Selective interaction of megalin with postsynaptic density-95 (PSD-95)-like membrane-associated guanylate kinase (MAGUK) proteins

Vitamin D status and its association with parathyroid hormone in 23,134 outpatients

In vitro studies indicate that 25-hydroxyvitamin D3 (25(OH)D3) and 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) inhibits the synthesis of parathyroid hormone (PTH). The degree of PTH inhibition in humans by circulating 25(OH)D and 1,25(OH)2D may be different. Moreover, age and sex as well as confounding factors like calcium and phosphate may likewise affect the relationship between vitamin D and PTH in humans. However, this was not done so far in adequately powered studies. We investigated the relationship between 25(OH)D as well as 1,25(OH)2D and intact parathyroid hormone (iPTH) in 23,134 outpatients (age mean: 59.81 years) from the Berlin-Brandenburg area of Germany with normal serum creatinine considering confounding factors like age, sex, calcium and phosphate. 25(OH)D and iPTH were inversely correlated (r = -0.17, p < 0.0001). The inverse linear correlation was observed over the entire spectrum of 25(OH)D concentrations - from low 25(OH)D concentrations to very high 25(OH)D concentrations. Multiple linear regression analysis revealed that this correlation was independent of age, sex, creatinine, calcium and phosphate (unstandardized coefficients B: -0.16, p < 0.0001). However, 1,25(OH)2D was only positively associated with iPTH in women (r = 0.05, p = 0.033) and in the subgroup of patients with lower 25(OH)D (25(OH)D< 40 ng/ml) (r = 0.09, p < 0.0001), which was also presented in multiple linear regression analysis (unstandardized coefficients B: 0.20, p = 0.001). Circulating 1,25(OH)2D does not contribute substantially to the regulation of PTH in middle aged and vitamin D sufficient outpatients from the Berlin-Brandenburg area of Germany with normal kidney function. Presumably, serum 25(OH)D that is converted to 1,25(OH)2D after uptake in the parathyroid chief cells plays the critical role.

Actionable Genomics in Clinical Practice: Paradigmatic Case Reports of Clinical and Therapeutic Strategies Based upon Genetic Testing

In clinical settings, the information provided by genetic testing can explain the triggers and processes underlying clinical presentations, such as neurodevelopmental disorders, in up to one third of affected individuals. However, translating this knowledge into better and more personalized clinical management to many appears a distant target. This article presents three paradigmatic cases to exemplify how this translational effort can, at least in some instances, be undertaken today with very positive results: (a) a young girl carrying a chr. 16p11.2 duplication can be screened using targeted exams and undertake therapeutic/preventive interventions related to her genetic diagnosis; (b) a 13-year-old boy with intellectual disability and autism spectrum disorder carries a chr. 11q14.1 deletion, partly spanning the DLG2 gene important for synaptic function, and gained over 20 I.Q. points ostensibly due to carbolithium, prescribed in the absence of affective symptoms, exclusively following the pathophysiology pointed out by the genetic results; (c) a 58-year-old woman carries a COL3A1 gene variant responsible for the vascular form of Ehler–Danlos syndrome with colon rupture. Detection of this variant in six members of her extended family allows for better clinical management of the proband and targeted genetic counselling for family members at risk of this connective tissue disorder. The unprecedented flow of genetic information available today through new technologies, if interpreted in the light of current knowledge in clinical diagnosis and care of those with connective tissue disorders and neurodevelopmental disturbances, in biology and in neuropsychopharmacology, can promote better clinical and pharmacological treatment, disease surveillance, and management provided and incorporated into the clinical setting.

