors the cluster size of homo-oligomers is not affected and Dab1 is not phosphorylated. Hetero-oligomerization, however, can be induced, but does not lead to Dab1 phosphorylation. Cells expressing only ApoER2 or VLDLR change their shape when stimulated with the central fragment. Cells expressing ApoER2 produce filopodia/lamellipodia and cell size increases, whereas VLDLR-expressing cells decrease in size. These findings demonstrate that the primary event in the canonical Reelin pathway is the rearrangement of preformed receptor homo-oligomers to higher order clusters. In addition the possibility of yet another signaling mechanism which is mediated by the central Reelin fragment independent of Dab1 phosphorylation became apparent.

The Reelin Receptors Apolipoprotein E receptor 2 (ApoER2) and VLDL Receptor

Apolipoprotein E receptor 2 (ApoER2) and VLDL receptor belong to the low density lipoprotein receptor family and bind apolipoprotein E. These receptors interact with the clathrin machinery to mediate endocytosis of macromolecules but also interact with other adapter proteins to perform as signal transduction receptors. The best characterized signaling pathway in which ApoER2 and VLDL receptor (VLDLR) are involved is the Reelin pathway. This pathway plays a pivotal role in the development of laminated structures of the brain and in synaptic plasticity of the adult brain. Since Reelin and apolipoprotein E, are ligands of ApoER2 and VLDLR, these receptors are of interest with respect to Alzheimer’s disease. We will focus this review on the complex structure of ApoER2 and VLDLR and a recently characterized ligand, namely clusterin.

The ferroxidase ceruloplasmin influences Reelin processing, cofilin phosphorylation and neuronal organization in the developing brain

Ceruloplasmin (Cp) is an important extracellular regulator of iron metabolism. We showed previously that it stimulates Reelin proteolytic processing and cell aggregation in cultures of developing neurons. Reelin is a secreted protein required for the correct positioning of neurons in the brain. It is cleaved in vivo into N-terminally-derived 300K and 180K fragments through incompletely known mechanisms. One of Reelin signaling targets is the actin-binding protein cofilin, the phosphorylation of which is diminished in Reelin-deficient mice. This work looked for in vivo evidence of a relationship between Cp, Reelin and neuronal organization during brain development by analyzing wild-type and Cp-null mice. Cp as well as the full-length, 300K and 180K Reelin species appeared together in wild-type brains at embryonic day (E) 12.5 by immunoblotting. In wild-type compared to Cp-null brains, there was more 300K Reelin from E12.5 to E17.5, a period characterized by extensive, radially directed neuronal migration in the cerebral cortex. Immunofluorescence labeling of tissue sections at E16.5 revealed the localization of Cp with radial glia and meningeal cells adjacent to Reelin-producing Cajal-Retzius neurons, underlining the proximity of Cp and Reelin. Cofilin phosphorylation was seen starting at E10.5-E12.5 and lasted longer until postnatal day 7 in wild-type than Cp-null mice. Finally, using CUX1 as a marker revealed defective accumulation of neurons in layers II/III in neonatal and adult Cp-null mice. These results combined with our earlier work point to a potentially new role of Cp in Reelin processing and signaling and neuronal organization in the cerebral cortex in vivo.

Rare RELN variants affect Reelin-DAB1 signal transduction in autism spectrum disorder

The Reelin-DAB1 signaling pathway plays a crucial role in regulating neuronal migration and synapse function. Although many rare heterozygous variants in the Reelin gene (RELN) have been identified in patients with autism spectrum disorder (ASD), most variants are still of unknown clinical significance. Also, genetic data suggest that heterozygous variants in RELN alone appear to be insufficient to cause ASD. Here, we describe the identification and functional characterization of rare compound heterozygous missense variants in RELN in a patient with ASD in whom we have previously reported hyperfunctional mTORC1 signaling of yet unknown etiology. Using iPSC-derived neural progenitor cells (NPCs) from this patient, we provide experimental evidence that the identified variants are deleterious and lead to diminished Reelin secretion and impaired Reelin-DAB1 signal transduction. Also, our results suggest that mTORC1 pathway overactivation may function as a second hit event contributing to downregulation of the Reelin-DAB1 cascade in patient-derived NPCs, and that inhibition of mTORC1 by rapamycin attenuates Reelin-DAB1 signaling impairment. Taken together, our findings point to an abnormal interplay between Reelin-DAB1 and mTORC1 networks in nonsyndromic ASD.

