Activation and functional characterization of the mosaic receptor SorLA/LR11

Mapping, genetic isolation, and characterization of genetic loci that determine resistance to atherosclerosis in C3H mice

OBJECTIVE

C3H/HeJ (C3H) mice are extremely resistant to atherosclerosis. To identify the genetic factors involved in lesion initiation, we studied a cross between C3H and the susceptible strain C57BL/6J (B6) on a hyperlipidemic (apolipoprotein E-null) background.

METHODS AND RESULTS

Whereas a previous cross in mice fed a Western diet for 16 weeks revealed a very complex inheritance pattern with many significant lesion QTLs, the present cross, on a chow diet, revealed a single major locus on chromosome 9 (lod=5.0, Ath29*), and a suggestive locus on chromosome 4 (lod=2.6, Ath8). QTLs for plasma HDL, total cholesterol, and triglyceride levels were found on chromosome 1 over the ApoA2 gene. Neither of the lesion QTLs were associated with differences in plasma lipid levels or other systemic risk factors, consistent with the concept that genetic factors affecting cellular functions of the vessel wall are important determinants of atherosclerosis susceptibility. We generated a congenic strain for Ath29 and confirmed its contribution to lesion development. Toll-like receptor 4 (Tlr4), the lipopolysaccharide (LPS) receptor, is located in the Ath8 region and is known to be defective in C3H/HeJ mice. We constructed a congenic strain carrying a normal Tlr4 gene on the C3H Apoe-null background and found that the defective Tlr4 does not contribute significantly to lesion resistance during early lesion development.

CONCLUSIONS

We identified one major QTL on chromosome 9, Ath29, for early lesion development in the BXH ApoE(-/-) cross fed on a chow diet and confirmed its contribution in congenic mice. We have also determined that Tlr4 on the C3H ApoE(-/-) background does not contribute to early lesion development. *Ath29 is referred to as Ath22 in Su et al 2006.

SorLA/LR11 regulates processing of amyloid precursor protein via interaction with adaptors GGA and PACS-1

SorLA has been recognized as a novel sorting receptor that regulates trafficking and processing of the amyloid precursor protein (APP) and that represents a significant risk factor for sporadic Alzheimer disease. Here, we investigated the cellular mechanisms that control intracellular trafficking of sorLA and their relevance for APP processing. We demonstrate that sorLA acts as a retention factor for APP in trans-Golgi compartments/trans-Golgi network, preventing release of the precursor into regular processing pathways. Proper localization and activity of sorLA are dependent on functional interaction with GGA and PACS-1, adaptor proteins involved in protein transport to and from the trans-Golgi network. Aberrant targeting of sorLA to the recycling compartment or the plasma membrane causes faulty APP trafficking and imbalance in non-amyloidogenic and amyloidogenic processing fates. Thus, our findings identified altered routing of sorLA as a major cellular mechanism contributing to abnormal APP processing and enhanced amyloid beta-peptide formation.

Sorting by the cytoplasmic domain of the amyloid precursor protein binding receptor SorLA

SorLA/LR11 (250 kDa) is the largest and most composite member of the Vps10p-domain receptors, a family of type 1 proteins preferentially expressed in neuronal tissue. SorLA binds several ligands, including neurotensin, platelet-derived growth factor-bb, and lipoprotein lipase, and via complex-formation with the amyloid precursor protein it downregulates generation of Alzheimer's disease-associated Abeta-peptide. The receptor is mainly located in vesicles, suggesting a function in protein sorting and transport. Here we examined SorLA's trafficking using full-length and chimeric receptors and find that its cytoplasmic tail mediates efficient Golgi body-endosome transport, as well as AP-2 complex-dependent endocytosis. Functional sorting sites were mapped to an acidic cluster-dileucine-like motif and to a GGA binding site in the C terminus. Experiments in permanently or transiently AP-1 mu1-chain-deficient cells established that the AP-1 adaptor complex is essential to SorLA's transport between Golgi membranes and endosomes. Our results further implicate the GGA proteins in SorLA trafficking and provide evidence that SNX1 and Vps35, as parts of the retromer complex or possibly in a separate context, are engaged in retraction of the receptor from endosomes.

