Presenilin-1 deficiency leads to loss of Cajal-Retzius neurons and cortical dysplasia similar to human type 2 lissencephaly

Essential roles for the FE65 amyloid precursor protein-interacting proteins in brain development

Targeted deletion of two members of the FE65 family of adaptor proteins, FE65 and FE65L1, results in cortical dysplasia. Heterotopias resembling those found in cobblestone lissencephalies in which neuroepithelial cells migrate into superficial layers of the developing cortex, aberrant cortical projections and loss of infrapyramidal mossy fibers arise in FE65/FE65L1 compound null animals, but not in single gene knockouts. The disruption of pial basal membranes underlying the heterotopias and poor organization of fibrillar laminin by isolated meningeal fibroblasts from double knockouts suggests that FE65 proteins are involved in basement membrane assembly. A similar phenotype is observed in triple mutant mice lacking the APP family members APP, APLP1 and APLP2, all of which interact with FE65 proteins, suggesting that this phenotype may be caused by decreased transmission of an APP-dependent signal through the FE65 proteins. The defects observed in the double knockout may also involve the family of Ena/Vasp proteins, which participate in actin cytoskeleton remodeling and interact with the WW domains of FE65 proteins.

A Two Decade Contribution of Molecular Cell Biology to the Centennial of Alzheimer's Disease: Are We Progressing Toward Therapy?

Alzheimer's disease (AD), described for the first time 100 years ago, is a neurodegenerative disease characterized by two neuropathological hallmarks: neurofibrillary tangles containing hyperphosphorylated tau and senile plaques. These lesions are likely initiated by an imbalance between production and clearance of amyloid beta, leading to increased oligomerization of these peptides, formation of amyloid plaques in the brain of the patient, and final dementia. Amyloid beta is generated from amyloid precursor protein (APP) by subsequent beta- and gamma-secretase cleavage, the latter being a multiprotein complex consisting of presenilin-1 or -2, nicastrin, APH-1, and PEN-2. Alternatively, APP can be cleaved by alpha- and gamma-secretase, precluding the production of Abeta. In this review, we discuss the major breakthroughs during the past two decades of molecular cell biology and the current genetic and cell biological state of the art on APP proteolysis, including structure-function relationships and subcellular localization. Finally, potential directions for cell biological research toward the development of AD therapies are briefly discussed.

Presenilins in the developing, adult, and aging cerebral cortex

Mutations in presenilins are the major cause of familial Alzheimer disease. The involvement of presenilins in the pathogenesis of Alzheimer disease, therefore, has been the subject of intense investigation during the past decade. Genetic analysis of phenotypes associated with presenilin mutations in invertebrate and vertebrate systems has greatly advanced our understanding of the in vivo functions of presenilins. In this review, the authors will summarize the current understanding of presenilin function, with an emphasis on the mammalian cerebral cortex. During development, presenilins play crucial roles in the maintenance of neural progenitor cell proliferation, the temporal control of neuronal differentiation, the survival of Cajal-Retzius neurons, and proper neuronal migration in the developing cerebral cortex. Analysis of presenilin function in the adult cerebral cortex has revealed essential roles for presenilins in synaptic plasticity, long-term memory, and neuronal survival. The authors will also discuss the molecular mechanisms through which presenilins may mediate these functions, including the Notch, CREB, and NMDA receptor-mediated signaling pathways. These diverse functions of presenilins in cortical development and function and neuronal survival have important implications for the pathogenesis of neurodegenerative dementia.

Selective expression of presenilin 1 in neural progenitor cells rescues the cerebral hemorrhages and cortical lamination defects in presenilin 1-null mutant mice

Mice with a null mutation of the presenilin 1 gene (Psen1(-/-)) die during late intrauterine life or shortly after birth and exhibit multiple CNS and non-CNS abnormalities, including cerebral hemorrhages and altered cortical development. The cellular and molecular basis for the developmental effects of Psen1 remain incompletely understood. Psen1 is expressed in neural progenitors in developing brain, as well as in postmitotic neurons. We crossed transgenic mice with either neuron-specific or neural progenitor-specific expression of Psen1 onto the Psen1(-/-) background. We show that neither neuron-specific nor neural progenitor-specific expression of Psen1 can rescue the embryonic lethality of the Psen1(-/-) embryo. Indeed neuron-specific expression rescued none of the abnormalities in Psen1(-/-) mice. However, Psen1 expression in neural progenitors rescued the cortical lamination defects, as well as the cerebral hemorrhages, and restored a normal vascular pattern in Psen1(-/-) embryos. Collectively, these studies demonstrate that Psen1 expression in neural progenitor cells is crucial for cortical development and reveal a novel role for neuroectodermal expression of Psen1 in development of the brain vasculature.

Notch signaling in the mammalian central nervous system: insights from mouse mutants

The Notch pathway, although originally identified in fruit flies, is now among the most heavily studied in mammalian biology. In mice, loss-of-function and gain-of-function work has demonstrated that Notch signaling is essential both during development and in the adult in a multitude of tissues. Prominent among these is the CNS, where Notch has been implicated in processes ranging from neural stem cell regulation to learning and memory. Here we review the role of Notch in the mammalian CNS by focusing specifically on mutations generated in mice. These mutations have provided critical insight into Notch function in the CNS and have led to the identification of promising new directions that are likely to generate important discoveries in the future.

Notch signaling, brain development, and human disease

The Notch signaling pathway is central to a wide array of developmental processes in a number of organ systems, including hematopoiesis, somitogenesis, vasculogenesis, and neurogenesis. These processes involve maintenance of stem cell self-renewal, proliferation, specification of cell fate or differentiation, and apoptosis. Recent studies have led to the recognition of the role of the Notch pathway in early neurodevelopment, learning, and memory, as well as late-life neurodegeneration. This review summarizes what is currently known about the role of the Notch pathway in neural stem cells, gliogenesis, learning and memory, and neurologic disease.

Nucleokinesis in Neuronal Migration

Neuronal migration is a critical phase of nervous system development and can be divided into two distinct phases: extension of the leading process and movement of the cell body and nucleus (nucleokinesis). Nucleokinesis appears to require many of the same cytoskeletal and signaling molecules used in cell mitosis. Converging studies suggest it requires cytoplasmic dynein, cell polarity genes, and microtubule-associated proteins that coordinate microtubule remodeling. These coordinate first the positioning of the centrosome (microtubule organizing center) in the leading process in front of the nucleus and then the movement of the nucleus towards the centrosome. The positioning of the centrosome and the dynamic regulation that couples and uncouples the nucleus underlies directed migration of neurons.