The recycling regulation of sodium-hydrogen exchanger isoform 3(NHE3) in epithelial cells

As the main exchanger of electroneutral NaCl absorption, sodium-hydrogen exchanger isoform 3 (NHE3) circulates in the epithelial brush border (BB) and intracellular compartments in a multi-protein complex. The size of the NHE3 complex changes during rapid regulation events. Recycling regulation of NHE3 in epithelial cells can be roughly divided into three stages. First, when stimulated by Ca²⁺, cGMP, and cAMP-dependent signaling pathways, NHE3 is converted from an immobile complex found at the apical microvilli (MV) into an easily internalized and mobile form that relocates to a compartment near the base of the MV. Second, NHE3 is internalized by clathrin and albumin-dependent pathways into cytoplasmic endosomal compartments, where the complex is reprocessed and reassembled. Finally, NHE3 is translocated from the recycling endosomes (REs) to the apex of epithelial cells, a process that can be stimulated by an increase in sodium-glucose cotransporter 1 (SGLT1) activity, epidermal growth factor receptor (EGFR) signaling, Ca²⁺ signaling, and binding to βPix and SH3 and multiple ankyrin repeat domains 2 (Shank2) proteins. This review describes the molecular steps and protein interactions involved in the recycling movement of NHE3 from the apex of epithelial cells, into vesicles, where it is reprocessed and reassembled, and returned to its original location on the plasma membrane, where it exerts its physiological function.

Cellular Receptors of Amyloid β Oligomers (AβOs) in Alzheimer's Disease

It is estimated that Alzheimer’s disease (AD) affects tens of millions of people, comprising not only suffering patients, but also their relatives and caregivers. AD is one of age-related neurodegenerative diseases (NDs) characterized by progressive synaptic damage and neuronal loss, which result in gradual cognitive impairment leading to dementia. The cause of AD remains still unresolved, despite being studied for more than a century. The hallmark pathological features of this disease are senile plaques within patients’ brain composed of amyloid beta (Aβ) and neurofibrillary tangles (NFTs) of Tau protein. However, the roles of Aβ and Tau in AD pathology are being questioned and other causes of AD are postulated. One of the most interesting theories proposed is the causative role of amyloid β oligomers (AβOs) aggregation in the pathogenesis of AD. Moreover, binding of AβOs to cell membranes is probably mediated by certain proteins on the neuronal cell surface acting as AβO receptors. The aim of our paper is to describe alternative hypotheses of AD etiology, including genetic alterations and the role of misfolded proteins, especially Aβ oligomers, in Alzheimer’s disease. Furthermore, in this review we present various putative cellular AβO receptors related to toxic activity of oligomers.

Estrogen-enhanced apical and basolateral secretion of apolipoprotein B-100 by polarized trophoblast-derived BeWo cells

Cholesterol is an important nutrient for fetal development and transplacental transport occurs at all stages of human pregnancy. Furthermore, cholesterol is required for membrane building as well as steroid hormone synthesis. Therefore, all placental cell types require cholesterol for proper function. In human term placenta, the syncytiotrophoblast (STB) faces the maternal circulation. Uptake of maternal-derived cholesterol at the apical membrane of the STB is well understood, but the route by which cholesterol exits at the basal side for subsequent transfer across the fetal endothelial cells (FEC) or to other placental cell types remains not well characterized. Our aim was to provide evidence for basal secretion of apolipoprotein B-100 (apoB) containing lipoproteins. Furthermore, we investigated the placental localization of apolipoprotein receptors (LRP2, LDLR and LRP1) to identify cell targets of lipoprotein particles secreted in a polarized fashion by the STB. In trophoblast-derived BeWo cells grown on permeable filter supports, we demonstrate by immunoprecipitation apical as well as basolateral apoB secretion, which was significantly upregulated by estrogen-treatment for 24 or 48 h. Furthermore, we showed by immunofluorescence microscopy apoB and microsomal triglyceride transfer protein subunits localization in the STB and placental stromal cells in situ. All investigated receptors were detected by RT-qPCR and western blot in BeWo cells, but only expression of LRP2 was estrogen-inducible. In situ, the multi-ligand receptor LRP2 was expressed exclusively in the cytotrophoblast (CTB), the STB precursor cell type. LDLR and LRP1 localized to trophoblasts as well as stromal cells in situ. In summary, basal apoB secretion by BeWo cells supports the concept of basal lipoprotein particle secretion by placental STB. These lipoprotein particles may serve as cholesterol source for STB precursor cells, the CTBs, as well as all stromal cells of the chorionic villi including FECs, which were herein demonstrated to express apoB receptors, LRP2 and LDLR, respectively.

