ApoE receptor 2 controls neuronal survival in the adult brain

The Reelin receptor ApoER2 is a cargo for the adaptor protein complex AP-4: Implications for Hereditary Spastic Paraplegia

Adaptor protein complex 4 (AP-4) is a heterotetrameric complex that promotes export of selected cargo proteins from the trans-Golgi network. Mutations in each of the AP-4 subunits cause a complicated form of Hereditary Spastic Paraplegia (HSP). Herein, we report that ApoER2, a receptor in the Reelin signaling pathway, is a cargo of the AP-4 complex. We identify the motif ISSF/Y within the ApoER2 cytosolic domain as necessary for interaction with the canonical signal-binding pocket of the µ4 (AP4M1) subunit of AP-4. AP4E1- knock-out (KO) HeLa cells and hippocampal neurons from Ap4e1-KO mice display increased co-localization of ApoER2 with Golgi markers. Furthermore, hippocampal neurons from Ap4e1-KO mice and AP4M1-KO human iPSC-derived cortical i3Neurons exhibit reduced ApoER2 protein expression. Analyses of biosynthetic transport of ApoER2 reveal differential post-Golgi trafficking of the receptor, with lower axonal distribution in KO compared to wild-type neurons, indicating a role of AP-4 and the ISSF/Y motif in the axonal localization of ApoER2. Finally, analyses of Reelin signaling in mouse hippocampal and human cortical KO neurons show that AP4 deficiency causes no changes in Reelin-dependent activation of the AKT pathway and only mild changes in Reelin-induced dendritic arborization, but reduces Reelin-induced ERK phosphorylation, CREB activation, and Golgi deployment. This work thus establishes ApoER2 as a novel cargo of the AP-4 complex, suggesting that defects in the trafficking of this receptor and in the Reelin signaling pathway could contribute to the pathogenesis of HSP caused by mutations in AP-4 subunits.

The Reelin Receptor ApoER2 is a Cargo for the Adaptor Protein Complex AP-4: Implications for Hereditary Spastic Paraplegia

Adaptor protein complex 4 (AP-4) is a heterotetrameric complex that promotes protein export from the trans -Golgi network. Mutations in each of the AP-4 subunits cause a complicated form of Hereditary Spastic Paraplegia (HSP). Herein, we report that ApoER2, a receptor in the Reelin signaling pathway, is a cargo of the AP-4 complex. We identify the motif ISSF/Y within the ApoER2 cytosolic domain as necessary for interaction with the canonical signal-binding pocket of the µ4 (AP4M1) subunit of AP-4. AP4E1 -knock-out (KO) HeLa cells and hippocampal neurons from Ap4e1 -KO mice display increased Golgi localization of ApoER2. Furthermore, hippocampal neurons from Ap4e1 -KO mice and AP4M1 -KO human iPSC-derived cortical i3Neurons exhibit reduced ApoER2 protein expression. Analyses of biosynthetic transport of ApoER2 reveal differential post-Golgi trafficking of the receptor, with lower axonal distribution in KO compared to wild-type neurons, indicating a role of AP-4 and the ISSF/Y motif in the axonal localization of ApoER2. Finally, analyses of Reelin signaling in mouse hippocampal and human cortical KO neurons show that AP4 deficiency causes no changes in Reelin-dependent activation of the AKT pathway and only mild changes in Reelin-induced dendritic arborization, but reduces Reelin-induced ERK phosphorylation, CREB activation, and Golgi deployment. Altogether, this work establishes ApoER2 as a novel cargo of the AP-4 complex, suggesting that defects in the trafficking of this receptor and in the Reelin signaling pathway could contribute to the pathogenesis of HSP caused by mutations in AP-4 subunits.

Roles of ApoE4 on the Pathogenesis in Alzheimer's Disease and the Potential Therapeutic Approaches

The Apolipoprotein E ε4 (ApoE ε4) allele, encoding ApoE4, is the strongest genetic risk factor for late-onset Alzheimer's disease (LOAD). Emerging epidemiological evidence indicated that ApoE4 contributes to AD through influencing β-amyloid (Aβ) deposition and clearance. However, the molecular mechanisms of ApoE4 involved in AD pathogenesis remains unclear. Here, we introduced the structure and functions of ApoE isoforms, and then we reviewed the potential mechanisms of ApoE4 in the AD pathogenesis, including the effect of ApoE4 on Aβ pathology, and tau phosphorylation, oxidative stress; synaptic function, cholesterol transport, and mitochondrial dysfunction; sleep disturbances and cerebrovascular integrity in the AD brains. Furthermore, we discussed the available strategies for AD treatments that target to ApoE4. In general, this review overviews the potential roles of ApoE4 in the AD development and suggests some therapeutic approaches for AD. ApoE4 is genetic risk of AD. ApoE4 is involved in the AD pathogenesis. Aβ deposition, NFT, oxidative stress, abnormal cholesterol, mitochondrial dysfunction and neuroinflammation could be observed in the brains with ApoE4. Targeting the interaction of ApoE4 with the AD pathology is available strategy for AD treatments.

TransIntegrator: capture nearly full protein-coding transcript variants via integrating Illumina and PacBio transcriptomes

Genes have the ability to produce transcript variants that perform specific cellular functions. However, accurately detecting all transcript variants remains a long-standing challenge, especially when working with poorly annotated genomes or without a known genome. To address this issue, we have developed a new computational method, TransIntegrator, which enables transcriptome-wide detection of novel transcript variants. For this, we determined 10 Illumina sequencing transcriptomes and a PacBio full-length transcriptome for consecutive embryo development stages of amphioxus, a species of great evolutionary importance. Based on the transcriptomes, we employed TransIntegrator to create a comprehensive transcript variant library, namely iTranscriptome. The resulting iTrancriptome contained 91 915 distinct transcript variants, with an average of 2.4 variants per gene. This substantially improved current amphioxus genome annotation by expanding the number of genes from 21 954 to 38 777. Further analysis manifested that the gene expansion was largely ascribed to integration of multiple Illumina datasets instead of involving the PacBio data. Moreover, we demonstrated an example application of TransIntegrator, via generating iTrancriptome, in aiding accurate transcriptome assembly, which significantly outperformed other hybrid methods such as IDP-denovo and Trinity. For user convenience, we have deposited the source codes of TransIntegrator on GitHub as well as a conda package in Anaconda. In summary, this study proposes an affordable but efficient method for reliable transcriptomic research in most species.

Alternative splicing in neurodegenerative disease and the promise of RNA therapies

Alternative splicing generates a myriad of RNA products and protein isoforms of different functions from a single gene. Dysregulated alternative splicing has emerged as a new mechanism broadly implicated in the pathogenesis of neurodegenerative diseases such as Alzheimer disease, amyotrophic lateral sclerosis, frontotemporal dementia, Parkinson disease and repeat expansion diseases. Understanding the mechanisms and functional outcomes of abnormal splicing in neurological disorders is vital in developing effective therapies to treat mis-splicing pathology. In this Review, we discuss emerging research and evidence of the roles of alternative splicing defects in major neurodegenerative diseases and summarize the latest advances in RNA-based therapeutic strategies to target these disorders.

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.