Neuronal migration and the role of reelin during early development of the cerebral cortex

During development, neurons migrate to the cortex radially from periventricular germinative zones as well as tangentially from ganglionic eminences. The vast majority of cortical neurons settle radially in the cortical plate. Neuronal migration requires an exquisite regulation of leading edge extension, nuclear translocation (nucleokinesis), and retraction of trailing processes. During the past few years, several genes and proteins have been identified that are implicated in neuronal migration. Many have been characterized by reference to known mechanisms of neuronal and non-neuronal cell migration in culture; however, probably the most interesting have been identified by gene inactivation or modification in mice and by positional cloning of brain malformation genes in humans and mice. Although it is impossible to provide a fully integrated view, some patterns clearly emerge and are the subject of this article. Specific emphasis is placed on three aspects: first, the role of the actin treadmill, with cyclic formation of filopodial and lamellipodial extensions, in relation to surface events that occur at the leading edge of radially migrating neurons; second, the regulation of microtubule dynamics, which seems to play a key role in nucleokinesis; and third, the mechanisms by which the extracellular protein Reelin regulates neuronal positioning at the end of migration.

Finally, a sense of closure? Animal models of human ventral body wall defects

Malformations concerning the ventral body wall constitute one of the leading categories of human birth defects and are present in about one out of every 2000 live births. Although the occurrence of these defects is relatively common, few detailed experimental studies exist on the development and closure of the ventral body wall in mouse and human. This field is further complicated by the array of theories on the pathogenesis of body wall defects and the likelihood that there is no single cause for these abnormalities. In this review, we summarize what is known concerning the mechanisms of normal ventral body wall closure in humans and mice. We then outline the theories that have been proposed concerning human body wall closure abnormalities and examine the growing number of mouse mutations that impact normal ventral body wall closure. Finally, we speculate how studies in animal models such as mouse and Drosophila are beginning to provide a much-needed mechanistic framework with which to identify and characterize the genes and tissues required for this vital aspect of human embryogenesis.

Neural progenitor cells do not differentiate prematurely in presenilin-1 null mutant mice

Mice with a null mutation of the presenilin-1 (PS1-/-) gene die during late intrauterine life or shortly after birth and exhibit defects in cortical development. A previous report suggested that neurons differentiate prematurely in PS1-/- brain [M. Handler, X. Yang, J. Shen, Presenilin-1 regulates neuronal differentiation during neurogenesis, Development 127 (2000) 2593-2606]. Here we reexamined the issue of whether premature neuronal differentiation occurs in PS1-/- brain using fresh cell suspensions from embryonic E11.5 and E13.5 telencephalon where individual cell phenotypes can be easily determined with cell type specific markers. Immunostaining with seven neuronal specific markers (MAP2, beta-III tubulin, GABA, reelin, GluR2/3, calbindin, and calretinin) failed to reveal any evidence of premature neuronal differentiation in PS1-/- telencephalon. We also determined the fraction of cells expressing the neural progenitor marker nestin and found no evidence for premature depletion of neural progenitor cells in PS1-/- telencephalon. Moreover, based on MAP2 staining of tissue sections from E12.5 embryos the topography of newly generated neurons also appeared to be undisturbed in the telencephalon of PS1-/- embryos. These studies thus argue that premature neuronal differentiation is unlikely to be a core pathophysiological feature underlying the aberrant cortical development that occurs in PS1-/- brain.

Role of presenilin-1 in cortical lamination and survival of Cajal-Retzius neurons

Presenilin-1 (PS1), the major causative gene of familial Alzheimer disease, regulates neuronal differentiation and Notch signaling during early neural development. To investigate the role of PS1 in neuronal migration and cortical lamination of the postnatal brain, we circumvented the perinatal lethality of PS1-null mice by generating a conditional knockout (cKO) mouse in which PS1 inactivation is restricted to neural progenitor cells (NPCs) and NPC-derived neurons and glia. BrdU birthdating analysis revealed that many late-born neurons fail to migrate beyond the early-born neurons to arrive at their appropriate positions in the superficial layer, while the migration of the early-born neurons is largely normal. The migration defect of late-born neurons coincides with the progressive reduction of radial glia in PS1 cKO mice. In contrast to the premature loss of Cajal-Retzius (CR) neurons in PS1-null mice, generation and survival of CR neurons are unaffected in PS1 cKO mice. Furthermore, the number of proliferating meningeal cells, which have been shown to be important for the survival of CR neurons, is increased in PS1-null mice but not in PS1 cKO mice. These findings show a cell-autonomous role for PS1 in cortical lamination and radial glial development, and a non-cell-autonomous role for PS1 in CR neuron survival.

Ryanodine receptor-mediated interference of neuronal cell differentiation by presenilin 2 mutation

Neuronal cell differentiation alterations induced by mutant presenilin 2 (PS2) were investigated in transgenic mice expressing wild-type or mutant-type PS2. Progressive increases in differentiation and marker protein expression were found in neuronal cells expressing wild-type PS2, whereas these processes were much perturbed in mutant-type PS2 with elevated ryanodine-receptor (RyR) expression and intracellular calcium levels. Moreover, dantrolene, a blocker of RyR reduced the PS2 mutation-induced interference of cell differentiation and calcium release, but caffeine, an activator of RyR, exacerbated PS2 mutation-induced interference with cell differentiation. Our results indicate that mutant PS2 inhibits normal neuronal cell differentiation and that RyR-mediated calcium overrelease may be a significant factor.