The Role of Megalin in the Transport of Gentamicin Across BeWo Cells, an In Vitro Model of the Human Placenta

Aminoglycosides (AG) are known to readily cross the placenta, although the mechanisms responsible for placental transport have not been characterized. Megalin is expressed in human placenta, and it is reasonable to speculate, given its role in renal AG uptake, that it is similarly involved in placental transport. However, the role of megalin in placental AG uptake has not been established. An in vitro model to study megalin-mediated placental transport has also not been previously described. The objectives of this study, therefore, were to evaluate the human choriocarcinoma (BeWo) cell line as a model to study megalin-mediated placental transport and to assess the uptake kinetics of gentamicin, an AG antibiotic, using this in vitro model. BeWo cells were grown on Transwell® plates, and megalin expression and functional activity were assessed. Uptake of (3)H-gentamicin was also evaluated in the presence and absence of megalin inhibitors. Expression of megalin protein and mRNA in BeWo cells were confirmed via immunoblot and qPCR analysis. Uptake of fluorescein isothiocyanate (FITC)-labeled bovine serum albumin (BSA) (a megalin substrate) was time-, concentration-, and temperature-dependent consistent with a transporter-mediated process. FITC-BSA uptake was also significantly reduced in the presence of unlabeled gentamicin (a megalin substrate) and sodium maleate (to induce megalin shedding) suggesting that megalin is functionally active in BeWo cells. Gentamicin uptake exhibited time and temperature dependence, saturability and Michaelis-Menten kinetics, all of which suggest a transporter-mediated process. Gentamicin uptake was also significantly reduced in the presence of the megalin inhibitors RAP and EDTA suggesting that megalin is likely involved in gentamicin uptake.

Megalin expression in human term and preterm placental villous tissues: effect of gestational age and sample processing and storage time

INTRODUCTION

The aims of this study were to characterize megalin expression in human term and preterm placental villous tissues and to assess the impact of gestational age and sample storage on receptor expression.

METHODS

Placental tissue samples were collected from pregnant women undergoing term and preterm Cesarean deliveries. Placental villous tissues were used to quantify megalin protein and mRNA expression by western blotting and quantitative polymerase chain reaction (q-PCR), respectively. Stability of megalin expression was also evaluated under various processing and storage conditions.

RESULTS

Megalin mRNA was detected in term and preterm placental villous tissues. Expression in early preterm samples was 6-fold higher than in late preterm and term samples. Refrigeration of processed term samples at 4°C for up to 18h had a slight impact on megalin mRNA expression with stored samples exhibiting mRNA levels approximately 1.5-fold lower than those frozen immediately after processing. A greater decrease in mRNA expression (up to 33-fold) was observed when processed samples were snap-frozen immediately and thawed at 4°C. Processing of samples prior to refrigeration also appeared to improve mRNA stability with significantly higher expression levels noted in processed vs. unprocessed samples at all points for up to 48h.

DISCUSSION

These data suggest that expression of megalin mRNA in term placental villous tissue is relatively stable for up to 18h when samples are processed immediately and refrigerated at 4°C prior to freezing. Processing prior to storage also appears to improve mRNA stability. This paper demonstrates the practical feasibility of analyzing stored tissue samples, thus, it will help with placental mRNA analysis. Additionally, megalin expression appears to vary inversely with gestational age with the greatest expression noted in the most premature samples. Age-dependent differences in placental megalin may therefore influence fetal exposure.

The endocytic receptor megalin and its associated proteins in proximal tubule epithelial cells

Receptor-mediated endocytosis in renal proximal tubule epithelial cells (PTECs) is important for the reabsorption and metabolization of proteins and other substances, including carrier-bound vitamins and trace elements, in glomerular filtrates. Impairment of this endocytic process results in the loss of such substances and development of proteinuria, which is an important clinical indicator of kidney diseases and is also a risk marker for cardiovascular disease. Megalin, a member of the low-density lipoprotein receptor gene family, is a multiligand receptor expressed in the apical membrane of PTECs and plays a central role in the endocytic process. Megalin interacts with various intracellular adaptor proteins for intracellular trafficking and cooperatively functions with other membrane molecules, including the cubilin-amnionless complex. Evidence suggests that megalin and the cubilin-amnionless complex are involved in the uptake of toxic substances into PTECs, which leads to the development of kidney disease. Studies of megalin and its associated molecules will be useful for future development of novel strategies for the diagnosis and treatment of kidney diseases.

