Petra May

Phosphatidylinositol 3-kinase interacts with the adaptor protein Dab1 in response to Reelin signaling and is required for normal cortical lamination

Reelin is a large secreted signaling protein that binds to two members of the low density lipoprotein receptor family, the apolipoprotein E receptor 2 and the very low density lipoprotein receptor, and regulates neuronal positioning during brain development. Reelin signaling requires activation of Src family kinases as well as tyrosine phosphorylation of the intracellular adaptor protein Disabled-1 (Dab1). This results in activation of phosphatidylinositol 3-kinase (PI3K), the serine/threonine kinase Akt, and the inhibition of glycogen synthase kinase 3β, a protein that is implicated in the regulation of axonal transport. Here we demonstrate that PI3K activation by Reelin requires Src family kinase activity and depends on the Reelin-triggered interaction of Dab1 with the PI3K regulatory subunit p85α. Because the Dab1 phosphotyrosine binding domain can interact simultaneously with membrane lipids and with the intracellular domains of apolipoprotein E receptor 2 and very low density lipoprotein receptor, Dab1 is preferentially recruited to the neuronal plasma membrane, where it is phosphorylated. Efficient Dab1 phosphorylation and activation of the Reelin signaling cascade is impaired by cholesterol depletion of the plasma membrane. Using a neuronal migration assay, we also show that PI3K signaling is required for the formation of a normal cortical plate, a step that is dependent upon Reelin signaling.

The LDL receptor-related protein (LRP) family: an old family of proteins with new physiological functions.

The low‐density lipoprotein (LDL) receptor is the founding member of a family of seven structurally closely related transmembrane proteins (LRP1, LRP1b, megalin/LRP2, LDL receptor, very low‐density lipoprotein receptor, MEGF7/LRP4, LRP8/apolipoprotein E receptor2). These proteins participate in a wide range of physiological processes, including the regulation of lipid metabolism, protection against atherosclerosis, neurodevelopment, and transport of nutrients and vitamins. While currently available data suggest that the role of the LDL receptor is limited to the regulation of cholesterol homeostasis by receptor‐mediated endocytosis of lipoprotein particles, there is growing experimental evidence that the other members of the gene family have additional physiological functions as signal transducers. In this review, we focus on the latest discovered functions of two major members of this family, LRP1 and megalin/LRP2, and on the newly elucidated physiological role of a third member of the family, MEGF7/LRP4, which can also function as a modulator of diverse signaling pathways during development.

Neuronal LRP1 Functionally Associates with Postsynaptic Proteins and Is Required for Normal Motor Function in Mice

ABSTRACT The LDL receptor-related protein 1 (LRP1) is a multifunctional cell surface receptor that is highly expressed on neurons. Neuronal LRP1 in vitro can mediate ligand endocytosis, as well as modulate signal transduction processes. However, little is known about its role in the intact nervous system. Here, we report that mice that lack LRP1 selectively in differentiated neurons develop severe behavioral and motor abnormalities, including hyperactivity, tremor, and dystonia. Since their central nervous systems appear histoanatomically normal, we suggest that this phenotype is likely attributable to abnormal neurotransmission. This conclusion is supported by studies of primary cultured neurons that show that LRP1 is present in close proximity to the N-methyl-d-aspartate (NMDA) receptor in dendritic synapses and can be coprecipitated with NMDA receptor subunits and the postsynaptic density protein PSD-95 from neuronal cell lysates. Moreover, treatment with NMDA, but not dopamine, reduces the interaction of LRP1 with PSD-95, indicating that LRP1 participates in transmitter-dependent postsynaptic responses. Together, these findings suggest that LRP1, like other ApoE receptors, can modulate synaptic transmission in the brain.

Molecular mechanisms of lipoprotein receptor signalling

Abstract.The low-density lipoprotein (LDL) receptor is the prototype of a classical endocytosis receptor that mediates the uptake of extracellular ligands. Other members of the LDL receptor gene family, on the other hand, have been shown to regulate intracellular signalling cascades. Among these are the LDL receptor-related protein 1, LRP1, a promiscuous and ubiquitously expressed receptor which is critically involved in a multitude of diverse physiological processes; the Reelin receptors ApoER2 and VLDL receptor, which participate in neuronal development; and megalin, a multifunctional receptor expressed in various epithelia. In this review, we focus on recent developments that highlight similarities and differences between these related receptors and their biological function, and discuss open questions as to the underlying molecular mechanisms.

Differential glycosylation regulates processing of lipoprotein receptors by γ-secretase

The low density lipoprotein (LDL) receptor-related protein 1 (LRP1) belongs to a growing number of cell surface proteins that undergo regulated proteolytic processing that culminates in the release of their intracellular domain (ICD) by the intramembranous protease γ-secretase. Here we show that LRP1 is differentially glycosylated in a tissue-specific manner and that carbohydrate addition reduces proteolytic cleavage of the extracellular domain and, concomitantly, ICD release. The apolipoprotein E (apoE) receptor-2 (apoER2), another member of the LDL receptor family with functions in cellular signal transmission, also undergoes sequential proteolytic processing, resulting in intracellular domain release into the cytoplasm. The penultimate processing step also involves cleavage of the apoER2 extracellular domain. The rate at which this cleavage step occurs is determined by the glycosylation state of the receptor, which in turn is regulated by the alternative splicing of an exon encoding several O-linked sugar attachment sites. These findings suggest a role for differential and tissue-specific glycosylation as a physiological switch that modulates the diverse biological functions of these receptors in a cell-type specific manner.