Proprotein convertase subtilisin/kexin type 9 (PCSK9) in the central nervous system

The gene encoding proprotein convertase subtilisin/kexin type 9 (PCSK9) and its protein product have been widely studied for their role in cholesterol and lipid metabolism. PCSK9 increases the rate of metabolic degradation of low-density lipoprotein receptors, preventing the diffusion of low-density lipoprotein (LDL) from plasma into cells and contributes to high lipoprotein-bound cholesterol levels in the plasma. While most research has focused on the regulation and disease relevance of PCSK9 to the cardiovascular system and lipid metabolism, there is a growing body of evidence that PCSK9 plays a crucial role in pathogenic processes in other organ systems, including the central nervous system. PCSK9's impact on the brain is not yet fully understood, though several recent studies have sought to illuminate its impact on various neurodegenerative and psychiatric disorders, as well as its connection with ischemic stroke. Cerebral PCSK9 expression is low but is highly upregulated during disease states. Among others, PCSK9 is known to play a role in neurogenesis, neural cell differentiation, central LDL receptor metabolism, neural cell apoptosis, neuroinflammation, Alzheimer's Disease, Alcohol Use Disorder, and stroke. The PCSK9 gene contains several polymorphisms, including both gain-of-function and loss-of-function mutations which profoundly impact normal PCSK9 signaling and cholesterol metabolism. Gain-of-function mutations lead to persistent hypercholesterolemia and poor health outcomes, while loss-of-function mutations generally lead to hypocholesterolemia and may serve as a protective factor against diseases of the liver, cardiovascular system, and central nervous system. Recent genomic studies have sought to identify the end-organ effects of such mutations and continue to identify evidence of a much broader role for PCSK9 in extrahepatic organ systems. Despite this, there remain large gaps in our understanding of PCSK9, its regulation, and its effects on disease risk outside the liver. This review, which incorporates data from a wide range of scientific disciplines and experimental paradigms, is intended to describe PCSK9's role in the central nervous system as it relates to cerebral disease and neuropsychiatric disorders, and to examine the clinical potential of PCSK9 inhibitors and genetic variation in the PCSK9 gene on disease outcomes, including neurological and neuropsychiatric disease.

Low-Density Lipoprotein Receptor-Related Protein 8 at the Crossroad between Cancer and Neurodegeneration

The low-density-lipoprotein receptors represent a family of pleiotropic cell surface receptors involved in lipid homeostasis, cell migration, proliferation and differentiation. The family shares common structural features but also has significant differences mainly due to tissue-specific interactors and to peculiar proteolytic processing. Among the receptors in the family, recent studies place low-density lipoprotein receptor-related protein 8 (LRP8) at the center of both neurodegenerative and cancer-related pathways. From one side, its overexpression has been highlighted in many types of cancer including breast, gastric, prostate, lung and melanoma; from the other side, LRP8 has a potential role in neurodegeneration as apolipoprotein E (ApoE) and reelin receptor, which are, respectively, the major risk factor for developing Alzheimer’s disease (AD) and the main driver of neuronal migration, and as a γ-secretase substrate, the main enzyme responsible for amyloid formation in AD. The present review analyzes the contributions of LDL receptors, specifically of LRP8, in both cancer and neurodegeneration, pointing out that depending on various interactions and peculiar processing, the receptor can contribute to both proliferative and neurodegenerative processes.

Human APOER2 Isoforms Have Differential Cleavage Events and Synaptic Properties

Human apolipoprotein E receptor 2 (APOER2) is a type I transmembrane protein with a large extracellular domain (ECD) and a short cytoplasmic tail. APOER2-ECD contains several ligand-binding domains (LBDs) that are organized into exons with aligning phase junctions, which allows for in-frame exon cassette splicing events. We have identified 25 human APOER2 isoforms from cerebral cortex using gene-specific APOER2 primers, where the majority are exon-skipping events within the N-terminal LBD regions compared with six identified in the heart. APOER2 undergoes proteolytic cleavage in response to ligand binding that releases a C-terminal fragment (CTF) and transcriptionally active intracellular domain (ICD). We tested whether the diversity of human brain-specific APOER2 variants affects APOER2 cleavage. We found isoforms with differing numbers of ligand-binding repeats generated different amounts of CTFs compared with full-length APOER2 (APOER2-FL). Specifically, APOER2 isoforms lacking exons 5-8 (Δex5-8) and lacking exons 4-6 (Δex4-6) generated the highest and lowest amounts of CTF generation, respectively, in response to APOE peptide compared with APOER2-FL. The differential CTF generation of Δex5-8 and Δex4-6 coincides with the proteolytic release of the ICD, which mediates transcriptional activation facilitated by the Mint1 adaptor protein. Functionally, we demonstrated loss of mouse Apoer2 decreased miniature event frequency in excitatory synapses, which may be because of a decrease in the total number of synapses and/or VAMP2 positive neurons. Lentiviral infection with human APOER2-FL or Δex4-6 isoform in Apoer2 knockout neurons restored the miniature event frequency but not Δex5-8 isoform. These results suggest that human APOER2 isoforms have differential cleavage events and synaptic properties.SIGNIFICANCE STATEMENT Humans and mice share virtually the same number of protein-coding genes. However, humans have greater complexity of any higher eukaryotic organisms by encoding multiple protein forms through alternative splicing modifications. Alternative splicing allows pre-mRNAs transcribed from genes to be spliced in different arrangements, producing structurally and functionally distinct protein variants that increase proteomic diversity and are particularly prevalent in the human brain. Here, we identified 25 distinct human APOER2 splice variants from the cerebral cortex using gene-specific APOER2 primers, where the majority are exon-skipping events that exclude N-terminal ligand-binding regions of APOER2. We show that some of the APOER2 variants have differential proteolytic properties in response to APOE ligand and exhibit distinct synaptic properties.

Single molecule, long-read Apoer2 sequencing identifies conserved and species-specific splicing patterns

Apolipoprotein E receptor 2 (Apoer2) is a synaptic receptor in the brain that binds disease-relevant ligand Apolipoprotein E (Apoe) and is highly alternatively spliced. We examined alternative splicing (AS) of conserved Apoer2 exons across vertebrate species and identified gain of exons in mammals encoding functional domains such as the cytoplasmic and furin inserts, and loss of an exon in primates encoding the eighth LDLa repeat, likely altering receptor surface levels and ligand-binding specificity. We utilized single molecule, long-read RNA sequencing to profile full-length Apoer2 isoforms and identified 68 and 48 unique full-length Apoer2 transcripts in the mouse and human cerebral cortex, respectively. Furthermore, we identified two exons encoding protein functional domains, the third EGF-precursor like repeat and glycosylation domain, that are tandemly skipped specifically in mouse. Our study provides new insight into Apoer2 isoform complexity in the vertebrate brain and highlights species-specific differences in splicing decisions that support functional diversity.

ExonSkipAD provides the functional genomic landscape of exon skipping events in Alzheimer's disease

Exon skipping (ES), the most common alternative splicing event, has been reported to contribute to diverse human diseases due to the loss of functional domains/sites or frameshifting of the open reading frame (ORF) and noticed as therapeutic targets. Accumulating transcriptomic studies of aging brains show the splicing disruption is a widespread hallmark of neurodegenerative diseases such as Alzheimer's disease (AD). Here, we built ExonSkipAD, the ES annotation database aiming to provide a resource/reference for functional annotation of ES events in AD and identify therapeutic targets in exon units. We identified 16 414 genes that have ~156 K, ~ 69 K, ~ 231 K ES events from the three representative AD cohorts of ROSMAP, MSBB and Mayo, respectively. For these ES events, we performed multiple functional annotations relating to ES mechanisms or downstream. Specifically, through the functional feature retention studies followed by the open reading frames (ORFs), we identified 275 important cellular regulators that might lose their cellular regulator roles due to exon skipping in AD. ExonSkipAD provides twelve categories of annotations: gene summary, gene structures and expression levels, exon skipping events with PSIs, ORF annotation, exon skipping events in the canonical protein sequence, 3'-UTR located exon skipping events lost miRNA-binding sites, SNversus in the skipped exons with a depth of coverage, AD stage-associated exon skipping events, splicing quantitative trait loci (sQTLs) in the skipped exons, correlation with RNA-binding proteins, and related drugs & diseases. ExonSkipAD will be a unique resource of transcriptomic diversity research for understanding the mechanisms of neurodegenerative disease development and identifying potential therapeutic targets in AD. Significance AS the first comprehensive resource of the functional genomics of the alternative splicing events in AD, ExonSkipAD will be useful for many researchers in the fields of pathology, AD genomics and precision medicine, and pharmaceutical and therapeutic researches.