Kinetic and mechanistic differences in the interactions between caldendrin and calmodulin with AKAP79 suggest different roles in synaptic function

The kinetic and mechanistic details of the interaction between caldendrin, calmodulin and the B-domain of AKAP79 were determined using a biosensor-based approach. Caldendrin was found to compete with calmodulin for binding at AKAP79, indicating overlapping binding sites. Although the AKAP79 affinities were similar for caldendrin (K(D) = 20 nM) and calmodulin (K(D) = 30 nM), their interaction characteristics were different. The calmodulin interaction was well described by a reversible one-step model, but was only detected in the presence of Ca(2+). Caldendrin interacted with a higher level of complexity, deduced to be an induced fit mechanism with a slow relaxation back to the initial encounter complex. It interacted with AKAP79 also in the absence of Ca(2+), but with different kinetic rate constants. The data are consistent with a similar initial Ca(2+)-dependent binding step for the two proteins. For caldendrin, a second Ca(2+)-independent rearrangement step follows, resulting in a stable complex. The study shows the importance of establishing the mechanism and kinetics of protein-protein interactions and that minor differences in the interaction of two homologous proteins can have major implications in their functional characteristics. These results are important for the further elucidation of the roles of caldendrin and calmodulin in synaptic function.

Endocytic Receptors in the Renal Proximal Tubule

Protein reabsorption is a predominant feature of the renal proximal tubule. Animal studies show that the ability to rescue plasma proteins relies on the endocytic receptors megalin and cubilin. Recently, studies of patients with syndromes caused by dysfunctional receptors have supported the importance of these for protein clearance of human ultrafiltrate. This review focuses on the molecular biology and physiology of the receptors and their involvement in renal pathological conditions.

LRP-1 and LRP-2 receptors function in the membrane neuron. Trafficking mechanisms and proteolytic processing in Alzheimer's disease

Low density lipoprotein receptor-related protein (LRP) belongs to the low-density lipoprotein receptor family, generally recognized as cell surface endocytic receptors, which bind and internalize extracellular ligands for degradation in lysosomes. Neurons require cholesterol to function and keep the membrane rafts stable. Cholesterol uptake into the neuron is carried out by ApoE via LRPs receptors on the cell surface. In neurons the most important are LRP-1 and LRP-2, even it is thought that a causal factor in Alzheimer's disease (AD) is the malfunction of this process which cause impairment intracellular signaling as well as storage and/or release of nutrients and toxic compounds. Both receptors are multifunctional cell surface receptors that are widely expressed in several tissues including neurons and astrocytes. LRPs are constituted by an intracellular (ICD) and extracellular domain (ECD). Through its ECD, LRPs bind at least 40 different ligands ranging from lipoprotein and protease inhibitor complex to growth factors and extracellular matrix proteins. These receptors has also been shown to interact with scaffolding and signaling proteins via its ICD in a phosphorylation-dependent manner and to function as a co-receptor partnering with other cell surface or integral membrane proteins. Thus, LRPs are implicated in two major physiological processes: endocytosis and regulation of signaling pathways, which are both involved in diverse biological roles including lipid metabolism, cell growth processes, degradation of proteases, and tissue invasion. Interestingly, LRPs were also localized in neurons in different stages, suggesting that both receptors could be implicated in signal transduction during embryonic development, neuronal outgrowth or in the pathogenesis of AD.

Oral administration of a GSK3 inhibitor increases brain insulin-like growth factor I levels