Reelin controls the positioning of brainstem serotonergic raphe neurons

Serotonin (5-HT) acts as both a morphogenetic factor during early embryonic development and a neuromodulator of circuit plasticity in the mature brain. Dysregulation of serotonin signaling during critical periods is involved in developmental neurological disorders, such as schizophrenia and autism. In this study we focused on the consequences of defect reelin signaling for the development of the brainstem serotonergic raphe system. We observed that reelin signaling components are expressed by serotonergic neurons during the critical period of their lateral migration. Further, we found that reelin signaling is important for the normal migration of rostral, but not caudal hindbrain raphe nuclei and that reelin deficiency results in the malformation of the paramedian raphe nucleus and the lateral wings of the dorsal raphe nuclei. Additionally, we showed that serotonergic neurons projections to laminated brain structures were severely altered. With this study, we propose that the perturbation of canonical reelin signaling interferes with the orientation of tangentially, but not radially, migrating brainstem 5-HT neurons. Our results open the window for further studies on the interaction of reelin and serotonin and the pathogenesis of neurodevelopmental disorders.

The Candidate Schizophrenia Risk Gene DGCR2 Regulates Early Steps of Corticogenesis

BACKGROUND

Alterations in early steps of cortical circuit assembly are thought to play a critical role in vulnerability to schizophrenia (SZ), but the pathogenic impact of SZ-risk mutations on corticogenesis remains to be determined. DiGeorge syndrome critical region 2 (DGCR2) is located in the 22q11.2 locus, whose deletion is a major risk factor for SZ. Moreover, exome sequencing of individuals with idiopathic SZ identified a rare missense mutation in DGCR2, further suggesting that DGCR2 is involved in SZ.

METHODS

Here we investigated the function of Dgcr2 and the pathogenic impact of the SZ-risk DGCR2 mutation in mouse corticogenesis using in utero electroporation targeted to projection neurons.

RESULTS

Dgcr2 knockdown impaired radial locomotion and final translocation of projection neurons, leading to persistent laminar positioning alterations. The DGCR2 missense SZ-risk mutation had a pathogenic impact on projection neuron laminar allocation by reducing protein expression. Mechanistically, we identified Dgcr2 as a novel member of the Reelin complex, regulating the phosphorylation of Reelin-dependent substrates and the expression of Reelin-dependent transcriptional targets.

CONCLUSIONS

Overall, this study provides biological evidence that the SZ-risk gene DGCR2 regulates critical steps of early corticogenesis possibly through a Reelin-dependent mechanism. Additionally, we found that the SZ-risk mutation in DGCR2 has a pathogenic impact on cortical formation by reducing protein expression level, suggesting a functional role for DGCR2 haploinsufficiency in the 22q11.2 deletion syndrome.

Proteolytic cleavage of transmembrane cell adhesion molecule L1 by extracellular matrix molecule Reelin is important for mouse brain development

The cell adhesion molecule L1 and the extracellular matrix protein Reelin play crucial roles in the developing nervous system. Reelin is known to activate signalling cascades regulating neuronal migration by binding to lipoprotein receptors. However, the interaction of Reelin with adhesion molecules, such as L1, has remained poorly explored. Here, we report that full-length Reelin and its N-terminal fragments N-R2 and N-R6 bind to L1 and that full-length Reelin and its N-terminal fragment N-R6 proteolytically cleave L1 to generate an L1 fragment with a molecular mass of 80 kDa (L1-80). Expression of N-R6 and generation of L1-80 coincide in time at early developmental stages of the cerebral cortex. Reelin-mediated generation of L1-80 is involved in neurite outgrowth and in stimulation of migration of cultured cortical and cerebellar neurons. Morphological abnormalities in layer formation of the cerebral cortex of L1-deficient mice partially overlap with those of Reelin-deficient reeler mice. In utero electroporation of L1-80 into reeler embryos normalised the migration of cortical neurons in reeler embryos. The combined results indicate that the direct interaction between L1 and Reelin as well as the Reelin-mediated generation of L1-80 contribute to brain development at early developmental stages.