Apolipoprotein E receptor 2 deficiency decreases endothelial adhesion of monocytes and protects against autoimmune encephalomyelitis

Under normal conditions, the blood-brain barrier effectively regulates the passage of immune cells into the central nervous system (CNS). However, under pathological conditions such as multiple sclerosis (MS), leukocytes, especially monocytes, infiltrate the CNS where they promote inflammatory demyelination, resulting in paralysis. Therapies targeting the immune cells directly and preventing leukocyte infiltration exist for MS but may compromise the immune system. Here, we explore how apolipoprotein E receptor 2 (ApoER2) regulates vascular adhesion and infiltration of monocytes during inflammation. We induced experimental autoimmune encephalitis in ApoER2 knockout mice and in mice carrying a loss-of-function mutation in the ApoER2 cytoplasmic domain. In both models, paralysis and neuroinflammation were largely abolished as a result of greatly diminished monocyte adherence due to reduced expression of adhesion molecules on the endothelial surface. Our findings expand our mechanistic understanding of the vascular barrier, the regulation of inflammation and vascular permeability, and the therapeutic potential of ApoER2-targeted therapies.

SFRS11 Loss Leads to Aging-Associated Cognitive Decline by Modulating LRP8 and ApoE

RNA binding proteins, the key regulators in gene expression at the posttranscriptional level, remain largely uncharacterized with respect to aging and relevant cognitive deterioration. Here, we report that the levels of SFRS11 are substantially decreased in the prefrontal cortex (PFC) of aged brains. Notably, mice with SFRS11 deficiency in the PFC show impaired learning and memory. We demonstrate that SFRS11 directly binds to the 3' UTR of LRP8 mRNA, as well as to the third exon of apoE mRNA, resulting in stabilization of these mRNAs, eventually deactivating JNK signaling. Importantly, restoration of LRP8 and apoE reduces JNK signaling that is significantly enhanced in SFRS11-deficient cells. In addition, LRP8 and apoE rescue aging-like phenotypes induced by SFRS11 loss. Our findings demonstrate that age-dependent loss of SFRS11 in the PFC reduces levels of apoE and LRP8, leading to activation of the JNK pathway, ultimately influencing cognitive deficits.

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.

Proprotein Convertase Subtilisin/Kexin Type 9 (PCSK9) in the Brain and Relevance for Neuropsychiatric Disorders

Proprotein convertase subtilisin/kexin type 9 (PCSK9) has long been studied in the liver due to its regulation of plasma low-density lipoprotein cholesterol (LDL-C) and its causal role in familial hypercholesterolemia. Although PCSK9 was first discovered in cerebellar neurons undergoing apoptosis, its function in the central nervous system (CNS) is less clear. PCSK9 has been shown to be involved in neuronal differentiation, LDL receptor family metabolism, apoptosis, and inflammation in the brain, but in vitro and in vivo studies offer contradictory findings. PCSK9 expression in the adult brain is low but is highly upregulated during disease states. Cerebral spinal fluid (CSF) PCSK9 concentrations are correlated with neural tube defects and neurodegenerative diseases in human patients. Epigenetic studies reveal that chronic alcohol use may modulate methylation of the PCSK9 gene and genetic studies show that patients with gain-of-function PCSK9 variants have higher LDL-C and an increased risk of ischemic stroke. Early safety studies of the PCSK9 inhibitors evolocumab and alirocumab, used to treat hypercholesterolemia, hinted that PCSK9 inhibition may negatively impact cognition but more recent, longer-term clinical trials found no adverse neurocognitive events. The purpose of this review is to elucidate the role of PCSK9 in the brain, particularly its role in disease pathogenesis.

Therapeutic IDOL Reduction Ameliorates Amyloidosis and Improves Cognitive Function in APP/PS1 Mice

Brain lipoprotein receptors have been shown to regulate the metabolism of ApoE and β-amyloid (Aβ) and are potential therapeutic targets for Alzheimer's disease (AD). Previously, we identified E3 ubiquitin ligase IDOL as a negative regulator of brain lipoprotein receptors. Genetic ablation of Idol increases low-density lipoprotein receptor protein levels, which facilitates Aβ uptake and clearance by microglia. In this study, we utilized an antisense oligonucleotide (ASO) to reduce IDOL expression therapeutically in the brains of APP/PS1 male mice. ASO treatment led to decreased Aβ pathology and improved spatial learning and memory. Single-cell transcriptomic analysis of hippocampus revealed that IDOL inhibition upregulated lysosomal/phagocytic genes in microglia. Furthermore, clustering of microglia revealed that IDOL-ASO treatment shifted the composition of the microglia population by increasing the prevalence of disease-associated microglia. Our results suggest that reducing IDOL expression in the adult brain promotes the phagocytic clearance of Aβ and ameliorates Aβ-dependent pathology. Pharmacological inhibition of IDOL activity in the brain may represent a therapeutic strategy for the treatment of AD.

Amyloid toxicity in Alzheimer's disease

A major feature of Alzheimer's disease (AD) pathology is the plaque composed of aggregated amyloid-β (Aβ) peptide. Although these plaques may have harmful properties, there is much evidence to implicate soluble oligomeric Aβ as the primary noxious form. Aβ oligomers can be generated both extracellularly and intracellularly. Aβ is toxic to neurons in a myriad of ways. It can cause pore formation resulting in the leakage of ions, disruption of cellular calcium balance, and loss of membrane potential. It can promote apoptosis, cause synaptic loss, and disrupt the cytoskeleton. Current treatments for AD are limited and palliative. Much research and effort is being devoted to reducing Aβ production as an approach to slowing or preventing the development of AD. Aβ formation results from the amyloidogenic cleavage of human amyloid precursor protein (APP). Reconfiguring this process to disfavor amyloid generation might be possible through the reduction of APP or inhibition of enzymes that convert the precursor protein to amyloid.

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.