Reduced brain input of serum insulin-like growth factor I (IGF-I), a potent neurotrophic peptide, may be associated with neurodegenerative processes. Thus, analysis of the mechanisms involved in passage of blood-borne IGF-I into the brain may shed light onto pathological mechanisms in neurodegeneration and provide new drug targets. A site of entrance of serum IGF-I into the brain is the choroid plexus. The transport mechanism for IGF-I in this specialized epithelium involves the IGF-I receptor and the membrane multicargo transporter megalin/LRP2. We have now analyzed this process in greater detail and found that the IGF-I receptor interacts with the transmembrane region of megalin, whereas the perimembrane domain of megalin is required for IGF-I internalization. Furthermore, a GSK3 site within the Src homology 3 domain of the C-terminal region of megalin is a key regulator of IGF-I transport. Thus, inhibition of GSK3 markedly increased internalization of IGF-I, whereas mutation of this GSK3 site abrogated this increase. Notably, oral administration of a GSK3 inhibitor to adult wild-type mice or to amyloid precursor protein/presenilin 1 mice modeling Alzheimer amyloidosis significantly increased brain IGF-I content. These results indicate that pharmacological modulation of IGF-I transport by megalin may be used to increase brain availability of serum IGF-I. Interestingly, GSK3 inhibitors such as those under development to treat Alzheimer disease may show therapeutic efficacy in part by increasing brain IGF-I levels, an effect already reported for other neuroprotective compounds.

Molecular mechanisms of receptor-mediated endocytosis in the renal proximal tubular epithelium

Receptor-mediated endocytosis is a pivotal function of renal proximal tubule epithelial cells (PTECs) to reabsorb and metabolize substantial amounts of proteins and other substances in glomerular filtrates. The function accounts for the conservation of nutrients, including carrier-bound vitamins and trace elements, filtered by glomeruli. Impairment of the process results in a loss of such substances and development of proteinuria, an important clinical sign of kidney disease and a risk marker for cardiovascular disease. Megalin is a multiligand endocytic receptor expressed at clathrin-coated pits of PTEC, playing a central role in the process. Megalin cooperates with various membrane molecules and interacts with many intracellular adaptor proteins for endocytic trafficking. Megalin is also involved in signaling pathways in the cells. Megalin-mediated endocytic overload leads to damage of PTEC. Further studies are needed to elucidate the mechanism of megalin-mediated endocytosis and develop strategies for preventing the damage of PTEC.

ClC transporters: discoveries and challenges in defining the mechanisms underlying function and regulation of ClC-5

The involvement of several members of the chloride channel (ClC) family of membrane proteins in human disease highlights the need to define the mechanisms underlying their function and the consequences of disease-causing mutations. Despite the utility of high-resolution structural models, our understanding of the molecular basis for function of the chloride channels and transporters in the family remains incomplete. In this review, we focus on recent discoveries regarding molecular mechanisms underlying the regulated chloride:proton antiporter activity of ClC-5, the protein mutated in the Dent's disease-a kidney disease presenting with proteinuria and renal failure in severe cases. We discuss the putative role of ClC-5 in receptor-mediated endocytosis and protein uptake by the proximal renal tubule and the possible molecular and cellular consequences of disease-causing mutations. However, validation of these models will require future study of the intrinsic function of this transporter, in situ, in the membranes of recycling endosomes in proximal tubule epithelial cells.

Receptor-mediated endocytosis in renal proximal tubule

Proteins filtered in renal glomeruli are removed from the tubular fluid by endocytosis in the proximal tubule mediated by the two receptors megalin and cubilin. After endocytic uptake, the proteins are transferred to lysosomes for degradation, while the receptors are returned to the apical cell membrane by receptor recycling in dense apical tubules. In the renal proximal tubule, there is no significant transcellular transport of protein. The reabsorptive process is extremely efficient as evidenced by the virtual protein free urine in humans. The two receptors bind a variety of ligands. The process serves not only to remove the proteins from the ultrafiltrate but also to conserve a variety of essential substances such as vitamins and trace elements carried by plasma proteins. The endocytic apparatus is highly developed in the proximal tubule demonstrating the high capacity of the cells; however, under certain circumstances like diseases affecting the glomeruli, the system is overloaded resulting in proteinuria.

PDZ domains: folding and binding

The PDZ domain is one of the most common protein-protein interaction domains in humans, and it is found in all kingdoms of life. We will review recent progress in the understanding of biophysical aspects of PDZ domains with emphasis on the folding and binding reactions. Finally, we discuss an intriguing correlation between stability and binding of peptide for PDZ2 from PTP-BL.

