Reelin stabilizes the actin cytoskeleton of neuronal processes by inducing n-cofilin phosphorylation at serine3

Serine phosphorylation regulates disabled-1 early isoform turnover independently of Reelin

The Reelin-Disabled 1 (Dab1) signaling pathway plays an important role in neuronal cell migration during brain development. Dab1, an intracellular adapter protein which is tyrosine phosphorylated upon Reelin stimulation, has been directly implicated in the transmission and termination of Reelin-mediated signaling. Two main forms of Dab1 have been identified in the developing chick retina, an early isoform (Dab1-E) expressed in progenitor cells and a late isoform (Dab1-L, a.k.a. Dab1) expressed in differentiated cells. Dab1-E is missing two Src family kinase (SFK) phosphorylation sites that are critical for Reelin-Dab1 signaling and is not tyrosine phosphorylated. We have recently demonstrated a role for Dab1-E in the maintenance of retinal progenitor cells. Here, we report that Dab1-E is phosphorylated at serine/threonine residues independent of Reelin. Cdk2, highly expressed in retinal progenitor cells, mediates Dab1-E phosphorylation at serine 475 which in turn promotes ubiquitination-triggered proteasome degradation of Dab1-E. Inhibition of protein phosphatase 1 and/or protein phosphatase 2A leads to increased Dab1-E instability. We propose that Dab1 turnover is regulated by both Reelin-independent serine/threonine phosphorylation and Reelin-dependent tyrosine phosphorylation.

Emerging topics in Reelin function

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

Role for Reelin-induced cofilin phosphorylation in the assembly of sympathetic preganglionic neurons in the murine intermediolateral column

Sympathetic preganglionic neurons (SPNs) are located in the intermediolateral column (IMLC) of the spinal cord. This specific localization results from primary and secondary migratory processes during spinal cord development. Thus, following neurogenesis in the neuroepithelium, SPNs migrate first in a ventrolateral direction and then, in a secondary step, dorsolaterally to reach the IMLC. These migratory processes are controlled, at least in part, by the glycoprotein Reelin, which is known to be important for the development of laminated brain structures. In reeler mutants deficient in Reelin, SPNs initially migrate ventrolaterally as normal. However, most of them then migrate medially to become eventually located near the central canal. Here, we provide evidence that in wild-type animals this aberrant medial migration towards the central canal is prevented by Reelin-induced cytoskeletal stabilization, brought about by phosphorylation of cofilin. Cofilin plays an important role in actin depolymerization, a process required for the changes in cell shape during migration. Phosphorylation of cofilin renders it unable to depolymerize F-actin, thereby stabilizing the cytoskeleton. Using immunostaining for phosphorylated cofilin (p-cofilin), we demonstrate that SPNs in wild-type animals, but not in reeler mutants and other mutants of the Reelin signalling cascade, are immunoreactive for p-cofilin. These findings suggest that Reelin near the central canal induces cofilin phosphorylation in SPNs, thereby preventing them from aberrant migration towards the central canal. The results extend our previous studies on cortical neurons in which Reelin in the marginal zone was found to stabilize the leading processes of migrating neurons and terminate the migration process.

Leading tip drives soma translocation via forward F-actin flow during neuronal migration

Neuronal migration involves coordinated extension of the leading process and translocation of the soma, but the relative contribution of different subcellular regions, including the leading process and cell rear, in driving soma translocation remains unclear. By local manipulation of cytoskeletal components in restricted regions of cultured neurons, we examined the molecular machinery underlying the generation of traction force for soma translocation during neuronal migration. In actively migrating cerebellar granule cells in culture, a growth cone (GC)-like structure at the leading tip exhibits high dynamics, and severing the tip or disrupting its dynamics suppressed soma translocation within minutes. Soma translocation was also suppressed by local disruption of F-actin along the leading process but not at the soma, whereas disrupting microtubules along the leading process or at the soma accelerated soma translocation. Fluorescent speckle microscopy using GFP-alpha-actinin showed that a forward F-actin flow along the leading process correlated with and was required for soma translocation, and such F-actin flow depended on myosin II activity. In migrating neurons, myosin II activity was high at the leading tip but low at the soma, and increasing or decreasing this front-to-rear difference accelerated or impeded soma advance. Thus, the tip of the leading process actively pulls the soma forward during neuronal migration through a myosin II-dependent forward F-actin flow along the leading process.