Inhibition of proprotein convertase subtilisin/kexin type 9 attenuates neuronal apoptosis following focal cerebral ischemia via apolipoprotein E receptor 2 downregulation in hyperlipidemic mice

The inhibition of proprotein convertase subtilisin/kexin type 9 (PCSK9) protects a variety of cell types against neuronal apoptosis by binding to apolipoprotein E receptor 2 (ApoER2). The present study aimed to determine the association between PCSK9/ApoER2 signaling and neuronal apoptosis following middle cerebral artery occlusion (MCAO) injury in hyperlipidemic mice. For this purpose, C57BL/6 mice fed with a high-fat diet (HFD) for 6 weeks were exposed to MCAO. Subsequently, PCSK9 was inhibited by a lentiviral vector harboring short-hairpin RNA (shRNA) targeting PCSK9, which was stereotaxically injected into the cerebral cortex of mice. At 48 h post-ischemia, hematoxylineosin staining and a terminal deoxynucleotidyl transferase dUTP nick end labeling assay were performed to determine cerebral tissue injury and apoptosis. PCSK9 and ApoER2 expression levels were assessed by reverse transcription-quantitative polymerase chain reaction, immunohistochemistry and western blotting. The results indicated that hyperlipidemia and increased PCSK9 expression were evident in HFD mice. Cerebral histological injury and neuronal apoptosis, as well as PCSK9 and ApoER2 levels, which were increased upon ischemia in hyperlipidemic mice, were attenuated by PCSK9 shRNA treatment. These protective effects of PCSK9 shRNA interference were associated with decreased neuronal apoptosis and a reduced level of ApoER2 expression in the hippocampus and cortex. The data of the present study demonstrated that the PCSK9 shRNA-mediated anti-apoptotic effect induced by MCAO in hyperlipidemic mice is associated with ApoER2 downregulation, which may be a potential new therapy for stroke treatment in patients with hyperlipidemia.

Genome-wide association for testis weight in the diversity outbred mouse population

Testis weight is a genetically mediated trait associated with reproductive efficiency across numerous species. We sought to evaluate the genetically diverse, highly recombinant Diversity Outbred (DO) mouse population as a tool to identify and map quantitative trait loci (QTLs) associated with testis weight. Testis weights were recorded for 502 male DO mice and the mice were genotyped on the GIGAMuga array at ~ 143,000 SNPs. We performed a genome-wide association analysis and identified one significant and two suggestive QTLs associated with testis weight. Using bioinformatic approaches, we developed a list of candidate genes and identified those with known roles in testicular size and development. Candidates of particular interest include the RNA demethylase gene Alkbh5, the cyclin-dependent kinase inhibitor gene Cdkn2c, the dynein axonemal heavy chain gene Dnah11, the phospholipase D gene Pld6, the trans-acting transcription factor gene Sp4, and the spermatogenesis-associated gene Spata6, each of which has a human ortholog. Our results demonstrate the utility of DO mice in high-resolution genetic mapping of complex traits, enabling us to identify developmentally important genes in adult mice. Understanding how genetic variation in these genes influence testis weight could aid in the understanding of mechanisms of mammalian reproductive function.

Potential Link Between Proprotein Convertase Subtilisin/Kexin Type 9 and Alzheimer's Disease

Alzheimer's disease [AD] is not only the most common neurodegenerative disease but is also currently incurable. Proprotein convertase subtilisin/kexin-9 [PCSK9] is an indirect regulator of plasma low density lipoprotein [LDL] levels controlling LDL receptor expression at the plasma membrane. PCSK9 also appears to regulate the development of glucose intolerance, insulin resistance, abdominal obesity, inflammation, and hypertension, conditions that have been identified as risk factors for AD. PCSK9 levels also depend on age, sex, and ethnic background, factors associated with AD. Herein, we will review indirect evidence that suggests a link between PCSK9 levels and AD.

Therapeutic correction of ApoER2 splicing in Alzheimer's disease mice using antisense oligonucleotides

Apolipoprotein E receptor 2 (ApoER2) is an apolipoprotein E receptor involved in long‐term potentiation, learning, and memory. Given its role in cognition and its association with the Alzheimer's disease (AD) risk gene, apoE, ApoER2 has been proposed to be involved in AD, though a role for the receptor in the disease is not clear. ApoER2 signaling requires amino acids encoded by alternatively spliced exon 19. Here, we report that the balance of ApoER2 exon 19 splicing is deregulated in postmortem brain tissue from AD patients and in a transgenic mouse model of AD. To test the role of deregulated ApoER2 splicing in AD, we designed an antisense oligonucleotide (ASO) that increases exon 19 splicing. Treatment of AD mice with a single dose of ASO corrected ApoER2 splicing for up to 6 months and improved synaptic function and learning and memory. These results reveal an association between ApoER2 isoform expression and AD, and provide preclinical evidence for the utility of ASOs as a therapeutic approach to mitigate Alzheimer's disease symptoms by improving ApoER2 exon 19 splicing.

New Insights in the Pathophysiology of Antiphospholipid Syndrome

The antiphospholipid syndrome (APS) is an autoimmune disorder characterized by an elevated risk for arterial and venous thrombosis and pregnancy-related morbidity. Since the discovery of the disease in 1980s, numerous studies in cell culture systems, in animal models, and in patient populations have been reported, leading to a deeper understanding of the pathogenesis of APS. These studies have determined that circulating autoantibodies, collectively called antiphospholipid antibodies (aPL), the majority of which recognize cell surface proteins attached to the plasma membrane phospholipids, play a causal role in the development of the disease. The binding of aPL to the cell surface antigens triggers interaction of the complex with transmembrane receptors to initiate intracellular signaling in critical cell types, including platelets, monocytes, endothelial cells, and trophoblasts. Subsequent alteration of various cell functions results in inflammation, thrombus formation, and pregnancy complications. Apolipoprotein E receptor 2 (apoER2), a lipoprotein receptor family member, has been implicated as a mediator for aPL actions in platelets and endothelial cells. Nitric oxide (NO) is a signaling molecule known to exert potent antithrombotic, anti-inflammatory, and anti-atherogenic effects. NO insufficiency and oxidative stress have been linked to APS pathogenesis. This review will focus on the recent findings on how apoER2 and dysregulation of NO production contribute to aPL-mediated pathologies in APS.

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.

Improvement of neuronal cell survival by astrocyte-derived exosomes under hypoxic and ischemic conditions depends on prion protein

Prion protein (PrP) protects neural cells against oxidative stress, hypoxia, ischemia, and hypoglycemia. In the present study we confirm that cultured PrP-deficient neurons are more sensitive to oxidative stress than wild-type neurons and present the novel findings that wild-type, but not PrP-deficient astrocytes protect wild-type cerebellar neurons against oxidative stress and that exosomes released from stressed wild-type, but not from stressed PrP-deficient astrocytes reduce neuronal cell death induced by oxidative stress. We show that neuroprotection by exosomes of stressed astrocytes depends on exosomal PrP but not on neuronal PrP and that astrocyte-derived exosomal PrP enters into neurons, suggesting neuronal uptake of astrocyte-derived exosomes. Upon exposure of wild-type astrocytes to hypoxic or ischemic conditions PrP levels in exosomes were increased. By mass spectrometry and Western blot analysis, we detected increased levels of 37/67 kDa laminin receptor, apolipoprotein E and the ribosomal proteins S3 and P0, and decreased levels of clusterin/apolipoprotein J in exosomes from wild-type astrocytes exposed to oxygen/glucose deprivation relative to exosomes from astrocytes maintained under normoxic conditions. The levels of these proteins were not altered in exosomes from stressed PrP-deficient astrocytes relative to unstressed PrP-deficient astrocytes. These results indicate that PrP in astrocytes is a sensor for oxidative stress and mediates beneficial cellular responses, e.g. release of exosomes carrying PrP and other molecules, resulting in improved survival of neurons under hypoxic and ischemic conditions.