A cytoplasmic PPPSP motif determines megalin's phosphorylation and regulates receptor's recycling and surface expression

Megalin is a large endocytic receptor expressed at the apical surface of several absorptive epithelia. It binds multiple ligands including apolipoproteins, vitamin and hormone carrier proteins and signaling molecules such as parathyroid hormone and the morphogen sonic hedgehog. An important characteristic of megalin is its high endocytic activity, which is mediated by tyrosine-based endocytic motifs within the receptor's cytoplasmic tail. This domain also harbors several putative consensus phosphorylation motifs for protein kinase (PK) C and casein kinase-II and one consensus motif for PKA and glycogen synthase kinase-3 (GSK3). Here we report that the cytoplasmic domain of megalin is constitutively phosphorylated depending on the integrity of a PPPSP motif, a putative GSK3 site, with a minor participation of the other phosphorylation motifs. Mutation of the serine residue within the PPPSP motif as well as blocking GSK3 activity, with two different inhibitors, significantly decreased the phosphorylation levels of the receptor. Both the megalin PPPAP mutant and the underphosphorylated wild-type receptor, by inhibition of GSK3 activity, were more expressed at the cell surface and more efficiently recycled, but they were not inhibited in their initial endocytosis rates. Altogether, these results show that the PPPSP motif and the GSK3 activity are critical to allow megalin phosphorylation and also negatively regulate the receptor's recycling.

The role of megalin (LRP-2/Gp330) during development

Megalin (LRP-2/GP330), a member of the LDL receptor family, is an endocytic receptor expressed mainly in polarised epithelial cells. Identified as the pathogenic autoantigen of Heymann nephritis in rats, its functions have been studied in greatest detail in adult mammalian kidney, but there is increasing recognition of its involvement in embryonic development. The megalin homologue LRP-1 is essential for growth and development in Caenorhabditis elegans and megalin plays a role in CNS development in zebrafish. There is now also evidence for a homologue in Drosophila. However, most research concerns mammalian embryogenesis; it is widely accepted to be important during forebrain development and the developing renal proximal tubule. Megalin is also expressed in lung, eye, intestine, uterus, oviduct, and male reproductive tract. It is found in yolk sacs and the outer cells of pre-implantation mouse embryos, where interactions with cubilin result in nutrient endocytosis, and it may be important during implantation. Models for megalin interaction(s) with Sonic Hedgehog (Shh) have been proposed. The importance of Shh signalling during embryogenesis is well established; how and when megalin interacts with Shh is becoming a pertinent question in developmental biology.

Low density lipoprotein receptor-related protein-2/megalin is expressed in oligodendrocytes in the mouse spinal cord white matter

Lipoprotein receptor-related protein-2 (LRP2)/megalin is a member of the low density lipoprotein receptor (LDLR) family, and is essential in absorptive epithelia for endocytosis of lipoproteins, low molecular weight proteins, cholesterol and vitamins, as well as in cellular signaling. Previous studies have shown megalin expression in ependymal cells and choroid plexus. We have investigated megalin expression in the spinal cord of postnatal mice with immunohistochemistry and immunoblot. Antibodies recognizing either the cytoplasmic tail (MM6) or the extracellular domain (E11) of megalin labeled oligodendrocytes in the spinal cord white matter, in parallel with myelination. MM6 antibodies, predominantly labeled the nuclei, whereas E11 antibodies labeled the cytoplasm of these cells. MM6 antibodies labeled also nuclei of oligodendrocytes cultured from embryonic mouse spinal cord. Immunoblots of spinal cord showed intact megalin, as well as its carboxyterminal fragment, the part remaining after shedding of the extracellular domain of megalin. Megalin-immunoreactive oligodendrocytes also expressed presenilin 1, an enzyme responsible for gamma-secretase mediated endodomain cleavage. These findings show that spinal cord oligodendrocytes are phenotypically different from those in the brain, and indicate that megalin translocates signals from the cell membrane to the nucleus of oligodendrocytes during the formation and maintenance of myelin of long spinal cord pathways.