Reelin-mediated Signaling during Normal and Pathological Forms of Aging

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

The early isoform of disabled-1 functions independently of Reelin-mediated tyrosine phosphorylation in chick retina

The Reelin-Disabled-1 (Dab1) signaling pathway plays a key role in the positioning of neurons during brain development. Two alternatively spliced Dab1 isoforms have been identified in chick retina and brain: Dab1-E, expressed at early stages of development, and Dab1-L (commonly referred to as Dab1), expressed at later developmental stages. The well-studied Dab1-L serves as an adaptor protein linking Reelin signal to its downstream effectors; however, nothing is known regarding the role of Dab1-E. Here we show that Dab1-E is primarily expressed in proliferating retinal progenitor cells whereas Dab1-L is found exclusively in differentiated neuronal cells. In contrast to Dab1-L, which is tyrosine phosphorylated upon Reelin stimulation, Dab1-E is not tyrosine phosphorylated and may function independently of Reelin. Knockdown of Dab1-E in chick retina results in a significant reduction in the number of proliferating cells and promotes ganglion cell differentiation. Our results demonstrate a role for Dab1-E in the maintenance of the retinal progenitor pool and determination of cell fate.

Role for Reelin in stabilizing cortical architecture

Reelin controls the migration of neurons and layer formation during brain development. However, recent studies have shown that disrupting Reelin function in the adult hippocampus induces repositioning of fully differentiated neurons, suggesting a stabilizing effect of Reelin on mature neuronal circuitry. Indeed, Reelin was recently found to stabilize the actin cytoskeleton by inducing cofilin phosphorylation. When unphosphorylated, cofilin acts as an actin-depolymerizing protein that promotes the disassembly of F-actin. Here, a novel hypothesis is proposed whereby decreased Reelin expression in the mature brain causes destabilization of neurons and their processes, leading to aberrant plasticity and aberrant wiring of brain circuitry. This has implications for brain disorders, such as epilepsy and schizophrenia, in which deficiencies in Reelin expression occur.

Expression of COUP-TFII nuclear receptor in restricted GABAergic neuronal populations in the adult rat hippocampus

The COUP-TFII nuclear receptor, also known as NR2F2, is expressed in the developing ventral telencephalon and modulates the tangential migration of a set of subpallial neuronal progenitors during forebrain development. Little information is available about its expression patterns in the adult brain. We have identified the cell populations expressing COUP-TFII and the contribution of some of them to network activity in vivo. Expression of COUP-TFII by hippocampal pyramidal and dentate granule cells, as well as neurons in the neocortex, formed a gradient increasing from undetectable in the dorsal to very strong in the ventral sectors. In the dorsal hippocampal CA1 area, COUP-TFII was restricted to GABAergic interneurons and expressed in several, largely nonoverlapping neuronal populations. Immunoreactivity was present in calretinin-, neuronal nitric oxide synthase-, and reelin-expressing cells, as well as in subsets of cholecystokinin- or calbindin-expressing or radiatum-retrohippocampally projecting GABAergic cells, but not in parvalbumin- and/or somatostatin-expressing interneurons. In vivo recording and juxtacellular labeling of COUP-TFII-expressing cells revealed neurogliaform cells, basket cells in stratum radiatum and tachykinin-expressing radiatum dentate innervating interneurons, identified by their axodendritic distributions. They showed cell type-selective phase-locked firing to the theta rhythm but no activation during sharp wave/ripple oscillations. These basket cells in stratum radiatum and neurogliaform cells fired at the peak of theta oscillations detected extracellularly in stratum pyramidale, unlike previously reported ivy cells, which fired at the trough. The characterization of COUP-TFII-expressing neurons suggests that this developmentally important transcription factor plays cell type-specific role(s) in the adult hippocampus.