Neurotensin receptor mechanisms and its modulation of glutamate transmission in the brain: relevance for neurodegenerative diseases and their treatment

The extracellular accumulation of glutamate and the excessive activation of glutamate receptors, in particular N-methyl-D-aspartate (NMDA) receptors, have been postulated to contribute to the neuronal cell death associated with chronic neurodegenerative disorders such as Parkinson's disease. Findings are reviewed indicating that the tridecaptide neurotensin (NT) via activation of NT receptor subtype 1 (NTS1) promotes and reinforces endogenous glutamate signalling in discrete brain regions. The increase of striatal, nigral and cortical glutamate outflow by NT and the enhancement of NMDA receptor function by a NTS1/NMDA interaction that involves the activation of protein kinase C may favour the depolarization of NTS1 containing neurons and the entry of calcium. These results strengthen the hypothesis that NT may be involved in the amplification of glutamate-induced neurotoxicity in mesencephalic dopamine and cortical neurons. The mechanisms involved may include also antagonistic NTS1/D2 interactions in the cortico-striatal glutamate terminals and in the nigral DA cell bodies and dendrites as well as in the nigro-striatal DA terminals. The possible increase in NT levels in the basal ganglia under pathological conditions leading to the NTS1 enhancement of glutamate signalling may contribute to the neurodegeneration of the nigro-striatal dopaminergic neurons found in Parkinson's disease, especially in view of the high density of NTS1 receptors in these neurons. The use of selective NTS1 antagonists together with conventional drug treatments could provide a novel therapeutic approach for treatment of Parkinson's disease.

A cyclic peptide that binds p75(NTR) protects neurones from beta amyloid (1-40)-induced cell death

The current study determined the ability of a p75(NTR) antagonistic cyclic peptide to rescue cells from beta amyloid (Abeta) (1-40)-induced death. p75(NTR)-, p140(trkA)-NIH-3T3 cells or E17 foetal rat cortical neurones were incubated with 125I-NGF or 125I-Abeta (1-40) and increasing concentrations of the cyclic peptide (CATDIKGAEC). Peptide ability to displace 125I-NGF or 125I-Abeta (1-40) binding was determined. Duplicate cultures were preincubated with CATDIKGAEC (250 nM) or diluent and then stimulated with Abeta (1-40). Peptide ability to displace Abeta (1-40) binding, interfere with Abeta (1-40)-induced signalling and rescue cells from Abeta-mediated toxicity was determined by immunoprecipitation and autoradiography, Northern blotting, JNK activation, MTT and trypan blue assays. The peptide inhibited NGF and Abeta (1-40) binding to p75(NTR), but not to p140(trkA). Abeta (1-40) induced c-jun transcription (57.3% +/- 0.07%) in diluent-treated p75(NTR)-cells, but not in cells preincubated with the cyclic peptide. Also, at 250 nM, the peptide reduced Abeta (1-40)-induced phosphorylation of JNK by 71.8% +/- 0.03% and protected neurones against Abeta-induced toxicity as determined by: trypan blue exclusion assay (53% +/- 11% trypan blue-positive cells in diluent pretreated cultures vs. 28% +/- 5% in cyclic peptide-pretreated cultures); MTT assay (0.09 +/-0.03 units in diluent-pretreated cells vs. 0.12 +/- 0.004 units in cyclic peptide-pretreated cells); and visualization of representative microscopic fields. Our data suggest that a cyclic peptide homologous to amino acids 28-36 of NGF known to mediate binding to p75(NTR) can interfere with Abeta (1-40) signalling and rescue neurones from Abeta (1-40)-induced toxicity.

N-Linked Oligosaccharides on the Low Density Lipoprotein Receptor Homolog SorLA/LR11 Are Modified with Terminal GalNAc-4-SO4 in Kidney and Brain