Choroid plexus megalin is involved in neuroprotection by serum insulin-like growth factor I

The involvement of circulating insulin-like growth factor I (IGF-I) in the beneficial effects of physical exercise on the brain makes this abundant serum growth factor a physiologically relevant neuroprotective signal. However, the mechanisms underlying neuroprotection by serum IGF-I remain primarily unknown. Among many other neuroprotective actions, IGF-I enhances clearance of brain amyloid beta (Abeta) by modulating transport/production of Abeta carriers at the blood-brain interface in the choroid plexus. We found that physical exercise increases the levels of the choroid plexus endocytic receptor megalin/low-density lipoprotein receptor-related protein-2 (LRP2), a multicargo transporter known to participate in brain uptake of Abeta carriers. By manipulating choroid plexus megalin levels through viral-directed overexpression and RNA interference, we observed that megalin mediates IGF-I-induced clearance of Abeta and is involved in IGF-I transport into the brain. Through this dual role, megalin participates in the neuroprotective actions of IGF-I including prevention of tau hyperphosphorylation and maintenance of cognitive function in a variety of animal models of cognitive loss. Because we found that in normal aged animals, choroid plexus megalin/LRP2 is decreased, an attenuated IGF-I/megalin input may contribute to increased risk of neurodegeneration, including late-onset Alzheimer's disease.

Low-density lipoprotein receptor-related protein (LRP)-2/megalin is transiently expressed in a subpopulation of neural progenitors in the embryonic mouse spinal cord

The lipoprotein receptor LRP2/megalin is expressed by absorptive epithelia and involved in receptor-mediated endocytosis of a wide range of ligands. Megalin is expressed in the neuroepithelium during central nervous system (CNS) development. Mice with homozygous deletions of the megalin gene show severe forebrain abnormalities. The possible role of megalin in the developing spinal cord, however, is unknown. Here we examined the spatial and temporal expression pattern of megalin in the embryonic mouse spinal cord using an antibody that specifically recognizes the cytoplasmic part of the megalin molecule. In line with published data, we show expression of megalin in ependymal cells of the central canal from embryonic day (E)11 until birth. In addition, from E11 until E15 a population of cells was found in the dorsal part of the developing spinal cord strongly immunoreactive against megalin. Double labeling showed that most of these cells express vimentin, a marker for immature astrocytes and radial glia, but not brain lipid binding protein (BLBP), a marker for radial glial cells, or glial fibrillary acidic protein (GFAP), a marker for mature astrocytes. These findings indicate that the majority of the megalin-positive cells are astroglial precursors. Megalin immunoreactivity was mainly localized in the nuclei of these cells, suggesting that the cytoplasmic part of the megalin molecule can be cleaved following ligand binding and translocated to the nucleus to act as a transcription factor or regulate other transcription factors. These findings suggest that megalin has a crucial role in the development of astrocytes of the spinal cord.

Role of megalin and cubilin in renal physiology and pathophysiology

Megalin and cubilin are endocytic receptors highly expressed in the endocytic apparatus of the renal proximal tubule. These receptors appear to be responsible for the tubular clearance of most proteins filtered in the glomeruli. Cubilin is a peripheral membrane protein, and therefore it does not have an endocytosis signaling sequence. It appears that megalin is responsible for internalization of cubilin and its ligands in addition to internalizing its own ligands. The proteinuria observed in megalin-deficient mice, in dogs lacking functional cubilin, and in patients with distinct mutations of the cubilin gene illustrates the importance of the receptors.

Synapse associated protein 102 is a novel binding partner to the cytoplasmic terminus of neurone-glial related cell adhesion molecule

Neurone glial-related cell adhesion molecule (NrCAM) is a member of the L1 family of transmembrane cell adhesion receptors which are involved in the development and function of the mammalian nervous system. How these receptors interact with intracellular signalling pathways is not understood. To date the only identified binding partner to the cytoplasmic terminus of NrCAM is ankyrin G. We screened a developing rat brain cDNA yeast two-hybrid library with the cytoplasmic domain of NrCAM to identify further intracellular binding partners. We identified synapse associated protein 102 (SAP102) as a new binding partner for NrCAM. The interaction was confirmed biochemically using glutathione S-transferase (GST)-pull-down and tandem affinity purification, and also immunocytochemically as NrCAM and SAP102 co-localized in COS-7 and cerebellar granule cells. Binding was specific to NrCAM as neither neurofascin nor L1 bound SAP102, and this interaction was reliant on the last three amino acids of NrCAM. Additionally, NrCAM constructs whose last three amino acids had been deleted appeared to have a dominant negative effect on neurite extension of cerebellar granule cells. This is the first interaction reported for NrCAM, and its association with SAP102 suggests that it is part of a larger complex which can interact with many different signalling pathways.