Altered morphological and electrophysiological properties of Cajal-Retzius cells in cerebral cortex of embryonic Presenilin-1 knockout mice

Mutations of Presenilin-1 are the major cause of familial Alzheimer's disease. Presenilin-1 knockout (PS1-/-) mice develop severe cortical dysplasia related to human type 2 lissencephaly. This overmigration syndrome has been attributed to the premature loss of Cajal-Retzius cells (CRcs), pioneer neurons required for the termination of radial neuronal migration. To elucidate the potential cellular mechanisms responsible for this premature neuronal loss, we investigated the morphological and electrophysiological properties of visually identified CRcs of wild-type (WT) and PS1-/- mouse brains at embryonic day 16.5. The density of CRcs was substantially reduced in the cerebral cortex of PS1-/-. In PS1-/- CRcs the number of axonal branches was significantly increased to 12.5 +/- 4.9 (n = 8; WT, 4.0 +/- 1.4, n = 12), while no differences in dendritic branching and total length of dendritic and axonal compartments were observed. Additionally, the resting membrane potential of PS1-/- CRcs was significantly depolarized (-48.3 +/- 1.7 mV; n = 23) in contrast to WT CRcs (-57.9 +/- 2.1 mV; n = 38). Active membrane properties were not affected by PS1 deficiency. CRcs of both genotypes showed spontaneous postsynaptic currents that could be completely blocked by 100 microM bicuculline and were unaffected by glutamatergic antagonists, suggesting that they were mediated by GABAA receptors. These results demonstrate that axonal branching and resting membrane potential of CRcs was affected in embryonic cerebral cortex of PS1-/- mice. The depolarized membrane potential observed in PS1-/- CRcs may increase the susceptibility to neuronal death, thus facilitating the premature loss of CRcs in PS1-/- mice.

Cortical dysplasia resembling human type 2 lissencephaly in mice lacking all three APP family members

The Alzheimer's disease beta-amyloid precursor protein (APP) is a member of a larger gene family that includes the amyloid precursor-like proteins, termed APLP1 and APLP2. We previously documented that APLP2-/-APLP1-/- and APLP2-/-APP-/- mice die postnatally, while APLP1-/-APP-/- mice and single mutants were viable. We now report that mice lacking all three APP/APLP family members survive through embryonic development, and die shortly after birth. In contrast to double-mutant animals with perinatal lethality, 81% of triple mutants showed cranial abnormalities. In 68% of triple mutants, we observed cortical dysplasias characterized by focal ectopic neuroblasts that had migrated through the basal lamina and pial membrane, a phenotype that resembles human type II lissencephaly. Moreover, at E18.5 triple mutants showed a partial loss of cortical Cajal Retzius (CR) cells, suggesting that APP/APLPs play a crucial role in the survival of CR cells and neuronal adhesion. Collectively, our data reveal an essential role for APP family members in normal brain development and early postnatal survival.

Presenilin 1 in migration and morphogenesis in the central nervous system

Morphogenesis of the central nervous system relies in large part upon the correct migration of neuronal cells from birthplace to final position. Two general modes of migration govern CNS morphogenesis: radial, which is mostly glia-guided and topologically relatively simple; and tangential, which often involves complex movement of neurons in more than one direction. We describe the consequences of loss of function of presenilin 1 on these fundamental processes. Previous studies of the central nervous system in presenilin 1 homozygote mutant embryos identified a premature neuronal differentiation that is transient and localized, with cortical dysplasia at later stages. We document widespread effects on CNS morphogenesis that appear strongly linked to defective neuronal migration. Loss of presenilin 1 function perturbs both radial and tangential migration in cerebral cortex, and several tangential migratory pathways in the brainstem. The inability of cells to execute their migratory trajectories affects cortical lamination, formation of the facial branchiomotor nucleus, the spread of cerebellar granule cell precursors to form the external granule layer and development of the pontine nuclei. Finally, overall morphogenesis of the mid-hindbrain region is abnormal,resulting in incomplete midline fusion of the cerebellum and overgrowth of the caudal midbrain. These observations indicate that in the absence of presenilin 1 function, the ability of a cell to move can be severely impaired regardless of its mode of migration, and, at a grosser level, brain morphogenesis is perturbed. Our results demonstrate that presenilin 1 plays a much more important role in brain development than has been assumed, consistent with a pleiotropic involvement of this molecule in cellular signaling.

Neuropathological and clinical phenotype of an Italian Alzheimer family with M239V mutation of presenilin 2 gene

Presenilin 1 and 2 are 2 highly homologous genes involved in familial Alzheimer disease. While more than 100 mutations in presenilin 1 are known to segregate with the disease in familial Alzheimer disease, only 9 mutations of presenilin 2 have been identified to date. We report the clinical and neuropathological phenotype of FLO10, the large Italian Alzheimer kindred associated with methionine to valine substitution at residue 239 of presenilin 2. The patients showed a remarkable variability in age of onset of symptoms, disease duration, and clinical presentation. The neuropathological study of 2 patients revealed peculiar features in addition to neurofibrillary changes and A beta amyloid deposits in the neuropil and vessel wall. Ectopic neurons in the subcortical white matter, often containing neurofibrillary tangles, were found in both patients, one of whom presented with epilepsy. Furthermore, 1 patient showed an unusually high number of ghost tangles in the cerebral cortex. These observations indicate that the Alzheimer kindred FLO10 associated with M239V mutation of presenilin 2 is characterized by some peculiarities of the clinical and neuropathologic phenotype compared to sporadic Alzheimer disease.

Presenilins in Memory, Alzheimer's Disease, and Therapy

Presenilins are considered to be the catalytic subunits of the gamma-secretase complex and are therefore drug targets for Alzheimer's disease. They are also essential for the fine tuning of the immunological system and for memory and synaptic plasticity. Genetic ablation in the forebrain results in a progressive neurodegenerative process that is independent from Abeta generation. The question arises as to what extent these observations should influence our thinking on the pathogenesis of Alzheimer's disease and on strategies to further develop gamma-secretase inhibitors.

Human neurodegenerative disease modeling using Drosophila

A number of approaches have been taken to recreate and to study the role of genes associated with human neurodegenerative diseases in the model organism Drosophila. These studies encompass the polyglutamine diseases, Parkinson's disease, Alzheimer's disease, and tau-associated pathologies. The findings highlight Drosophila as an important model system in which to study the fundamental pathways influenced by these genes and have led to new insights into aspects of pathogenesis and modifier mechanisms.

DNA microarray profiling of developing PS1-deficient mouse brain reveals complex and coregulated expression changes

Presenilin 1 (PS1) plays a critical role in the nervous system development and PS1 mutations have been associated with familial Alzheimer's disease. PS1-deficient mice exhibit alterations in neural and vascular development and die in late embryogenesis. The present study was aimed at uncovering transcript networks that depend on intact PS1 function in the developing brain. To achieve this, we analyzed the brains of PS1-deficient and control animals at embryonic ages E12.5 and E14.5 using MG_U74Av2 oligonucleotide microarrays by Affymetrix. Based on the microarray data, overall molecular brain development appeared to be comparable between the E12.5 and E14.5 PS1-deficient and control embryos. However, in brains of PS1-deficient mice, we observed significant differences in the expression of genes encoding molecules that are associated with neural differentiation, extracellular matrix, vascular development, Notch-related signaling and lipid metabolism. Many of the expression differences between wild-type and PS1-deficient animals were present at both E12.5 and E14.5, whereas other transcript alterations were characteristic of only one developmental stage. The results suggest that the role of PS1 in development includes influences on a highly co-regulated transcript network; some of the genes participating in this expression network may contribute to the pathophysiology of Alzheimer's disease.