γ-Secretase Limits the Inflammatory Response Through the Processing of LRP1

Cleavage of the intracellular domain of the lipoprotein receptor LRP1 allows it to transcriptionally inhibit inflammatory responses. Feedback Required Inflammatory responses, such as those elicited by bacterial lipopolysaccharide (LPS), are critical for clearance of infection, but these responses must then be reined in to prevent the occurrence of side effects such as tissue damage. Low-density lipoprotein receptor–related protein 1 (LRP1) is a multifunctional lipoprotein receptor that plays a role in endocytosis and signal transduction. Processing of LRP1 by the γ-secretase complex releases the intracellular domain (ICD) of LRP1 from the plasma membrane. Zurhove et al. now show that LPS-stimulated processing of LRP1 by γ-secretase results in ICD-mediated feedback inhibition of the inflammatory response, in part by inhibiting the activity of the transcription factor interferon regulatory factor 3. As well as linking γ-secretase and LRP1 to the innate immune response, these data are potentially of clinical relevance; the therapeutic use of γ-secretase inhibitors may have the undesirable outcome of shutting down a pathway that prevents uncontrollable inflammation. Inflammation is a potentially self-destructive process that needs tight control. We have identified a nuclear signaling mechanism through which the low-density lipoprotein receptor–related protein 1 (LRP1) limits transcription of lipopolysaccharide (LPS)–inducible genes. LPS increases the proteolytic processing of the ectodomain of LRP1, which results in the γ-secretase–dependent release of the LRP1 intracellular domain (ICD) from the plasma membrane and its translocation to the nucleus, where it binds to and represses the interferon-γ promoter. Basal transcription of LPS target genes and LPS-induced secretion of proinflammatory cytokines are increased in the absence of LRP1. The interaction between LRP1-ICD and interferon regulatory factor 3 (IRF-3) promotes the nuclear export and proteasomal degradation of IRF-3. Feedback inhibition of the inflammatory response through intramembranous processing of LRP1 thus defines a physiological role for γ-secretase.

LRP1 Controls Intracellular Cholesterol Storage and Fatty Acid Synthesis through Modulation of Wnt Signaling

The low-density lipoprotein receptor-related protein LRP1 is a cell surface receptor with functions in diverse physiological pathways, including lipid metabolism. Here we show that LRP1-deficient fibroblasts accumulate high levels of intracellular cholesterol and cholesteryl-ester when stimulated for adipocyte differentiation. We demonstrate that LRP1 stimulates a canonical Wnt5a signaling pathway that prevents cholesterol accumulation. Moreover, we show that LRP1 is required for lipolysis and stimulates fatty acid synthesis independently of the noradrenergic pathway, through inhibition of GSK3β and its previously unknown target acetyl-CoA carboxylase (ACC). As a result of ACC inhibition, mature LRP1-deficient adipocytes of adult mice are hypotrophic, and lower uptake of fatty acids into adipose tissue leads to their redistribution to the liver. These results establish LRP1 as a novel integrator of adipogenic differentiation and fat storage signals.

Origin, maturation, and astroglial transformation of secondary radial glial cells in the developing dentate gyrus

The dentate gyrus is a brain region where neurons are continuously born throughout life. In the adult, the role of its radial glia in neurogenesis has attracted much attention over the past years; however, little is known about the generation and differentiation of glial cells and their relationship to radial glia during the ontogenetic development of this brain structure. Here, we combine immunohistochemical phenotyping using antibodies against glial marker proteins with BrdU birthdating to characterize the development of the secondary radial glial scaffold in the dentate gyrus and its potential to differentiate into astrocytes. We demonstrate that the expression of brain lipid‐binding protein, GLAST, and glial fibrillary acidic protein (GFAP) characterizes immature differentiating cells confined to an astrocytic fate in the early postnatal dentate gyrus. On the basis of our studies, we propose a model where immature astrocytes migrate radially through the granule cell layer to adopt their final positions in the molecular layer of the dentate gyrus. Time‐lapse imaging of acute hippocampal slices from hGFAP‐eGFP transgenic mice provides direct evidence for such a migration mode of differentiating astroglial cells in the developing dentate gyrus.

LDL Receptor-Related Proteins in Neurodevelopment

Low‐density lipoprotein receptor‐related proteins (LRPs) are evolutionarily ancient cell‐surface receptors with diverse biological functions. All are expressed in the central nervous system and, for most receptors, animal models have shown that they are indispensable for successful neurodevelopment. The mechanisms by which they regulate the formation of the nervous system are varied and include the transduction of extracellular signals and the modulation of intracellular signal propagation, as well as cargo transport, the function most commonly attributed to this gene family. Here, we will summarize recent advances in our understanding of the molecular basis on which these receptors function during development.

Integration of Endocytosis and Signal Transduction by Lipoprotein Receptors

The members of the low density lipoprotein receptor (LDLR) gene family are cell surface molecules with diverse functions in cellular metabolism. All LDLR family members are endocytic receptors that mediate the uptake of extracellular cargo into the cell; recent research indicates that they also participate directly in signal transduction. Regulated proteolytic release of the intracellular domain of one of these lipoprotein receptors, the LDLR-related protein 1 (LRP1), has been described, along with the possible role of the released domain in transcriptional regulation. A recent study suggests that megalin, a member of the LDLR gene family that mediates the cellular uptake of vitamin D carrier protein, may also modulate vitamin D-related gene transcription through sequestration of a component of the vitamin D receptor transcriptional complex. May et al. discuss this research in the context of the integration of endocytosis and signaling by this receptor family.