Stress-induced upregulation of VLDL receptor alters Wnt-signaling in neurons

Lipoprotein receptor family members hold multiple roles in the brain, and alterations in lipoprotein receptor expression and function are implicated in neuronal stress, developmental disorders and neurodegenerative diseases, such as Alzheimer's disease. Berberine (BBR), a nutraceutical shown to have both neuroprotective and neurotoxic properties, is suggested to regulate lipoprotein receptor expression. We show that subtoxic concentration of BBR regulates neuronal lipoprotein receptor expression in a receptor- and time-dependent fashion in cerebellar granule neurons (CGN). Similarly to BBR, subtoxic concentrations of neuronal stressors cobalt chloride, thapsigargin and rotenone increased very-low-density lipoprotein receptor (VLDLR) mRNA and protein expression in CGN suggesting a conserved pathway for stress-induced upregulation of VLDLR in neurons. We also show that VLDLR upregulation is accompanied by transiently increased stabilization of hypoxia-induced factor 1 alpha (HIF-1α) and decreased β-catenin levels affecting the Wnt pathway through GSK3β phosphorylation, a crucial player in neurodegenerative processes. Our results indicate that neuronal stress differentially regulates lipoprotein receptor expression in neurons, with VLDLR upregulation as a common element as a modulator of neuronal Wnt signaling.

Differential splicing and glycosylation of Apoer2 alters synaptic plasticity and fear learning

Apoer2 is an essential receptor in the central nervous system that binds to the apolipoprotein ApoE. Various splice variants of Apoer2 are produced. We showed that Apoer2 lacking exon 16, which encodes the O-linked sugar (OLS) domain, altered the proteolytic processing and abundance of Apoer2 in cells and synapse number and function in mice. In cultured cells expressing this splice variant, extracellular cleavage of OLS-deficient Apoer2 was reduced, consequently preventing γ-secretase-dependent release of the intracellular domain of Apoer2. Mice expressing Apoer2 lacking the OLS domain had increased Apoer2 abundance in the brain, hippocampal spine density, and glutamate receptor abundance, but decreased synaptic efficacy. Mice expressing a form of Apoer2 lacking the OLS domain and containing an alternatively spliced cytoplasmic tail region that promotes glutamate receptor signaling showed enhanced hippocampal long-term potentiation (LTP), a phenomenon associated with learning and memory. However, these mice did not display enhanced spatial learning in the Morris water maze, and cued fear conditioning was reduced. Reducing the expression of the mutant Apoer2 allele so that the abundance of the protein was similar to that of Apoer2 in wild-type mice normalized spine density, hippocampal LTP, and cued fear learning. These findings demonstrated a role for ApoE receptors as regulators of synaptic glutamate receptor activity and established differential receptor glycosylation as a potential regulator of synaptic function and memory.

Extracellular matrix control of dendritic spine and synapse structure and plasticity in adulthood

Dendritic spines are the receptive contacts at most excitatory synapses in the central nervous system. Spines are dynamic in the developing brain, changing shape as they mature as well as appearing and disappearing as they make and break connections. Spines become much more stable in adulthood, and spine structure must be actively maintained to support established circuit function. At the same time, adult spines must retain some plasticity so their structure can be modified by activity and experience. As such, the regulation of spine stability and remodeling in the adult animal is critical for normal function, and disruption of these processes is associated with a variety of late onset diseases including schizophrenia and Alzheimer's disease. The extracellular matrix (ECM), composed of a meshwork of proteins and proteoglycans, is a critical regulator of spine and synapse stability and plasticity. While the role of ECM receptors in spine regulation has been extensively studied, considerably less research has focused directly on the role of specific ECM ligands. Here, we review the evidence for a role of several brain ECM ligands and remodeling proteases in the regulation of dendritic spine and synapse formation, plasticity, and stability in adults.

Neurotrophins regulate ApoER2 proteolysis through activation of the Trk signaling pathway

Background

ApoER2 and the neurotrophin receptors Trk and p75^NTR are expressed in the CNS and regulate key functional aspects of neurons, including development, survival, and neuronal function. It is known that both ApoER2 and p75^NTR are processed by metalloproteinases, followed by regulated intramembrane proteolysis. TrkA activation by nerve growth factor (NGF) increases the proteolytic processing of p75^NTR mediated by ADAM17. Reelin induces the sheeding of ApoER2 ectodomain depending on metalloproteinase activity. However, it is not known if there is a common regulation mechanism for processing these receptors.

Results

We found that TrkA activation by NGF in PC12 cells induced ApoER2 processing, which was dependent on TrkA activation and metalloproteinases. NGF-induced ApoER2 proteolysis was independent of mitogen activated protein kinase activity and of phosphatidylinositol-3 kinase activity. In contrast, the basal proteolysis of ApoER2 increased when both kinases were pharmacologically inhibited. The ApoER2 ligand reelin regulated the proteolytic processing of its own receptor but not of p75^NTR. Finally, in primary cortical neurons, which express both ApoER2 and TrkB, we found that the proteolysis of ApoER2 was also regulated by brain-derived growth factor (BDNF).

Conclusions

Our results highlight a novel relationship between neurotrophins and the reelin-ApoER2 system, suggesting that these two pathways might be linked to regulate brain development, neuronal survival, and some pathological conditions.

Macrophage deficiency of Akt2 reduces atherosclerosis in Ldlr null mice

Macrophages play crucial roles in the formation of atherosclerotic lesions. Akt, a serine/threonine protein kinase B, is vital for cell proliferation, migration, and survival. Macrophages express three Akt isoforms, Akt1, Akt2, and Akt3, but the roles of Akt1 and Akt2 in atherosclerosis in vivo remain unclear. To dissect the impact of macrophage Akt1 and Akt2 on early atherosclerosis, we generated mice with hematopoietic deficiency of Akt1 or Akt2. After 8 weeks on Western diet, Ldlr^−/− mice reconstituted with Akt1^−/− fetal liver cells (Akt1^−/−→Ldlr^−/−) had similar atherosclerotic lesion areas compared with control mice transplanted with WT cells (WT→Ldlr^−/−). In contrast, Akt2^−/−→Ldlr^−/− mice had dramatically reduced atherosclerotic lesions compared with WT→Ldlr^−/− mice of both genders. Similarly, in the setting of advanced atherosclerotic lesions, Akt2^−/−→Ldlr^−/− mice had smaller aortic lesions compared with WT→Ldlr^−/− and Akt1^−/−→Ldlr^−/− mice. Importantly, Akt2^−/−→Ldlr^−/− mice had reduced numbers of proinflammatory blood monocytes expressing Ly-6C^hi and chemokine C-C motif receptor 2. Peritoneal macrophages isolated from Akt2^−/− mice were skewed toward an M2 phenotype and showed decreased expression of proinflammatory genes and reduced cell migration. Our data demonstrate that loss of Akt2 suppresses the ability of macrophages to undergo M1 polarization reducing both early and advanced atherosclerosis.