Syndecan 3 intramembrane proteolysis is presenilin/gamma-secretase-dependent and modulates cytosolic signaling

The syndecans play critical roles in several signal transduction pathways. The core proteins of these heparan sulfate proteoglycans are characterized by highly conserved transmembrane and intracellular domains which are required for signaling across the membrane and for interaction with cytosolic proteins. However, regulatory mechanisms controlling these functions remain largely unknown. Here we show that, upon ligand-induced primary proteolytic cleavage within the ectodomain, the intracellular domain of syndecan 3 is released by regulated intramembrane proteolysis. The cleavage is mediated by presenilin/gamma-secretase complex and negatively regulates the plasma membrane targeting of the transcriptional cofactor CASK.

A cell biological perspective on Alzheimer's disease

The amyloid precursor protein and the proteases cleaving this protein are important players in the pathogenesis of Alzheimer's disease via the generation of the amyloid peptide. Physiologically, the amyloid precursor protein is implied in axonal vesicular trafficking and the proteases are implicated in developmentally important signaling pathways, most significantly those involving regulated intramembrane proteolysis or RIP. We discuss the cell biology behind the amyloid and tangle hypothesis for Alzheimer's disease, drawing on the many links to the fields of cell biology and developmental biology that have been established in the recent years.

Regulation of lipocalin-type prostaglandin D synthase gene expression by Hes-1 through E-box and interleukin-1 beta via two NF-kappa B elements in rat leptomeningeal cells

The promoter function of the rat lipocalin-type prostaglandin D synthase (L-PGDS) gene was characterized in primary cultures of leptomeningeal cells prepared from the neonatal rat brain. Luciferase reporter assays with deletion and site-directed mutation of the promoter region (-1250 to +77) showed that an AP-2 element at -109 was required for activation and an E-box at +57, for repression. Binding of nuclear factors to each of these cis-elements was demonstrated by an electrophoretic mobility shift assay. Several components of the Notch-Hes signaling pathway, Jagged, Notch1, Notch3, and Hes-1, were expressed in the leptomeningeal cells. Human Hes-1 co-expressed in the leptomeningeal cells bound to the E-box of the rat L-PGDS gene, and repressed the promoter activity of the rat L-PGDS gene in a dose-dependent manner. The L-PGDS gene expression was up-regulated slowly by interleukin-1 beta to the maximum level at 24 h. The reporter assay with deletion and mutation revealed that two NF-kappa B elements at -1106 and -291 were essential for this up-regulation. Binding of two NF-kappa B subunits, p65 and c-Rel, to these two NF-kappa B elements occurred after the interleukin-1 beta treatment. Therefore, the L-PGDS gene is the first gene identified as the target for the Notch-Hes signal through the E-box among a variety of genes involved in the prostanoid biosynthesis, classified to the lipocalin family, and expressed in the leptomeninges. Moreover, the L-PGDS gene is a unique gene that is activated slowly by the NF-kappa B system.

Stromal cell-derived factor 1 is secreted by meningeal cells and acts as chemotactic factor on neuronal stem cells of the cerebellar external granular layer

The cerebellar external granular layer (EGL) is an unusually long-lasting neural proliferative zone positioned immediately beneath the pial surface. Its position and stability critically depend on meningeal cells, as their selective destruction leads to its rapid dispersal, creating massive cortical ectopia. Similar ectopias have recently been described as a side effect of deficiency for stromal cell-derived factor 1 (SDF-1), a chemoattractant for haematopoietic precursor cell migration. Here we show that SDF-1 is present in meningeal cells in vivo and in vitro, where it is secreted in functionally relevant concentrations into the medium. Correspondingly, the SDF-1 receptor (termed CXCR4) can be demonstrated on stem cells of the external granular layer, but is absent on postmitotic cells commencing their final inward migration. We show that SDF-1 is concentrated by heparan sulphate proteoglycans highly expressed in the EGL in a laminar fashion, which thus might act to locally restrict SDF-1 action to the EGL in a kind of step gradient. In vitro, SDF-1 chemotactically attracts neuronal cells isolated from the external, but not from the internal granular layer, in a Boyden chamber assay in concentrations found in meningeal cell-conditioned medium. Selective removal of SDF-1 from conditioned media by immunoprecipitation abolishes their chemoattractive action, which can be reconstituted again by the addition of recombinant SDF-1. Meningeal cells are thus an important source for the expression of SDF-1 during brain development, which--comparable to its role in haematopoiesis--appears to be a key factor attracting precursor cells to their proliferative compartment.

The surface of the developing cerebral cortex: still special cells one century later

The marginal zone of the developing cerebral cortex is formed by different types of neurons, some of which were described more than one century ago. It is the case of Cajal-Retzius cells, which are known to synthesize and secrete Reelin, an extracellular matrix glycoprotein critically involved in the radial migration and early cortical cytoarchitectonic organization. These cells do not emit projection axons, a characteristic that bespeaks against these cells being considered as pioneer neurons. The true pioneer neurons of the marginal zone are part of a distinct cell entity: these are cells that emit the earliest descending axonal projection from the cerebral cortex into the subpallium, even before than subplate neurons, the other population of pioneer neurons in the cortical anlage. Finally, the marginal zone is a territory where cohorts of undifferentiated cortical interneurons migrate into the upper layers of the cerebral cortex. Marginal zone neurons, including Cajal-Retzius cells, tend to distribute non-uniformly over the cortical surface. Such a mosaic structural configuration points towards more complexities regarding their possible functions during cortical development.

A critical function of the pial basement membrane in cortical histogenesis

Mice with a targeted deletion of the nidogen-binding site of laminin gamma1 were used to study the function of the pial basement membrane in cortical histogenesis. The pial basement membrane in the mutant embryos assembled but was unstable and disintegrated at random segments. In segments with a disrupted basement membrane, radial glia cells were retracted from the pial surface, and radially migrating neurons, including Cajal-Retzius cells and cortical plate neurons, passed the meninges or terminated their migration prematurely. By correlating the disruptions in the pial basal lamina with changes in the morphology of radial glia cells, the aberrant migration of Cajal-Retzius cells, and subsequent dysplasia of cortical plate neurons, the present data establish a causal relationship of proper cortical histogenesis with the presence of an intact pial basement membrane.

