Reelin is a signaling protein that plays a crucial role in synaptic function, which expression is influenced by β-amyloid (Aβ). We show that Reelin and Aβ oligomers co-immunoprecipitated in human brain extracts and were present in the same size-exclusion chromatography fractions. Aβ treatment of cells led to increase expression of Reelin, but secreted Reelin results trapped together with Aβ aggregates. In frontal cortex extracts an increase in Reelin mRNA, and in soluble and insoluble (guanidine-extractable) Reelin protein, was associated with late Braak stages of Alzheimer’s disease (AD), while expression of its receptor, ApoER2, did not change. However, Reelin-dependent induction of Dab1 phosphorylation appeared reduced in AD. In cells, Aβ reduced the capacity of Reelin to induce internalization of biotinylated ApoER2 and ApoER2 processing. Soluble proteolytic fragments of ApoER2 generated after Reelin binding can be detected in cerebrospinal fluid (CSF). Quantification of these soluble fragments in CSF could be a tool to evaluate the efficiency of Reelin signaling in the brain. These CSF-ApoER2 fragments correlated with Reelin levels only in control subjects, not in AD, where these fragments diminished. We conclude that while Reelin expression is enhanced in the Alzheimer’s brain, the interaction of Reelin with Aβ hinders its biological activity.

The human genome is a mosaic of isochores, which are long DNA segments (z.Gt;300 kbp) relatively homogeneous in G+C. Human isochores were first identified by density-gradient ultracentrifugation of bulk DNA, and differ in important features, e.g. genes are found predominantly in the GC-richest isochores. Here, we use a reliable segmentation method to partition the longest contigs in the human genome draft sequence into long homogeneous genome regions (LHGRs), thereby revealing the isochore structure of the human genome. The advantages of the isochore maps presented here are: (1) sequence heterogeneities at different scales are shown in the same plot; (2) pair-wise compositional differences between adjacent regions are all statistically significant; (3) isochore boundaries are accurately defined to single base pair resolution; and (4) both gradual and abrupt isochore boundaries are simultaneously revealed. Taking advantage of the wide sample of genome sequence analyzed, we investigate the correspondence between LHGRs and true human isochores revealed through DNA centrifugation. LHGRs show many of the typical isochore features, mainly size distribution, G+C range, and proportions of the isochore classes. The relative density of genes, Alu and long interspersed nuclear element repeats and the different types of single nucleotide polymorphisms on LHGRs also coincide with expectations in true isochores. Potential applications of isochore maps range from the improvement of gene-finding algorithms to the prediction of linkage disequilibrium levels in association studies between marker genes and complex traits. The coordinates for the LHGRs identified in all the contigs longer than 2 Mb in the human genome sequence are available at the online resource on isochore mapping: http://bioinfo2.ugr.es/isochores.

Increasing evidence indicates that altered reelin signaling could contribute to synaptic dysfunction in Alzheimer's disease (AD). We found that reelin protein and mRNA levels were increased in the AD brain (particularly at advanced Braak stages in apolipoprotein E4 noncarriers), compared with that of control subjects. The β-amyloid (Aβ) protein impairs reelin activity and increases reelin expression through a mechanism that is not yet understood. To explore that mechanism, we examined the effect of Aβ aa 1-42 (Aβ) on DNA methylation of the RELN promoter and the processing of reelin receptor apolipoprotein E receptor 2 (ApoER2) in differentiated SH-SY5Y cells because ApoER2 C-terminal fragments (CTFs), generated after reelin binding, regulate reelin expression. We found that Aβ decreased nuclear levels of DNA-methyltransferase 1. However, RELN promoter methylation did not change in Aβ-treated cells or in AD brain extracts. Instead, the levels of ApoER2-CTF appeared significantly lower in Aβ-treated cells and in AD extracts from advanced Braak stages of apolipoprotein E4 noncarriers. Our data show that ApoER2-CTF levels are decreased, whereas reelin expression is increased in AD brain at advanced Braak stages and after Aβ treatment, supporting the view that ApoER2-CTF exerts a modulatory role on reelin expression.-Mata-Balaguer, T., Cuchillo-Ibañez, I., Calero, M., Ferrer, I., Sáez-Valero, J. Decreased generation of C-terminal fragments of ApoER2 and increased reelin expression in Alzheimer's disease.