The functions of Reelin in membrane trafficking and cytoskeletal dynamics: implications for neuronal migration, polarization and differentiation

Reelin is a large extracellular matrix protein with relevant roles in mammalian central nervous system including neurogenesis, neuronal polarization and migration during development; and synaptic plasticity with its implications in learning and memory, in the adult. Dysfunctions in reelin signaling are associated with brain lamination defects such as lissencephaly, but also with neuropsychiatric diseases like autism, schizophrenia and depression as well with neurodegeneration. Reelin signaling involves a core pathway that activates upon reelin binding to its receptors, particularly ApoER2 (apolipoprotein E receptor 2)/LRP8 (low-density lipoprotein receptor-related protein 8) and very low-density lipoprotein receptor, followed by Src/Fyn-mediated phosphorylation of the adaptor protein Dab1 (Disabled-1). Phosphorylated Dab1 (pDab1) is a hub in the signaling cascade, from which several other downstream pathways diverge reflecting the different roles of reelin. Many of these pathways affect the dynamics of the actin and microtubular cytoskeleton, as well as membrane trafficking through the regulation of the activity of small GTPases, including the Rho and Rap families and molecules involved in cell polarity. The complexity of reelin functions is reflected by the fact that, even now, the precise mode of action of this signaling cascade in vivo at the cellular and molecular levels remains unclear. This review addresses and discusses in detail the participation of reelin in the processes underlying neurogenesis, neuronal migration in the cerebral cortex and the hippocampus; and the polarization, differentiation and maturation processes that neurons experiment in order to be functional in the adult brain. In vivo and in vitro evidence is presented in order to facilitate a better understanding of this fascinating system.

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.

Neuronal Polarity in the Embryonic Mammalian Cerebral Cortex

The cerebral cortex is composed of billions of neurons that can grossly be subdivided into two broad classes: inhibitory GABAergic interneurons and excitatory glutamatergic neurons. The majority of cortical neurons in mammals are the excitatory type and they are the main focus of this review article. Like many of the cells in multicellular organisms, fully differentiated neurons are both morphologically and functionally polarized. However, they go through several changes in polarity before reaching this final mature differentiated state. Neurons are derived from polarized neuronal progenitor/stem cells and their commitment to neuronal fate is decided by cellular and molecular asymmetry during their last division in the neurogenic zone. They migrate from their birthplace using so-called multipolar migration, during which they switch direction of movement several times, and repolarize for bipolar migration when the axon is specified. Therefore, neurons have to break their previous symmetry, change their morphology and adequately respond to polarizing signals during migration in order to reach the correct position in the cortex and start making connections. Finally, the dendritic tree is elaborated and the axon/dendrite morphological polarity is set. Here we will describe the function, establishment and maintenance of polarity during the different developmental steps starting from neural stem cell (NSC) division, neuronal migration and axon specification at embryonic developmental stages.

The de novo autism spectrum disorder RELN R2290C mutation reduces Reelin secretion and increases protein disulfide isomerase expression

Despite the recent identification of over 40 missense heterozygous Reelin gene (RELN) mutations in autism spectrum disorder (ASD), none of these has been functionally characterized. Reelin is an integral signaling ligand for proper brain development and post-natal synapse function - properties likely disrupted in ASD patients. We find that the R2290C mutation, which arose de novo in an affected ASD proband, and other analogous mutations in arginine-amino acid-arginine domains reduce protein secretion. Closer analysis of RELN R2290C heterozygous neurospheres reveals up-regulation of Protein Disulfide Isomerase A1, best known as an endoplasmic reticulum-chaperone protein, which has been linked to neuronal pathology. This effect is recapitulated in a heterozygous RELN mouse mutant that is characterized by defective Reelin secretion. These findings suggest that both a deficiency in Reelin signaling and pathologic impairment of Reelin secretion may contribute to ASD risk.

The ApoE receptors Vldlr and Apoer2 in central nervous system function and disease

The LDL receptor (LDLR) family has long been studied for its role in cholesterol transport and metabolism; however, the identification of ApoE4, an LDLR ligand, as a genetic risk factor for late-onset Alzheimer's disease has focused attention on the role this receptor family plays in the CNS. Surprisingly, it was discovered that two LDLR family members, ApoE receptor 2 (Apoer2) and VLDL receptor (Vldlr), play key roles in brain development and adult synaptic plasticity, primarily by mediating Reelin signaling. This review focuses on Apoer2 and Vldlr signaling in the CNS and its role in human disease.