Expression of p73 and Reelin in the developing human cortex

Cajal-Retzius (CR) cells of the developing neocortex secrete Reelin (Reln), a glycoprotein involved in neuronal migration. CR cells selectively express p73, a p53 family member implicated in cell survival and apoptosis. Immunocytochemistry in prenatal human telencephalon reveals a complex sequence of migration waves of p73- and Reln-immunoreactive (IR) neurons into the cortical marginal zone (MZ). At early preplate stages, p73/Reln-IR cells arise in distinct sectors of the telencephalon, including cortical primordium and ganglionic eminences. After the appearance of the cortical plate, further p73/Reln-IR cells originate in the medial periolfactory forebrain. In addition, p73 marks a novel cell population that appears at the choroid-cortical junction or cortical hem before the emergence of the dorsal hippocampus. A pronounced mediolateral gradient in the density of p73/Reln-IR neurons in the neocortical MZ at 8 gestational weeks suggests that a subset of CR cells migrate tangentially from cortical hem and taenia tecta into neocortical territory. This hypothesis is supported by the absence of p73-transcripts in prospective neocortex of p73-/-mice at embryonic day 12 (E12), whereas they are present in cortical hem and taenia tecta. In the p73-/- preplate, Reln is faintly expressed in a calretinin-positive cell population, not present in this form in the E12 wild-type cortex. At P2, Reln-IR CR cells are undetectable in the p73-/- cortex, whereas Reln-expression in interneurons is unchanged. Our results point to a close association between p73 and Reln in CR cells of the developing neocortex, with a partial dissociation in early preplate and basal telencephalon, and to a p73-mediated role of the cortical hem in neocortical development.

Overexpression of wild type but not an FAD mutant presenilin-1 promotes neurogenesis in the hippocampus of adult mice

Mutations in the presenilin-1 (PS-1) gene are one cause of familial Alzheimer's disease (FAD). However, the functions of the PS-1 protein as well as how PS-1 mutations cause FAD are incompletely understood. Here we investigated if neuronal overexpression of wild-type or FAD mutant PS-1 in transgenic mice affects neurogenesis in the hippocampus of adult animals. We show that either a wild-type or an FAD mutant PS-1 transgene reduces the number of neural progenitors in the dentate gyrus. However, the wild-type, but not the FAD mutant PS-1 promoted the survival and differentiation of progenitors leading to more immature granule cell neurons being generated in PS-1 wild type expressing animals. These studies suggest that PS-1 plays a role in regulating neurogenesis in adult hippocampus and that FAD mutants may have deleterious properties independent of their effects on amyloid deposition.

Neuronal deficiency of presenilin 1 inhibits amyloid plaque formation and corrects hippocampal long-term potentiation but not a cognitive defect of amyloid precursor protein [V717I] transgenic mice

In the brain of Alzheimer's disease (AD) patients, neurotoxic amyloid peptides accumulate and are deposited as senile plaques. A major therapeutic strategy aims to decrease production of amyloid peptides by inhibition of gamma-secretase. Presenilins are polytopic transmembrane proteins that are essential for gamma-secretase activity during development and in amyloid production. By loxP/Cre-recombinase-mediated deletion, we generated mice with postnatal, neuron-specific presenilin-1 (PS1) deficiency, denoted PS1(n-/-), that were viable and fertile, with normal brain morphology. In adult PS1(n-/-) mice, levels of endogenous brain amyloid peptides were strongly decreased, concomitant with accumulation of amyloid precursor protein (APP) C-terminal fragments. In the cross of APP[V717I]xPS1 (n-/-) double transgenic mice, the neuronal absence of PS1 effectively prevented amyloid pathology, even in mice that were 18 months old. This contrasted sharply with APP[V717I] single transgenic mice that all develop amyloid pathology at the age of 10-12 months. In APP[V717I]xPS1 (n-/-) mice, long-term potentiation (LTP) was practically rescued at the end of the 2 hr observation period, again contrasting sharply with the strongly impaired LTP in APP[V717I] mice. The findings demonstrate the critical involvement of amyloid peptides in defective LTP in APP transgenic mice. Although these data open perspectives for therapy of AD by gamma-secretase inhibition, the neuronal absence of PS1 failed to rescue the cognitive defect, assessed by the object recognition test, of the parent APP[V717I] transgenic mice. This points to potentially detrimental effects of accumulating APP C99 fragments and demands further study of the consequences of inhibition of gamma-secretase activity. In addition, our data highlight the complex functional relation of APP and PS1 to cognition and neuronal plasticity in adult and aging brain.

Identification and characterization of presenilin-independent Notch signaling

Presenilin (PS) proteins control the proteolytic cleavage that precedes nuclear access of the Notch intracellular domain. Here we observe that a partial activation of the HES1 promoter can be detected in PS1/PS2 (PS1/2) double null cells using Notch1 Delta E constructs or following Delta 1 stimulation, despite an apparent abolition of the production and nuclear accumulation of the Notch intracellular domain. PS1/2-independent Notch activation is sensitive to Numblike, a physiological inhibitor of Notch. PS1/2-independent Notch signaling is also inhibited by an active gamma-secretase inhibitor in the low micromolar range and is not inhibited by an inactive analogue, similar to PS-dependent Notch signaling. However, experiments using a Notch1-Gal4-VP16 fusion protein indicate that the PS1/2-independent activity does not release Gal4-VP16 and is therefore unlikely to proceed via an intramembranous cleavage. These data reveal that a novel PS1/2-independent mechanism plays a partial role in Notch signal transduction.

Ectopic white matter neurons, a developmental abnormality that may be caused by the PSEN1 S169L mutation in a case of familial AD with myoclonus and seizures

We report clinical, neuropathologic and molecular genetic data from an individual affected by a familial Alzheimer disease (AD) variant. The proband had an onset of dementia at age 29 followed by generalized seizures a year later. He died at age 40. Neuropathologically, he had severe brain atrophy and characteristic histopathologic lesions of AD. Three additional neuropathologic features need to be emphasized: 1) severe deposition of Abeta in the form of diffuse deposits in the cerebral and cerebellar cortices, 2) numerous Abeta deposits in the subcortical white matter and in the centrum semiovale, and 3) numerous ectopic neurons, often containing tau-immunopositive neurofibrillary tangles, in the white maner of the frontal and temporal lobes. A molecular genetic analysis of DNA extracted from brain tissue of the proband revealed a S169L mutation in the Presenilin 1 (PSEN1) gene. The importance of this case lies in the presence of ectopic neurons in the white matter, early-onset seizures, and a PSEN1 mutation. We hypothesize that the PSEN1 mutation may have a causal relationship with an abnormality in neuronal development.

