The Reelin signaling pathway promotes dendritic spine development in hippocampal neurons

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

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

Interaction of reelin with amyloid precursor protein promotes neurite outgrowth

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

Molecular regulation of neuronal migration during neocortical development

Neocortex, a distinct six-layered neural structure, is one of the most exquisite nerve tissues in the human body. Proper assembly of neocortex requires precise regulation of neuronal migration and abnormalities can result in severe neurological diseases. Three major types of neuronal migration have been implicated in corticogenesis: radial migration of excitatory neuron precursors and tangential migration of interneurons as well as Cajal-Retzius cells. In the past several years, significant progress has been made in understanding how these parallel events are regulated and coordinated during corticogenesis. New insights have been gained into regulation of radial neuron migration by the well-known Reelin signal. New pathways have also been identified that regulate radial as well as tangential migration. Equally important, better understandings have been obtained on the cellular and molecular mechanics of cell migration by both projection neurons and interneurons. These findings have not only enhanced our understanding of normal neuron migration but also revealed insights into the etiologies of several neurological diseases where these processes go awry.

Apolipoprotein E receptors in the nervous system

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

Apolipoprotein E and its receptors in Alzheimer's disease: pathways, pathogenesis and therapy

The vast majority of Alzheimer's disease (AD) cases are late-onset and their development is probably influenced by both genetic and environmental risk factors. A strong genetic risk factor for late-onset AD is the presence of the epsilon4 allele of the apolipoprotein E (APOE) gene, which encodes a protein with crucial roles in cholesterol metabolism. There is mounting evidence that APOE4 contributes to AD pathogenesis by modulating the metabolism and aggregation of amyloid-beta peptide and by directly regulating brain lipid metabolism and synaptic functions through APOE receptors. Emerging knowledge of the contribution of APOE to the pathophysiology of AD presents new opportunities for AD therapy.