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

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

Reelin promotes neuronal orientation and dendritogenesis during preplate splitting

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

Proper layering is important for precisely timed activation of hippocampal mossy cells

The mammalian cortex exhibits a laminated structure that may underlie optimal synaptic connectivity and support temporally precise activation of neurons. In 'reeler' mice, the lack of the extracellular matrix protein Reelin leads to abnormal positioning of cortical neurons and disrupted layering. To address how these structural changes impact neuronal function, we combined electrophysiological and neuroanatomical techniques to investigate the synaptic activation of hippocampal mossy cells (MCs), the cell type that integrates the output of dentate gyrus granule cells (GCs). While somatodendritic domains of wild-type (WT) MCs were confined to the hilus, the somata and dendrites of reeler MCs were often found in the molecular layer, where the perforant path (PP) terminates. Most reeler MCs received aberrant monosynaptic excitatory input from the PP, whereas the disynaptic input to MCs via GCs was decreased and inhibition was increased. In contrast to the uniform disynaptic discharge of WT MCs, many reeler cells discharged with short, monosynaptic latencies, while others fired with long latencies over a broad temporal window in response to PP activation. Thus, disturbed lamination results in aberrant synaptic connectivity and altered timing of action potential generation. These results highlight the importance of a layered cortical structure for information processing.

The neurodevelopmental implications of PI3K signaling

Our understanding of the mechanisms involved in the formation of the complex arrangement of neurons and their interconnections within the brain has made significant progress in recent years. Current research has uncovered a network of intracellular signaling events that provide precise coordination of a diverse array of cellular responses, including trafficking events, cytoskeletal remodeling, gene transcription, and protein ubiquitination and translation. This chapter considers the specific cellular responses controlled by the phosphatidylinositol 3-kinase (PI3K) signaling pathway, which is instructive with regard to a number of important steps involved in the development of the brain. These range from the mediation of extrinsic signals - such as growth factors, axon guidance cues, and extracellular matrix components - to intrinsic effectors, such as downstream signaling components that act, for example, at the translation level. PI3K signaling is, consequently, at the heart of controlling neuronal migration and neuronal morphogenesis, as well as dendrite and synapse development. Many neurobehavioral disorders arise as a consequence of subtle developmental abnormalities. Unsurprisingly, therefore, aberrant PI3K signaling has been indicated by many studies to be a contributing factor to the pathophysiology of disorders such as schizophrenia and autism. In this chapter, we will focus on the specific, yet divergent, cellular processes that are achieved through PI3K signaling in neurons and are key to brain development.

A new role for TIMP-1 in modulating neurite outgrowth and morphology of cortical neurons

Background

Tissue inhibitor of metalloproteinases-1 (TIMP-1) displays pleiotropic activities, both dependent and independent of its inhibitory activity on matrix metalloproteinases (MMPs). In the central nervous system (CNS), TIMP-1 is strongly upregulated in reactive astrocytes and cortical neurons following excitotoxic/inflammatory stimuli, but no information exists on its effects on growth and morphology of cortical neurons.

Principal Findings

We found that 24 h incubation with recombinant TIMP-1 induced a 35% reduction in neurite length and significantly increased growth cones size and the number of F-actin rich microprocesses. TIMP-1 mediated reduction in neurite length affected both dendrites and axons after 48 h treatment. The effects on neurite length and morphology were not elicited by a mutated form of TIMP-1 inactive against MMP-1, -2 and -3, and still inhibitory for MMP-9, but were mimicked by a broad spectrum MMP inhibitor. MMP-9 was poorly expressed in developing cortical neurons, unlike MMP-2 which was present in growth cones and whose selective inhibition caused neurite length reductions similar to those induced by TIMP-1. Moreover, TIMP-1 mediated changes in cytoskeleton reorganisation were not accompanied by modifications in the expression levels of actin, βIII-tubulin, or microtubule assembly regulatory protein MAP2c. Transfection-mediated overexpression of TIMP-1 dramatically reduced neuritic arbour extension in the absence of detectable levels of released extracellular TIMP-1.

Conclusions

Altogether, TIMP-1 emerges as a modulator of neuronal outgrowth and morphology in a paracrine and autrocrine manner through the inhibition, at least in part, of MMP-2 and not MMP-9. These findings may help us understand the role of the MMP/TIMP system in post-lesion pre-scarring conditions.