Interaction with telencephalin and the amyloid precursor protein predicts a ring structure for presenilins

The carboxyl terminus of presenilin 1 and 2 (PS1 and PS2) binds to the neuron-specific cell adhesion molecule telencephalin (TLN) in the brain. PS1 deficiency results in the abnormal accumulation of TLN in a yet unidentified intracellular compartment. The first transmembrane domain and carboxyl terminus of PS1 form a binding pocket with the transmembrane domain of TLN. Remarkably, APP binds to the same regions via part of its transmembrane domain encompassing the critical residues mutated in familial Alzheimer's disease. Our data surprisingly indicate a spatial dissociation between the binding site and the proposed catalytic site near the critical aspartates in PSs. They provide important experimental evidence to support a ring structure model for PS.

Relationship between beta amyloid peptide generating molecules and neprilysin in Alzheimer disease and normal brain

beta-Amyloid peptide (Abeta) is generated by two cleavages of amyloid precursor protein (APP). The initial cleavage by BACE is followed by gamma-secretase cleavage of the C-terminal APP fragment. Presenilin-1 (PS-1) is intimately related to gamma-secretase. Once formed, Abeta is mainly broken down by neprilysin. To estimate vulnerability to Abeta senile plaque formation, we measured the relative mRNA levels of APP695, APP751, APP770, BACE, presenilin-1 (PS-1) and neprilysin in nine brain areas and in heart, liver, spleen and kidney in a series of Alzheimer disease (AD) and control cases. Each of the mRNAs was expressed in every tissue examined. APP695 was the dominant APP isoform in brain. Compared with controls, APP695 and PS-1 mRNA levels were significantly elevated in high plaque areas of AD brain, while neprilysin mRNA levels were significantly reduced. BACE levels were not significantly different in AD compared with control brain. In peripheral organs, there were no significant differences in any of the mRNAs between AD and control cases. APP isoforms were differently expressed in the periphery than in brain, with APP 751>770>695. Neprilysin mRNA levels were much higher, while APP695 and PS-1 mRNA levels were much lower in the periphery than in brain. The data suggest that, in the periphery, the capacity to degrade Abeta is srong, accounting for the failure of Abeta deposits to form. In plaque prone areas of AD brain, the capacity to degrade Abeta is weak, while the capacity to generate Ab is upregulated. In plaque resistant areas of brain, a closer balance exists, but there is some tendency towards lower degrading and higher synthesizing capacity in AD brain compared with control brain. Overall, the data indicate that effectiveness of degradation by neprilysin may be a key factor in determining whether Abeta deposits develop.

Role of the reelin signaling pathway in central nervous system development

The neurological mutant mouse reeler has played a critical role in the evolution of our understanding of normal brain development. From the earliest neuroanatomic studies of reeler, it was anticipated that the characterization of the gene responsible would elucidate important molecular and cellular principles governing cell positioning and the formation of synaptic circuits in the developing brain. Indeed, the identification of reelin has challenged many of our previous notions and has led to a new vision of the events involved in the migration of neurons. Several neuronal populations throughout the brain secrete Reelin, which binds to transmembrane receptors located on adjacent cells triggering a tyrosine kinase cascade. This allows neurons to complete migration and adopt their ultimate positions in laminar structures in the central nervous system. Recent studies have also suggested a role for the Reelin pathway in axonal branching, synaptogenesis, and pathology underlying neurodegeneration.

Neuropathology of mice carrying mutant APP(swe) and/or PS1(M146L) transgenes: alterations in the p75(NTR) cholinergic basal forebrain septohippocampal pathway

Cholinergic basal forebrain (CBF) projection systems are defective in late Alzheimer's disease (AD). We examined the brains of 12-month-old singly and doubly transgenic mice overexpressing mutant amyloid precursor protein (APP(swe)) and/or presenilin-1 (PS1(M146L)) to investigate the effects of these AD-related genes on plaque and tangle pathology, astrocytic expression, and the CBF projection system. Two types of beta-amyloid (Abeta)-immunoreactive (ir) plaques were observed: type 1 were darkly stained oval and elongated deposits of Abeta, and type 2 were diffuse plaques containing amyloid fibrils. APP(swe) and PS1(M146L) mouse brains contained some type 1 plaques, while the doubly transgenic (APP(swe)/PS1(M146L)) mice displayed a greater abundance of types 1 and 2 plaques. Sections immunostained for the p75 NGF receptor (p75(NTR)) revealed circular patches scattered throughout the cortex and hippocampus of the APP(swe)/PS1(M146L) mice that contained Abeta, were innervated by p75(NTR)-ir neurites, but displayed virtually no immunopositive neurons. Tau pathology was not seen in any transgenic genotype, although a massive glial response occurred in the APP(swe)/PS1(M146L) mice associated with amyloid plaques. Stereology revealed a significant increase in p75(NTR)-ir medial septal neurons in the APP(swe) and PS1(M146L) singly transgenic mice compared to the APP(swe)/PS1(M146L) mice. No differences in size or optical density of p75(NTR)-ir neurons were observed in these three mutants. p75(NTR)-ir fibers in hippocampus and cortex were more pronounced in the APP(swe) and PS1(M146L) mice, while the APP(swe)/PS1(M146L) mice showed the least p75(NTR)-ir fiber staining. These findings suggest a neurotrophic role for mutant APP and PS1 upon cholinergic hippocampal projection neurons at 12 months of age.

Human neocortical development: the importance of embryonic and early fetal events

The identification of numerous genes involved in the development of the cerebral cortex has led to an increased interest in the early stages of corticogenesis, when the first postmitotic neurons migrate into the cortical plate to form the foundation of the adult cortex. However, the cellular substrate of gene expression in early human cortical development is widely unknown. This article analyzes the complex sequence of events in the differentiation of the preplate, the predecessor of the neocortex, and discusses the possible origin and migratory routes of the neuronal populations involved in the transition from preplate to cortical plate. The neuronal classes present in embryonic and early fetal stages are redefined in terms of their relationship with the Reelin-Dab1 signaling pathway whose integrity is essential for successful migration into the cortex. A timetable of developmental steps is provided, and the peculiarities of the preplate derivatives in the human brain, marginal zone, and subplate are discussed. The results presented here may contribute to a deeper understanding of the pathogenesis of migration disorders.