Sorting protein-related receptor (SorLA/LR11) is a highly conserved mosaic receptor that is expressed by cells in a number of different tissues including principal cells of the collecting ducts in the kidney and neurons in the central and peripheral nervous systems. SorLA/LR11 has features that indicate it serves as a sorting receptor shuttling between the plasma membrane, endosomes, and the Golgi. We have found that a fraction of SorLA/LR11 that is synthesized in the kidney and the brain bears N-linked oligosaccharides that are modified with terminal beta1,4-linked GalNAc-4-SO(4). Oligosaccharides located in the vacuolar sorting (Vps) 10p domain (Vps10p domain) are modified with beta1,4-linked GalNAc when the Vps10p domain is expressed in cells along with either of two recently cloned protein-specific beta1,4GalNAc-transferases, GalNAcTIII and GalNAcTIV. Either of two sequences with basic amino acids located within the Vps10p domain is able to mediate recognition by these beta1,4GalNAc-transferases. The highly specific modification of oligosaccharides in the Vps10p domain of SorLA/LR11 with terminal GalNAc-4-SO(4) suggests that this unusual modification may modulate the interaction of SorLA/LR11 with proteins and influence their trafficking.

Neurotensin agonists as an alternative to antipsychotics

Neurotensin (NT) is a 13 amino acid neuropeptide that is found in the central nervous system and in the gastrointestinal tract. In brain, this peptide is prominently associated anatomically with dopaminergic, as well as other neurotransmitter systems. Based on animal studies, already decades old, researchers have hypothesised that NT receptor agonists will have antipsychotic properties in patients. However, to date no one has obtained a non-peptide NT receptor agonist. Therefore, there has been great interest in obtaining peptide analogues of NT, that, unlike NT resist degradation by peptidases and cross the blood-brain barrier, yet have the pharmacological characteristics of native NT, for therapeutic use in the treatment of schizophrenia, as well as other neuropsychiatric diseases such as Parkinson's disease and addiction to psychostimulants. In this review, we present the rationale for development of NT receptor agonists for treatment of certain central nervous system diseases, as well as a review of those peptide agonists that are in early stages of development.

Binding areas of urokinase-type plasminogen activator?plasminogen activator inhibitor-1 complex for endocytosis receptors of the low-density lipoprotein receptor family, determined by site-directed mutagenesis

Some endocytosis receptors related to the low-density lipoprotein receptor, including low-density lipoprotein receptor-related protein-1A, very-low-density lipoprotein receptor, and sorting protein-related receptor, bind protease-inhibitor complexes, including urokinase-type plasminogen activator (uPA), plasminogen activator inhibitor-1 (PAI-1), and the uPA-PAI-1 complex. The unique capacity of these receptors for high-affinity binding of many structurally unrelated ligands renders mapping of receptor-binding surfaces of serpin and serine protease ligands a special challenge. We have mapped the receptor-binding area of the uPA-PAI-1 complex by site-directed mutagenesis. Substitution of a cluster of basic residues near the 37-loop and 60-loop of uPA reduced the receptor-binding affinity of the uPA-PAI-1 complex approximately twofold. Deletion of the N-terminal growth factor domain of uPA reduced the affinity 2-4-fold, depending on the receptor, and deletion of both the growth factor domain and the kringle reduced the affinity sevenfold. The binding affinity of the uPA-PAI-1 complex to the receptors was greatly reduced by substitution of basic and hydrophobic residues in alpha-helix D and alpha-helix E of PAI-1. The localization of the implicated residues in the 3D structures of uPA and PAI-1 shows that they form a continuous receptor-binding area spanning the serpin as well as the A-chain and the serine protease domain of uPA. Our results suggest that the 10-100-fold higher affinity of the uPA-PAI-1 complex compared with the free components depends on the bonus effect of bringing the binding areas on uPA and PAI-1 together on the same binding entity.

LR11/SorLA expression is reduced in sporadic Alzheimer disease but not in familial Alzheimer disease

LR11 is an ApoE receptor that is enriched in the brain. We have shown that LR11 is markedly downregulated in patients with sporadic Alzheimer disease (AD). This finding led us to explore whether reduced LR11 expression reflects a primary mechanism of disease or merely a secondary consequence of other AD-associated changes. Therefore, LR11 expression was assessed in a transgenic mouse model of AD and familial AD (FAD) brains. Immunohistochemistry and immunoblotting of LR11 in PS1/APP transgenic and wild-type mice indicated that LR11 levels are not affected by genotype or accumulation of amyloid pathology. LR11 expression was also evaluated based on immunoblotting and LR11 immunostaining intensity in human frontal cortex in controls, sporadic AD, and FAD, including cases with presenilin-1 (PS1) and presenilin-2 (PS2) mutations. Although LR11 was reduced in sporadic AD, there was no difference in protein level or staining intensity between control and FAD cases. The finding that LR11 expression is unaffected in both a mouse model of AD and autosomal-dominant forms of AD suggests that LR11 is not regulated by amyloid accumulation or other AD neuropathologic changes. We hypothesize that LR11 loss may be specific to sporadic AD and influence amyloid pathology through mechanisms independent of substrate-enzyme interactions regulated by FAD mutations.