Apolipoprotein E receptor-2 deficiency enhances macrophage susceptibility to lipid accumulation and cell death to augment atherosclerotic plaque progression and necrosis

Genome-wide association studies have linked LRP8 polymorphisms to premature coronary artery disease and myocardial infarction in humans. However, the mechanisms by which dysfunctions of apolipoprotein E receptor-2 (apoER2), the protein encoded by LRP8 gene, influence atherosclerosis have not been elucidated completely. The current study focused on the role of apoER2 in macrophages, a cell type that plays an important role in atherosclerosis. Results showed that apoER2-deficient mouse macrophages accumulated more lipids and were more susceptible to oxidized LDL (oxLDL)-induced death compared to control cells. Consistent with these findings, apoER2 deficient macrophages also displayed defective serum-induced Akt activation and higher levels of the pro-apoptotic protein phosphorylated p53. Furthermore, the expression and activation of peroxisome proliferator-activated receptor γ (PPARγ) were increased in apoER2-deficient macrophages. Deficiency of apoER2 in hypercholesterolemic LDL receptor-null mice (Lrp8(-/-)Ldlr(-/-) mice) also resulted in accelerated atherosclerosis with more complex lesions and extensive lesion necrosis compared to Lrp8(+/+)Ldlr(-/-) mice. The atherosclerotic plaques of Lrp8(-/-)Ldlr(-/-) mice displayed significantly higher levels of p53-positive macrophages, indicating that the apoER2-deficient macrophages contribute to the accelerated atherosclerotic lesion necrosis observed in these animals. Taken together, this study indicates that apoER2 in macrophages limits PPARγ expression and protects against oxLDL-induced cell death. Thus, abnormal apoER2 functions in macrophages may at least in part contribute to the premature coronary artery disease and myocardial infarction in humans with LRP8 polymorphisms. Moreover, the elevated PPARγ expression in apoER2-deficient macrophages suggests that LRP8 polymorphism may be a genetic modifier of cardiovascular risk with PPARγ therapy.

Interdigital cell death in the embryonic limb is associated with depletion of Reelin in the extracellular matrix

Interdigital cell death is a physiological regression process responsible for sculpturing the digits in the embryonic vertebrate limb. Changes in the intensity of this degenerative process account for the different patterns of interdigital webbing among vertebrate species. Here, we show that Reelin is present in the extracellular matrix of the interdigital mesoderm of chick and mouse embryos during the developmental stages of digit formation. Reelin is a large extracellular glycoprotein which has important functions in the developing nervous system, including neuronal survival; however, the significance of Reelin in other systems has received very little attention. We show that reelin expression becomes intensely downregulated in both the chick and mouse interdigits preceding the establishment of the areas of interdigital cell death. Furthermore, fibroblast growth factors, which are cell survival signals for the interdigital mesoderm, intensely upregulated reelin expression, while BMPs, which are proapototic signals, downregulate its expression in the interdigit. Gene silencing experiments of reelin gene or its intracellular effector Dab-1 confirmed the implication of Reelin signaling as a survival factor for the limb undifferentiated mesoderm. We found that Reelin activates canonical survival pathways in the limb mesoderm involving protein kinase B and focal adhesion kinase. Our findings support that Reelin plays a role in interdigital cell death, and suggests that anoikis (apoptosis secondary to loss of cell adhesion) may be involved in this process.

Apolipoprotein E and apolipoprotein E receptors: normal biology and roles in Alzheimer disease

Apolipoprotein E (APOE) genotype is the major genetic risk factor for Alzheimer disease (AD); the ε4 allele increases risk and the ε2 allele is protective. In the central nervous system (CNS), apoE is produced by glial cells, is present in high-density-like lipoproteins, interacts with several receptors that are members of the low-density lipoprotein receptor (LDLR) family, and is a protein that binds to the amyloid-β (Aβ) peptide. There are a variety of mechanisms by which apoE isoform may influence risk for AD. There is substantial evidence that differential effects of apoE isoform on AD risk are influenced by the ability of apoE to affect Aβ aggregation and clearance in the brain. Other mechanisms are also likely to play a role in the ability of apoE to influence CNS function as well as AD, including effects on synaptic plasticity, cell signaling, lipid transport and metabolism, and neuroinflammation. ApoE receptors, including LDLRs, Apoer2, very low-density lipoprotein receptors (VLDLRs), and lipoprotein receptor-related protein 1 (LRP1) appear to influence both the CNS effects of apoE as well as Aβ metabolism and toxicity. Therapeutic strategies based on apoE and apoE receptors may include influencing apoE/Aβ interactions, apoE structure, apoE lipidation, LDLR receptor family member function, and signaling. Understanding the normal and disease-related biology connecting apoE, apoE receptors, and AD is likely to provide novel insights into AD pathogenesis and treatment.

Function of alternative splicing

Almost all polymerase II transcripts undergo alternative pre-mRNA splicing. Here, we review the functions of alternative splicing events that have been experimentally determined. The overall function of alternative splicing is to increase the diversity of mRNAs expressed from the genome. Alternative splicing changes proteins encoded by mRNAs, which has profound functional effects. Experimental analysis of these protein isoforms showed that alternative splicing regulates binding between proteins, between proteins and nucleic acids as well as between proteins and membranes. Alternative splicing regulates the localization of proteins, their enzymatic properties and their interaction with ligands. In most cases, changes caused by individual splicing isoforms are small. However, cells typically coordinate numerous changes in 'splicing programs', which can have strong effects on cell proliferation, cell survival and properties of the nervous system. Due to its widespread usage and molecular versatility, alternative splicing emerges as a central element in gene regulation that interferes with almost every biological function analyzed.

JNK3 perpetuates metabolic stress induced by Aβ peptides

Although Aβ peptides are causative agents in Alzheimer's disease (AD), the underlying mechanisms are still elusive. We report that Aβ42 induces a translational block by activating AMPK, thereby inhibiting the mTOR pathway. This translational block leads to widespread ER stress, which activates JNK3. JNK3 in turn phosphorylates APP at T668, thereby facilitating its endocytosis and subsequent processing. In support, pharmacologically blocking translation results in a significant increase in Aβ42 in a JNK3-dependent manner. Thus, JNK3 activation, which is increased in human AD cases and a familial AD (FAD) mouse model, is integral to perpetuating Aβ42 production. Concomitantly, deletion of JNK3 from FAD mice results in a dramatic reduction in Aβ42 levels and overall plaque loads and increased neuronal number and improved cognition. This reveals AD as a metabolic disease that is under tight control by JNK3.

Reduced hippocampal volume in healthy young ApoE4 carriers: an MRI study

The E4 allele of the ApoE gene has consistently been shown to be related to an increased risk of Alzheimer's disease (AD). The E4 allele is also associated with functional and structural grey matter (GM) changes in healthy young, middle-aged and older subjects. Here, we assess volumes of deep grey matter structures of 22 healthy younger ApoE4 carriers and 22 non-carriers (20-38 years). Volumes of the nucleus accumbens, amygdala, caudate nucleus, hippocampus, pallidum, putamen, thalamus and brain stem were calculated by FMRIB's Integrated Registration and Segmentation Tool (FIRST) algorithm. A significant drop in volume was found in the right hippocampus of ApoE4 carriers (ApoE4+) relative to non-carriers (ApoE4-), while there was a borderline significant decrease in the volume of the left hippocampus of ApoE4 carriers. The volumes of no other structures were found to be significantly affected by genotype. Atrophy has been found to be a sensitive marker of neurodegenerative changes, and our results show that within a healthy young population, the presence of the ApoE4+ carrier gene leads to volume reduction in a structure that is vitally important for memory formation. Our results suggest that the hippocampus may be particularly vulnerable to further degeneration in ApoE4 carriers as they enter middle and old age. Although volume reductions were noted bilaterally in the hippocampus, atrophy was more pronounced in the right hippocampus. This finding relates to previous work which has noted a compensatory increase in right hemisphere activity in ApoE4 carriers in response to preclinical declines in memory function. Possession of the ApoE4 allele may lead to greater predilection for right hemisphere atrophy even in healthy young subjects in their twenties.

Production of selenoprotein P (Sepp1) by hepatocytes is central to selenium homeostasis

BACKGROUND

Sepp1 transports selenium, but its complete role in selenium homeostasis is not known.

RESULTS

Deletion of Sepp1 in hepatocytes increases liver selenium at the expense of other tissues and decreases whole-body selenium by increasing excretion.