Neuronal migration

Like other motile cells, neurons migrate in three schematic steps, namely leading edge extension, nuclear translocation or nucleokinesis, and retraction of the trailing process. In addition, neurons are ordered into architectonic patterns at the end of migration. Leading edge extension can proceed at the extremity of the axon, by growth cone formation, or from the dendrites, by formation of dendritic tips. Among both categories of leading edges, variation seems to be related to the rate of extension of the leading process. Leading edge extension is directed by microfilament polymerization following integration of extracellular cues and is regulated by Rho-type small GTPases. In humans, mutations of filamin, an actin-associated protein, result in heterotopic neurons, probably due to defective leading edge extension. The second event in neuron migration is nucleokinesis, a process which is critically dependent on the microtubule network, as shown in many cell types, from slime molds to vertebrates. In humans, mutations in the PAFAH1B1 gene (more commonly called LIS1) or in the doublecortin (DCX) gene result in type 1 lissencephalies that are most probably due to defective nucleokinesis. Both the Lis1 and doublecortin proteins interact with microtubules, and two Lis1-interacting proteins, Nudel and mammalian NudE, are components of the dynein motor complex and of microtubule organizing centers. In mice, mutations of Cdk5 or of its activators p35 and p39 result in a migration phenotype compatible with defective nucleokinesis, although an effect on leading edge formation is also likely. The formation of architectonic patterns at the end of migration requires the integrity of the Reelin signalling pathway. Other known components of the pathway include members of the lipoprotein receptor family, the intracellular adaptor Dab1, and possibly integrin alpha 3 beta 1. Defective Reelin leads to poor lamination and, in humans, to a lissencephaly phenotype different from type 1 lissencephaly. Although the action of Reelin is unknown, it may trigger some recognition-adhesion among target neurons. Finally, pattern formation requires the integrity of the external limiting membrane, defects of which lead to overmigration of neurons in meninges and to human type 2 lissencephaly.

Alzheimer's disease as a disorder of mechanisms underlying structural brain self-organization

Mental function has as its cerebral basis a specific dynamic structure. In particular, cortical and limbic areas involved in "higher brain functions" such as learning, memory, perception, self-awareness and consciousness continuously need to be self-adjusted even after development is completed. By this lifelong self-optimization process, the cognitive, behavioural and emotional reactivity of an individual is stepwise remodelled to meet the environmental demands. While the presence of rigid synaptic connections ensures the stability of the principal characteristics of function, the variable configuration of the flexible synaptic connections determines the unique, non-repeatable character of an experienced mental act. With the increasing need during evolution to organize brain structures of increasing complexity, this process of selective dynamic stabilization and destabilization of synaptic connections becomes more and more important. These mechanisms of structural stabilization and labilization underlying a lifelong synaptic remodelling according to experience, are accompanied, however, by increasing inherent possibilities of failure and may, thus, not only allow for the evolutionary acquisition of "higher brain function" but at the same time provide the basis for a variety of neuropsychiatric disorders. It is the objective of the present paper to outline the hypothesis that it might be the disturbance of structural brain self-organization which, based on both genetic and epigenetic information, constantly "creates" and "re-creates" the brain throughout life, that is the defect that underlies Alzheimer's disease (AD). This hypothesis is, in particular, based on the following lines of evidence. (1) AD is a synaptic disorder. (2) AD is associated with aberrant sprouting at both the presynaptic (axonal) and postsynaptic (dendritic) site. (3) The spatial and temporal distribution of AD pathology follows the pattern of structural neuroplasticity in adulthood, which is a developmental pattern. (4) AD pathology preferentially involves molecules critical for the regulation of modifications of synaptic connections, i.e. "morphoregulatory" molecules that are developmentally controlled, such as growth-inducing and growth-associated molecules, synaptic molecules, adhesion molecules, molecules involved in membrane turnover, cytoskeletal proteins, etc. (5) Life events that place an additional burden on the plastic capacity of the brain or that require a particularly high plastic capacity of the brain might trigger the onset of the disease or might stimulate a more rapid progression of the disease. In other words, they might increase the risk for AD in the sense that they determine when, not whether, one gets AD. (6) AD is associated with a reactivation of developmental programmes that are incompatible with a differentiated cellular background and, therefore, lead to neuronal death. From this hypothesis, it can be predicted that a therapeutic intervention into these pathogenetic mechanisms is a particular challenge as it potentially interferes with those mechanisms that at the same time provide the basis for "higher brain function".

Presenilin function in APP processing

Familial Alzheimer's disease (FAD) is now linked to at least three genes encoding the amyloid precursor protein (APP) on chromosome 21, and presenilin 1 and 2 on chromosome 14 and 1, respectively. FAD cases in whom presenilin mutations occur are more frequent than those with APP mutations. However, altogether they only account for approximately 0.1% of all the people suffering from Alzheimer's disease (AD), and the causes of the remaining 99.9% of the sporadic form of AD or senile dementia remain unknown. Since FAD presents with the same neuropathological features as sporadic AD, i.e., cognitive impairments and the amyloid plaques and tangles in the brain, our working hypothesis is that similar molecular pathogenic mechanisms underly both sporadic and familial AD. It follows that APP and the presenilins must be key players in the disease. Detailed knowledge about the cell biology of these proteins will be a rich source of insight into the pathology of AD, but will also shed light on the fundamental neurobiology of these proteins.

The role of Alzheimer's disease-related presenilin 1 in intercellular adhesion

Most cases of familial early-onset Alzheimer's disease are caused by mutations in the presenilin 1 (PS1) gene. However, the cellular functions of PS1 are unknown. We showed predominant localization of PS1 to cell-cell contacts of the plasma membrane in human prostate epithelial tissue and in a human epithelial cell line HEp2 stably transfected with an inducible PS1 construct. PS1 co-immunoprecipitated with beta-catenin from cell lysates of stable transfectants. Conversely, PS1 lacking the PS1-beta-catenin interaction site did not co-immunoprecipitate with beta-catenin and was not recruited to the cell-cell contacts. L cells, which do not form tight intercellular contacts, formed clusters of adhered cells after stable transfection with GFP-PS1 cDNA and demonstrated a clear preference for independent aggregation in the mixed cultures. However, L cells transfected with mutant GFP-PS1 constructs, which had a truncated N-terminus of PS1 or deleted PS1-beta-catenin interaction site, failed to form intercellular contacts. In addition, in primary cultures of mouse cortical neurons PS1 was highly concentrated on the surface of extended growth cones. Taken together, our results suggest an important role of PS1 in intercellular adhesion in epithelial cells and neurons.