The cytoplasmic adaptor protein X11alpha and extracellular matrix protein Reelin regulate ApoE receptor 2 trafficking and cell movement

The goal of this study was to determine the effect of X11alpha on ApoE receptor 2 (ApoEr2) trafficking and the functional significance of this interaction on cell movement in MCF 10A epithelial cells. We found that X11alpha increased surface levels of ApoEr2 by 64% compared to vector control, as determined by surface protein biotinylation. To examine the functional significance of this effect, we tested whether ApoEr2 played a novel role in cell movement in a wound-healing assay. We found that overexpression of ApoEr2 in MCF 10A cells increased cell migration velocity by 87% (P<0.01, n=4) compared to GFP control. Cotransfection of X11alpha had an additive effect on average velocity compared to ApoEr2 alone (13%; P<0.05, n=4). In addition, we tested whether ApoEr2 ligands altered the effect of ApoEr2 on cell movement. We found that treatment with concentrated medium containing the extracellular matrix protein Reelin, but not control medium, further increased the velocity of ApoEr2- but not APP-transfected cells (20%; P<0.001, n=4). Similarly, Reelin treatment increased cell velocity in the presence of ApoEr2 and X11alpha (10%; P<0.05, n=4). In the present study, we are the first to demonstrate that ApoEr2 regulates cell movement, and both X11alpha and Reelin enhance this effect.

Reelin deficiency causes granule cell dispersion in epilepsy

Cortical migration defects are often associated with epilepsy. In mesial temporal lobe epilepsy (MTLE), granule cell dispersion (GCD), a migration defect of dentate granule cells, is frequently observed. Little is known how GCD develops and to which extent it contributes to the development of seizure activity. Since the reelin-deficient reeler mouse mutant shows a similar migration defect of dentate cells, we performed a series of studies investigating whether reelin deficiency is involved in GCD development. We show that in MTLE patients and in a mouse model of MTLE, the development of GCD correlates with a loss of the extracellular matrix protein reelin. In addition, we present evidence that GCD occurs in the absence of neurogenesis, thus representing a displacement of mature neurons due to a reelin deficiency. Accordingly, antibody blockade of reelin function in naïve, adult mice induced GCD. Finally, we show that GCD formation can be prevented by infusion of exogenous reelin. In summary, these studies show that in epilepsy reelin dysfunction causes GCD development and that reelin is important for the maintenance of layered structures in the adult brain.

Interaction of reelin with amyloid precursor protein promotes neurite outgrowth

The processing of amyloid precursor protein (APP) to Abeta is an important event in the pathogenesis of Alzheimer's disease, but the physiological function of APP is not well understood. Our previous work has shown that APP processing and Abeta production are regulated by the extracellular matrix protein Reelin. In the present study, we examined whether Reelin interacts with APP, and the functional consequences of that interaction in vitro. Using coimmunoprecipitation, we found that Reelin interacted with APP through the central domain of Reelin (repeats 3-6) and the E1 extracellular domain of APP. Reelin increased cell surface levels of APP and decreased endocytosis of APP in hippocampal neurons in vitro. In vivo, Reelin levels were increased in brains of APP knock-out mice and decreased in APP-overexpressing mice. RNA interference knockdown of APP decreased neurite outgrowth in vitro and prevented Reelin from increasing neurite outgrowth. Knock-out of APP or Reelin decreased dendritic arborization in cortical neurons in vivo, and APP overexpression increased dendritic arborization. APP and Reelin have previously been shown to promote neurite outgrowth through interactions with integrins. We confirmed that APP interacted with alpha3beta1 integrin, and alpha3beta1 integrin altered APP trafficking and processing. Addition of an alpha3beta1 integrin antibody prevented APP and Reelin-induced neurite outgrowth. These findings demonstrate that Reelin interacts with APP, potentially having important effects on neurite development.

Apolipoprotein E receptors in the nervous system

PURPOSE OF REVIEW

Over the past few years, apolipoprotein E (ApoE) receptors, also known as LDL receptor-related proteins, have distinguished themselves as functionally diverse signaling receptors with pivotal roles not only in the vascular system but also in the nervous system and during development.