CONCLUSION

Sepp1 production by hepatocytes retains selenium in the organism and distributes it from the liver to peripheral tissues.

SIGNIFICANCE

Sepp1 is central to selenium homeostasis. Sepp1 is a widely expressed extracellular protein that in humans and mice contains 10 selenocysteine residues in its primary structure. Extra-hepatic tissues take up plasma Sepp1 for its selenium via apolipoprotein E receptor-2 (apoER2)-mediated endocytosis. The role of Sepp1 in the transport of selenium from liver, a rich source of the element, to peripheral tissues was studied using mice with selective deletion of Sepp1 in hepatocytes (Sepp1(c/c)/alb-cre(+/-) mice). Deletion of Sepp1 in hepatocytes lowered plasma Sepp1 concentration to 10% of that in Sepp1(c/c) mice (controls) and increased urinary selenium excretion, decreasing whole-body and tissue selenium concentrations. Under selenium-deficient conditions, Sepp1(c/c)/alb-cre(+/-) mice accumulated selenium in the liver at the expense of extra-hepatic tissues, severely worsening clinical manifestations of dietary selenium deficiency. These findings are consistent with there being competition for metabolically available hepatocyte selenium between the synthesis of selenoproteins and the synthesis of selenium excretory metabolites. In addition, selenium deficiency down-regulated the mRNA of the most abundant hepatic selenoprotein, glutathione peroxidase-1 (Gpx1), to 15% of the selenium-replete value, while reducing Sepp1 mRNA, the most abundant hepatic selenoprotein mRNA, only to 61%. This strongly suggests that Sepp1 synthesis is favored in the liver over Gpx1 synthesis when selenium supply is limited, directing hepatocyte selenium to peripheral tissues in selenium deficiency. We conclude that production of Sepp1 by hepatocytes is central to selenium homeostasis in the organism because it promotes retention of selenium in the body and effects selenium distribution from the liver to extra-hepatic tissues, especially under selenium-deficient conditions.

SeP, ApoER2 and megalin as necessary factors to maintain Se homeostasis in mammals

Selenoprotein P (SeP) is an extracellular protein containing ten selenium atoms in the form of selenocysteine, secreted mainly from the liver. About 60% of the whole plasma selenium level is present in SeP, which makes it a useful biomarker of selenium nutritional status. The main functions of SeP are transport and storage of selenium in plasma. It is especially an important protein for the brain, testes and kidneys where the supplementation of the proper amount of Se ensures the synthesis of selenoenzymes with antioxidant properties.Recently, it has been found that SeP uptake in kidneys, testes and brain depends on the apolipoprotein receptor 2 (ApoER2) and lipoprotein megalin receptor (Lrp2). Megalin receptor represents a physiological SeP receptor in kidneys, mediating the re-uptake of secreted SeP from the primary urine. The absence of a functional megalin receptor causes a significant reduction of plasma selenium and the SeP levels as a result of Se excretion. ApoER2 is a SeP receptor in the brain and testes which uptakes Se from the extracellular fluid. Deletion of ApoER2 in mice leads to a lowered selenium level in the brain and testes, neurological dysfunction, production of abnormal spermatozoa, infertility and even death when the subjects are fed a low-selenium diet.

LRP8 mediates Wnt/β-catenin signaling and controls osteoblast differentiation

The Wnt/β-catenin signaling pathway plays a pivotal role in regulating osteoblast differentiation and bone formation. Here, we identify low-density lipoprotein (LDL) receptor-related protein 8 (LRP8) as a positive regulator of Wnt/β-catenin signaling. In a small interfering RNA (siRNA) screen, LRP8 was shown to be required for Wnt/β-catenin-induced transcriptional reporter activity. We found that ectopic expression of LRP8 increased Wnt-induced transcriptional responses, and promoted Wnt-induced β-catenin accumulation. Moreover, knockdown of LRP8 resulted in a decrease in β-catenin levels and suppression of Wnt/β-catenin-induced Axin2 transcription. Functional studies in KS483 osteoprogenitor cells showed that LRP8 depletion resulted in impaired activation of endogenous Wnt-induced genes and decreased osteoblast differentiation and mineralization, whereas LRP8 ectopic expression had the opposite effect. These results identify LRP8 as a novel positive factor of canonical Wnt signaling pathway and show its involvement in Wnt-induced osteoblast differentiation.

Loss of the Reelin-signaling pathway differentially disrupts heat, mechanical and chemical nociceptive processing

The Reelin-signaling pathway regulates neuronal positioning during embryonic development. Reelin, the extracellular matrix protein missing in reeler mutants, is secreted by neurons in laminae I, II and V, binds to Vldl and Apoer2 receptors on nearby neurons, and tyrosine phosphorylates the adaptor protein Disabled-1 (Dab1), which activates downstream signaling. We previously reported that reeler and dab1 mutants had significantly reduced mechanical and increased heat nociception. Here we extend our analysis to chemical, visceral, and cold pain and importantly, used Fos expression to relate positioning errors in mutant mouse dorsal horn to changes in neuronal activity. We found that noxious mechanical stimulation-induced Fos expression is reduced in reeler and dab1 laminae I-II, compared to wild-type mice. Additionally, mutants had fewer Fos-immunoreactive neurons in the lateral-reticulated area of the deep dorsal horn than wild-type mice, a finding that correlates with a 50% reduction and subsequent mispositioning of the large Dab1-positive cells in the mutant lateral-reticulated area. Furthermore, several of these Dab1 cells expressed Fos in wild-type mice but rarely in reeler mutants. By contrast, paralleling the behavioral observations, noxious heat stimulation evoked significantly greater Fos expression in laminae I-II of reeler and dab1 mutants. We then used the formalin test to show that chemical nociception is reduced in reeler and dab1 mutants and that there is a corresponding decrease in formalin-induced Fos expression. Finally, neither visceral pain nor cold-pain sensitivity differed between wild-type and mutant mice. As differences in the nociceptor distribution within reeler and dab1 mutant dorsal horn were not detected, these differential effects observed on distinct pain modalities suggest that dorsal horn circuits are organized along modality-specific lines.

Impact of the Reelin signaling cascade (ligands-receptors-adaptor complex) on cognition in schizophrenia

Our previous neurocognitive studies of schizophrenia outlined two clusters of affected subjects--cognitively spared (CS) and cognitive deficit (CD), the latter's characteristics pointing to developmental origins and impaired synaptic plasticity. Here we investigate the contribution of polymorphisms in major regulators of these processes to susceptibility to schizophrenia and to CD in patients. We examine variation in genes encoding proteins at the gateway of Reelin signaling: ligands RELN and APOE, their common receptors APOER2 and VLDLR, and adaptor DAB1. Association analysis with disease outcome and cognitive performance in the Western Australian Family Study of Schizophrenia (WAFSS) was followed by replication analysis in the Australian Schizophrenia Research Bank (ASRB) and in the Health in Men Study (HIMS) of normal aging males. In the WAFSS sample, we observed significant association of APOE, APOER2, VLDLR, and DAB1 SNPs with disease outcome in the case-control and CD-control datasets, and with pre-morbid intelligence and verbal memory in cases. HIMS replication analysis supported rs439401 (APOE regulatory region), and rs2297660 and rs3737983 (APOER2), with an effect on memory performance in normal aging subjects consistent with the findings in schizophrenia cases. APOER2 gene expression analysis revealed lower transcript levels in lymphoblastoid cells from cognitively impaired schizophrenia patients of the alternatively spliced exon 19, mediating Reelin signaling and synaptic plasticity in the adult brain. ASRB replication analysis produced marginally significant results, possibly reflecting a recruitment strategy biased toward CS patients. The data suggest a contribution of neurodevelopmental/synaptic plasticity genes to cognitive impairment in schizophrenia.