Developmental mechanisms in the pathogenesis of neurodegenerative diseases

Cellular genes that are mutated in neurodegenerative diseases code for proteins that are expressed throughout neural development. Genetic analysis suggests that these genes are essential for a broad range of normal neurodevelopmental processes. The proteins they code for interact with numerous other cellular proteins that are components of signaling pathways involved in patterning of the neural tube and in regional specification of neuronal subtypes. Further, pathogenetic mutations of these genes can cause progressive, sublethal alterations in the cellular homeostasis of evolving regional neuronal subpopulations, culminating in late-onset cell death. Therefore, as a consequence of the disease mutations, targeted cell populations may retain molecular traces of abnormal interactions with disease-associated proteins by exhibiting changes in a spectrum of normal cellular functions and enhanced vulnerability to a host of environmental stressors. These observations suggest that the normal functions of these disease-associated proteins are to ensure the fidelity and integration of developmental events associated with the progressive elaboration of neuronal subtypes as well as the maintenance of mature neuronal populations during adult life. The ability to identify alterations within vulnerable neuronal precursors present in pre-symptomatic individuals prior to the onset of irrevocable cellular injury may help foster the development of effective therapeutic interventions using evolving pharmacologic, gene and stem cell technologies.

Neuronal migration disorders: from genetic diseases to developmental mechanisms

Neurons that constitute the cerebral cortex must migrate hundreds of cell-body distances from their place of birth, and through several anatomical boundaries, to reach their final position within the correct cortical layer. Human neurological conditions associated with abnormal neuronal migration, together with spontaneous and engineered mouse mutants, define at least four distinct steps in cortical neuronal migration. Many of the genes that control neuronal migration have strong genetic or biochemical links to the cytoskeleton, suggesting that the field of neuronal migration might be closing in on the underlying cytoskeletal events.

Presenilin-1 regulates neuronal differentiation during neurogenesis

Mutations in Presenilin-1 (PS1) are a major cause of familial Alzheimer's disease. Our previous studies showed that PS1 is required for murine neural development. Here we report that lack of PS1 leads to premature differentiation of neural progenitor cells, indicating a role for PS1 in a cell fate decision between postmitotic neurons and neural progenitor cells. Neural proliferation and apoptotic cell death during neurogenesis are unaltered in PS1(−/−) mice, suggesting that the reduction in the neural progenitor cells observed in the PS1(−/−) brain is due to premature differentiation of progenitor cells, rather than to increased apoptotic cell death or decreased cell proliferation. In addition, the premature neuronal differentiation in the PS1(−/−) brain is associated with aberrant neuronal migration and disorganization of the laminar architecture of the developing cerebral hemisphere. In the ventricular zone of PS1(−/−) mice, expression of the Notch1 downstream effector gene Hes5 is reduced and expression of the Notch1 ligand Dll1 is elevated, whereas expression of Notch1 is unchanged. The level of Dll1 transcripts is also increased in the presomitic mesoderm of PS1(−/−) embryos, while the level of Notch1 transcripts is unchanged, in contrast to a previous report (Wong et al., 1997, Nature 387, 288–292). These results provide direct evidence that PS1 controls neuronal differentiation in association with the downregulation of Notch signalling during neurogenesis.

Proteolytic processing and cell biological functions of the amyloid precursor protein

Recent research has identified some key players involved in the proteolytic processing of amyloid precursor protein (APP) to amyloid beta-peptide, the principal component of the amyloid plaques in Alzheimer patients. Interesting parallels exists with the proteolysis of other proteins involved in cell differentiation, cholesterol homeostasis and stress responses. Since the cytoplasmic domain of APP is anchored to a complex protein network that might function in axonal elongation, dendritic arborisation and neuronal cell migration, the proteolysis of APP might be critically involved in intracellular signalling events.

Binding partners of Alzheimer's disease proteins: are they physiologically relevant?

Protein-protein interactions are a molecular basis for the structural and functional organization within cells. They are mediated by a growing number of protein modules that bind peptide targets. Alterations in binding affinities can have serious consequences for some essential cellular processes. The three proteins identified to have mutations in their corresponding genes leading to presenile Alzheimer dementia (AD)-the amyloid precursor protein (APP) and presenilin 1 and 2-all interact with other proteins. The nature and function of these interacting proteins may contribute to elucidating the proper physiological functions of the AD proteins. APP-interacting proteins are pointing toward a function of APP in cell adhesion and neurite outgrowth and signaling. Proteins interacting with the presenilins however are more diverse in nature linking presenilin function to regulation in different signaling pathways including Wnt and Notch but also in apoptosis and Ca(2+) homeostasis. Further research however is still needed to delineate the exact functional relevance of these interactions with respect to the physiological functions of the AD proteins in particular and the contribution of these proteins to AD pathogenesis in general.

Presenilin-1 differentially facilitates endoproteolysis of the beta-amyloid precursor protein and Notch

Mutations in the presenilin-1 (PS1) gene are associated with Alzheimer's disease and cause increased secretion of the neurotoxic amyloid-beta peptide (Abeta). Critical intramembraneous aspartates at residues 257 and 385 are required for the function of PS1 protein. Here we investigate the biological function of a naturally occurring PS1 splice variant (PS1 Deltaexon 8), which lacks the critical aspartate 257. Cell lines that stably express PS1 Deltaexon 8 or a PS1 protein in which aspartate residue 257 is mutated secrete significant levels of Abeta, whereas Abeta generation is severely reduced in cells transfected with PS1 containing a mutation of aspartate 385. In contrast, endoproteolytic processing of Notch is almost completely inhibited in cell lines expressing any of the PS1 variants that lack one of the critical aspartates. These data indicate that PS1 may differentially facilitate gamma-secretase-mediated generation of Abeta and endoproteolysis of Notch.

Cortical development: receiving reelin

Recent genetic and biochemical studies indicate that lipoprotein receptors are components of the neuronal receptor for Reelin, mediating the glycoprotein's essential function in cortical development. At least eight cadherin-related neuronal receptors may also play a part in this signalling system.