Embryonic mosaic deletion of APP results in displaced Reelin-expressing cells in the cerebral cortex

It is widely accepted that amyloid precursor protein (APP) plays a central role in the pathogenesis of Alzheimer's disease. In addition, APP has been proposed to have functions in numerous biological processes including neuronal proliferation, differentiation, migration, axon guidance, and neurite outgrowth, as well as in synapse formation and function. However, germline knockout of APP yields relatively subtle phenotypes, and brain development appears grossly normal. This is thought to be due in part to functional compensation by APP family members and other type I transmembrane proteins. Here, we have generated a conditional mouse knockout for APP that is controlled temporally using Cre^ER and tamoxifen administration. We show that total cortical expression of APP is reduced following tamoxifen administration during embryonic time points critical for cortical lamination, and that this results in displacement of Reelin-positive cells below the cortical plate with a concurrent elevation in Reelin protein levels. These results support a role for APP in cortical lamination and demonstrate the utility of a conditional knockout approach in which APP can be deleted with temporal control in vivo. This new tool should be useful for many different applications in the study of APP function across the mammalian life span.

Nonneuronal roles for the reelin signaling pathway

The reelin signaling pathway has been established as an important regulator of cell migration during development of the central nervous system, and disruptions in reelin signaling alter the positioning of many types of neurons. Reelin is a large extracellular matrix glycoprotein and governs cell migration through activation of multiple intracellular signaling events by means of the receptors ApoE receptor 2 (ApoER2) and very low density lipoprotein receptor (VLDLR), and the intracellular adaptor protein Disabled-1 (Dab1). Earlier studies reported expression of reelin in nonneuronal tissues, but the functions of this signaling pathway outside of the nervous system have not been studied until recently. A large body of evidence now suggests that reelin functions during development and disease of multiple nonneuronal tissues. This review addresses recent advances in the field of nonneuronal reelin signaling. Developmental Dynamics 246:217-226, 2017. © 2016 Wiley Periodicals, Inc.

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.

Seizure-Induced Motility of Differentiated Dentate Granule Cells Is Prevented by the Central Reelin Fragment

Granule cell dispersion (GCD) represents a pathological widening of the granule cell layer in the dentate gyrus and it is frequently observed in patients with mesial temporal lobe epilepsy (MTLE). Recent studies in human MTLE specimens and in animal epilepsy models have shown that a decreased expression and functional inactivation of the extracellular matrix protein Reelin correlates with GCD formation, but causal evidence is still lacking. Here, we used unilateral kainate (KA) injection into the mouse hippocampus, an established MTLE animal model, to precisely map the loss of reelin mRNA-synthesizing neurons in relation to GCD along the septotemporal axis of the epileptic hippocampus. We show that reelin mRNA-producing neurons are mainly lost in the hilus and that this loss precisely correlates with the occurrence of GCD. To monitor GCD formation in real time, we used organotypic hippocampal slice cultures (OHSCs) prepared from mice which express enhanced green fluorescent protein (eGFP) primarily in differentiated dentate granule cells. Using life cell microscopy we observed that increasing doses of KA resulted in an enhanced motility of eGFP-positive granule cells. Moreover, KA treatment of OHSC resulted in a rapid loss of Reelin-producing interneurons mainly in the hilus, as observed in vivo. A detailed analysis of the migration behavior of individual eGFP-positive granule cells revealed that the majority of these neurons actively migrate toward the hilar region, where Reelin-producing neurons are lost. Treatment with KA and subsequent addition of the recombinant R3–6 Reelin fragment significantly prevented the movement of eGFP-positive granule cells. Together, these findings suggest that GCD formation is indeed triggered by a loss of Reelin in hilar interneurons.

Canonical and Non-canonical Reelin Signaling

Reelin is a large secreted glycoprotein that is essential for correct neuronal positioning during neurodevelopment and is important for synaptic plasticity in the mature brain. Moreover, Reelin is expressed in many extraneuronal tissues; yet the roles of peripheral Reelin are largely unknown. In the brain, many of Reelin's functions are mediated by a molecular signaling cascade that involves two lipoprotein receptors, apolipoprotein E receptor-2 (Apoer2) and very low density-lipoprotein receptor (Vldlr), the neuronal phosphoprotein Disabled-1 (Dab1), and members of the Src family of protein tyrosine kinases as crucial elements. This core signaling pathway in turn modulates the activity of adaptor proteins and downstream protein kinase cascades, many of which target the neuronal cytoskeleton. However, additional Reelin-binding receptors have been postulated or described, either as coreceptors that are essential for the activation of the "canonical" Reelin signaling cascade involving Apoer2/Vldlr and Dab1, or as receptors that activate alternative or additional signaling pathways. Here we will give an overview of canonical and alternative Reelin signaling pathways, molecular mechanisms involved, and their potential physiological roles in the context of different biological settings.

Structural Insights into Reelin Function: Present and Future

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