Structural Characterization of the Intermolecular Interactions of Synapse-associated Protein-97 with the NR2B Subunit of N-Methyl-d-aspartate Receptors

The synapse-associated protein-97 (SAP97) is important in the proper trafficking and cell surface maintenance of the N-methyl-D-aspartate ionotropic glutamate receptor. The molecular scaffold/receptor interaction is mediated by the association of the C terminus of the NR2B subunit of the N-methyl-D-aspartate receptor with the PDZ domains of SAP97. Here, we characterize the binding of the C terminus of NR2B with the PDZ domains of SAP97 and determine the structure of the PDZ1-NR2B complex employing high-resolution NMR. Based on fluorescence anisotropy, the NR2B subunit binds to the first and second PDZ domains of SAP97, with higher affinity for PDZ2; no appreciable binding to PDZ3 could be measured. The structural features of the NR2B bound to PDZ1 is consistent with the canonical PDZ-binding motif with the glutamic acid at the -3 position of the C terminus (i.e. -E-S-D-V) interacting with the beta2/beta3 loop. Two sites within the loop of PDZ1 were replaced with the corresponding residue from PDZ2, D243G and P245Q. The former mutation, designed to remove a possible Coulombic repulsion between E(-3)(NR2B) and Asp-243 (PDZ1) has only a minimal effect on binding. The P245Q mutation leads to a 2-fold increase in binding affinity of NR2B, approaching that observed for wild-type PDZ2. These results indicate that modification of the beta2/beta3 loop provides an avenue for regulating the ligand specificity of PDZ domains.

Survey of the year 2003 commercial optical biosensor literature

In the year 2003 there was a 17% increase in the number of publications citing work performed using optical biosensor technology compared with the previous year. We collated the 962 total papers for 2003, identified the geographical regions where the work was performed, highlighted the instrument types on which it was carried out, and segregated the papers by biological system. In this overview, we spotlight 13 papers that should be on everyone's 'must read' list for 2003 and provide examples of how to identify and interpret high-quality biosensor data. Although we still find that the literature is replete with poorly performed experiments, over-interpreted results and a general lack of understanding of data analysis, we are optimistic that these shortcomings will be addressed as biosensor technology continues to mature.

The low-density lipoprotein receptor is regulated by estrogen and forms a functional complex with the estrogen-regulated protein ezrin in pituitary GH3 somatolactotropes

Estrogen regulates the function, growth, and proliferation of lactotropes in the pituitary. We report here that low-density lipoprotein (LDL) receptor (LDLR) gene expression and LDL uptake are strongly up-regulated by estrogen in pituitary somatolactotropic GH(3) cells. The uptake of LDL was significantly inhibited by the F-actin-severing drug, swinholide A, indicating that LDL uptake is dependent on the integrity of the cortical actin cytoskeleton in GH(3) cells. We examined whether the estrogen-inducible cytoskeletal linker protein, ezrin, interacts with the LDLR. The LDLR coimmunoprecipitated with ezrin, and fluorescently labeled LDL bound to regions of the cell membrane that colocalized with the active, phosphorylated form of ezrin (phosphoezrin). Evidence for a functional interaction between ezrin and the LDLR was obtained by transient transfection experiments using ezrin-green fluorescent protein (GFP) expression constructs. We observed that transient transfection of GH(3) cells with an ezrin N terminus-GFP dominant-negative construct prevented the uptake of LDL particles, whereas expression of GFP alone or an ezrin C terminus-GFP construct had no effect on LDL uptake. Transfection with the ezrin N terminus dominant- negative construct had no effect on the endocytosis of transferrin. Thus, estrogen stimulates the expression of ezrin and the LDLR in GH(3) cells, which interact physically and functionally to facilitate the endocytosis of LDL. We propose that the up-regulation and interaction of ezrin and the LDLR serves to augment the delivery of cholesterol and other lipids in support of the hypertrophic and proliferative response of cells to estrogen.