RECENT FINDINGS

The expanding roles of ApoE receptors for cellular signal transduction at the same time transcend and integrate their lipid transport roles into a larger biological and clinical context. ApoE receptors are essential for the development of the nervous system, the regulation of synaptic plasticity, neuroprotection and the innervation of the muscle. They also regulate the metabolism of the amyloid precursor protein on multiple levels, implicating them in the pathogenesis of Alzheimer's disease.

SUMMARY

ApoE, a common ligand for all members of the evolutionarily ancient LDL receptor gene family, is the major genetic modifier of the age of onset of Alzheimer's disease. The underlying molecular mechanisms remain shrouded in mystery, but the numerous critical functions of ApoE receptors within and outside the nervous system that have recently emerged make it likely that these multifunctional signal modulators participate in Alzheimer's disease pathogenesis. This review attempts to summarize the most recent and relevant findings in this area.

Disruption of normal cytoskeletal dynamics may play a key role in the pathogenesis of epilepsy

Epilepsy, a common disease affecting 1% to 2% of the population, is characterized by seizures, hyperexcitability at synapses, and aberrant extension of neurons following seizures. Much work has been done on the role of synaptic components in the pathogenesis of epilepsy, but relatively little attention has been given to the potential role of the cytoskeleton. The neuronal cytoskeleton consists of microtubules, actin filaments, intermediate filaments, and associated proteins. A number of mutations in both microtubule-associated proteins (MAPs) and actin-binding proteins, as well as altered expression levels of several cytoskeletal proteins, are known to be involved in epilepsy. These changes will affect the dynamics of the neuronal cytoskeleton and therefore are likely to contribute to the pathogenesis of epilepsy through mechanisms such as increased neurotrophic support to neurons and increased sprouting of mossy fibers. These changes may also contribute to hyperexcitability of neurons through an as yet unidentified mechanism.

Role of Reelin in the development and maintenance of cortical lamination

Reelin is a large extracellular matrix molecule, synthesized by early generated Cajal-Retzius cells in the marginal zone of the cortex. It plays an important role in the migration of cortical neurons and the development of cortical lamination. We recently discovered that Reelin is required not only for the formation of cortical layers during development but also for their maintenance in adulthood. Thus, decreased Reelin expression in a mouse model of epilepsy and in epileptic patients was accompanied by a loss of granule cell lamination, called granule cell dispersion, in the dentate gyrus of the hippocampal formation. Moreover, antibody blockade of Reelin in normal, adult mice resulted in granule cell dispersion. Collectively these findings point to a role for Reelin in the formation and maintenance of a laminated cortical structure. How does Reelin act on the cytoskeleton in the migration process of cortical neurons? It has been shown that Reelin signalling involves the lipoprotein receptors apolipoprotein E receptor 2 and very low density lipoprotein receptor, the adapter protein Disabled1, and phosphatidylinositol-3-kinase, but it has remained unclear how activation of the Reelin signalling cascade controls cytoskeletal reorganization. Here, we provide evidence that Reelin signalling leads to serine3 phosphorylation of cofilin, an actin-depolymerizing protein that promotes the disassembly of F-actin. Phosphorylation at serine3 renders cofilin unable to depolymerize F-actin, thereby stabilizing the cytoskeleton. Phosphorylation of cofilin in the leading processes of migrating neurons anchors them to the marginal zone containing Reelin. Our results indicate that Reelin-induced stabilization of the neuronal cytoskeleton is an important component of Reelin's function in the development and maintenance of cortical architecture.

Polarity regulation in migrating neurons in the cortex

The formation of the cerebral cortex requires migration of billions of cells from their birth position to their final destination. A motile cell must have internal polarity in order to move in a specified direction. Locomotory polarity requires the coordinated polymerization of cytoskeletal elements such as microtubules and actin combined with regulated activities of the associated molecular motors. This review is focused on migrating neurons in the developing cerebral cortex, which need to attain internal polarity in order to reach their proper target. The position and dynamics of the centrosome plays an important function in this directed motility. We highlight recent interesting findings connecting polarity proteins with neuronal migration events regulated by the microtubule-associated molecular motor, cytoplasmic dynein.