PCSK9 regulates neuronal apoptosis by adjusting ApoER2 levels and signaling

The secreted protease proprotein convertase subtilisin/kexin type 9 (PCSK9) binds to low-density lipid (LDL) receptor family members LDLR, very low density lipoprotein receptor (VLDLR) and apolipoprotein receptor 2 (ApoER2), and promotes their degradation in intracellular acidic compartments. In the liver, LDLR is a major controller of blood LDL levels, whereas VLDLR and ApoER2 in the brain mediate Reelin signaling, a critical pathway for proper development of the nervous system. Expression level of PCSK9 in the brain is highest in the cerebellum during perinatal development, but is also increased in the adult brain after ischemia. The mechanism of PCSK9 function and its involvement in neuronal apoptosis is poorly understood. We show here that RNAi-mediated knockdown of PCSK9 significantly reduced the death of potassium-deprived cerebellar granule neurons (CGN), as shown by reduced levels of nuclear phosphorylated c-Jun and activated caspase-3, as well as condensed apoptotic nuclei. ApoER2 protein levels were increased in PCSK9 RNAi cells. Knockdown of ApoER2 but not of VLDLR was sufficient to reverse the protection provided by PCSK9 RNAi, suggesting that proapoptotic signaling of PCSK9 is mediated by altered ApoER2 function. Pharmacological inhibition of signaling pathways associated with lipoprotein receptors suggested that PCSK9 regulates neuronal apoptosis independently of NMDA receptor function but in concert with ERK and JNK signaling pathways. PCSK9 RNAi also reduced staurosporine-induced CGN apoptosis and axonal degeneration in the nerve growth factor-deprived dorsal root ganglion neurons. We conclude that PCSK9 potentiates neuronal apoptosis via modulation of ApoER2 levels and related anti-apoptotic signaling pathways.

ApoER2 function in the establishment and maintenance of retinal synaptic connectivity

The cellular and molecular mechanisms responsible for the development of inner retinal circuitry are poorly understood. Reelin and apolipoprotein E (apoE), ligands of apoE receptor 2 (ApoER2), are involved in retinal development and degeneration, respectively. Here we describe the function of ApoER2 in the developing and adult retina. ApoER2 expression was highest during postnatal inner retinal synaptic development and was considerably lower in the mature retina. Both during development and in the adult, ApoER2 was expressed by A-II amacrine cells. ApoER2 knock-out (KO) mice had rod bipolar morphogenic defects, altered A-II amacrine dendritic development, and impaired rod-driven retinal responses. The presence of an intact ApoER2 NPxY motif, necessary for binding Disabled-1 and transducing the Reelin signal, was also necessary for development of the rod bipolar pathway, while the alternatively spliced exon 19 was not. Mice deficient in another Reelin receptor, very low-density lipoprotein receptor (VLDLR), had normal rod bipolar morphology but altered A-II amacrine dendritic development. VLDLR KO mice also had reductions in oscillatory potentials and delayed synaptic response intervals. Interestingly, age-related reductions in rod and cone function were observed in both ApoER2 and VLDLR KOs. These results support a pivotal role for ApoER2 in the establishment and maintenance of normal retinal synaptic connectivity.

Disorders of selenium metabolism and selenoprotein function

PURPOSE OF REVIEW

Inborn errors of metabolism are increasingly recognized as underlying causes in pediatric diseases. Selenium and selenoproteins have only recently been identified as causes of inherited defects. Respective case reports have broadened our understanding of selenoprotein function and their developmental importance. This review presents the characterized defects and tries to attract attention to the spectrum of potential phenotypes.

RECENT FINDINGS

The characterization of patients with inherited mutations in selenoprotein N has corroborated the physiological importance of selenium for muscle function. Individuals with inherited defects in selenocysteine insertion sequence (SECIS)-binding protein 2 display a syndrome of selenoprotein-related defects including abnormal thyroid hormone metabolism, delayed bone maturation, and other more individual phenotypes. The recent identification of mutations in selenocysteine synthase causing progressive cerebello-cerebral atrophy underlines the central role of selenoproteins in brain development and protection from neurodegeneration.

SUMMARY

The spectrum of diseases related to inborn defects of selenium utilization, transport, and metabolism is expanding. However, only few examples are already known, resulting from defects in one selenoprotein gene and two genes involved in selenoprotein biosynthesis, respectively. Complex syndromes with impaired muscle function, stunted growth, neurosensory and/or immune defects may point to the involvement of impaired selenium metabolism and selenoprotein function, necessitating specific diagnostic procedures.

Influence of brain-derived neurotrophic factor and apolipoprotein E genetic variants on hemispheric and lateral ventricular volume of young healthy adults

OBJECTIVE

Brain-derived neurotrophic factor (BDNF) and apolipoprotein E (ApoE) are thought to be implicated in a variety of neuronal processes, including cell growth, resilience to noxious stimuli and synaptic plasticity. A Val to Met substitution at codon 66 in the BDNF protein has been associated with a variety of neuropsychiatric conditions. The ApoE4 allele is considered a risk factor for late-onset Alzheimer's disease, but its effects on young adults are less clear. We sought to investigate the effects of those two polymorphisms on hemispheric and lateral ventricular volumes of young healthy adults.

METHODS

Hemispheric and lateral ventricular volumes of 144 healthy individuals, aged 19-35 years, were measured using high resolution magnetic resonance imaging and data were correlated with BDNF and ApoE genotypes.

RESULTS

There were no correlations between BDNF or ApoE genotype and hemispheric or lateral ventricular volumes.

CONCLUSION

These findings indicate that it is unlikely that either the BDNF Val66Met or ApoE polymorphisms exert any significant effect on hemispheric or lateral ventricular volume. However, confounding epistatic genetic effects as well as relative insensitivity of the volumetric methods used cannot be ruled out. Further imaging analyses are warranted to better define any genetic influence of the BDNF Val6Met and ApoE polymorphism on brain structure of young healthy adults.

Lipoprotein receptors--an evolutionarily ancient multifunctional receptor family

The evolutionarily ancient low-density lipoprotein (LDL) receptor gene family represents a class of widely expressed cell surface receptors. Since the dawn of the first primitive multicellular organisms, several structurally and functionally distinct families of lipoprotein receptors have evolved. In accordance with the now obsolete 'one-gene-one-function' hypothesis, these cell surface receptors were originally perceived as mere transporters of lipoproteins, lipids, and nutrients or as scavenger receptors, which remove other kinds of macromolecules, such as proteases and protease inhibitors from the extracellular environment and the cell surface. This picture has since undergone a fundamental change. Experimental evidence has replaced the perception that these receptors serve merely as cargo transporters. Instead it is now clear that the transport of macromolecules is inseparably intertwined with the molecular machinery by which cells communicate with each other. Lipoprotein receptors are essentially sensors of the extracellular environment that participate in a wide range of physiological processes by physically interacting and coevolving with primary signal transducers as co-regulators. Furthermore, lipoprotein receptors modulate cellular trafficking and localization of the amyloid precursor protein (APP) and the β-amyloid peptide (Aβ), suggesting a role in the pathogenesis of Alzheimer's disease. Moreover, compelling evidence shows that LDL receptor family members are involved in tumor development and progression.