Brain neurotensin, psychostimulants, and stress--emphasis on neuroanatomical substrates

Neurotensin (NT) is a peptide that is widely distributed throughout the brain. NT is involved in locomotion, reward, stress and pain modulation, and in the pathophysiology of drug addiction and depression. In its first part this review brings together relevant literature about the neuroanatomy of NT and its receptors. The second part focuses on functional-anatomical interactions between NT, the mesotelencephalic dopamine system and structures targeted by dopaminergic projections. Finally, recent data about the actions of NT in processes underlying behavioral sensitization to psychostimulant drugs and the involvement of NT in the regulation of the hypothalamo-pituitary-adrenal gland axis are considered.

Bioactive analogs of neurotensin: focus on CNS effects

Neurotensin (NT) is a 13-amino acid neuropeptide found in the central nervous system and in the gastrointestinal tract. It is closely associated anatomically with dopaminergic and other neurotransmitter systems, and evidence supports a role for NT agonists in the treatment of various neuropsychiatric disorders. However, NT is readily degraded by peptidases, so there is much interest in the development of stable NT agonists, that can be injected systemically, cross the blood-brain barrier (BBB), yet retains the pharmacological characteristics of native NT for therapeutic use in the treatment of diseases such as schizophrenia, Parkinson's disease and addiction.

Modulation of beta-amyloid precursor protein trafficking and processing by the low density lipoprotein receptor family

Amyloid-β peptide (Aβ) accumulation in the brain is an early, toxic event in the pathogenesis of Alzheimer's disease (AD). Aβ is produced by proteolytic processing of a transmembrane protein, β-amyloid precursor protein (APP), by β- and γ-secretases. Mounting evidence has demonstrated that alterations in APP cellular trafficking and localization directly impact its processing to Aβ. Recent studies have shown that members of the low-density lipoprotein receptor family, including LRP, LRP1B, SorLA/LR11, and apolipoprotein E (apoE) receptor 2, interact with APP and regulate its endocytic trafficking. Another common feature of these receptors is their ability to bind apoE, which exists in three isoforms in humans and the presence of the ε4 allele represents a genetic risk factor for AD. In this review, we summarize the current understanding of the function of these apoE receptors with a focus on their role in APP trafficking and processing. Knowledge of the interactions between these distinct low-density lipoprotein receptor family members and APP may ultimately influence future therapies for AD.

Positional cloning of Sorcs1, a type 2 diabetes quantitative trait locus

We previously mapped the type 2 diabetes mellitus-2 locus (T2dm2), which affects fasting insulin levels, to distal chromosome 19 in a leptin-deficient obese F2 intercross derived from C57BL/6 (B6) and BTBR T+ tf/J (BTBR) mice. Introgression of a 7-Mb segment of the B6 chromosome 19 into the BTBR background (strain 1339A) replicated the reduced insulin linked to T2dm2. The 1339A mice have markedly impaired insulin secretion in vivo and disrupted islet morphology. We used subcongenic strains derived from 1339A to localize the T2dm2 quantitative trait locus (QTL) to a 242-kb segment comprising the promoter, first exon and most of the first intron of the Sorcs1 gene. This was the only gene in the 1339A strain for which we detected amino acid substitutions and expression level differences between mice carrying B6 and BTBR alleles of this insert, thereby identifying variation within the Sorcs1 gene as underlying the phenotype associated with the T2dm2 locus. SorCS1 binds platelet-derived growth factor, a growth factor crucial for pericyte recruitment to the microvasculature, and may thus have a role in expanding or maintaining the islet vasculature. Our identification of the Sorcs1 gene provides insight into the pathway underlying the pathophysiology of obesity-induced type 2 diabetes mellitus.

The lipoprotein receptor LR11 regulates amyloid beta production and amyloid precursor protein traffic in endosomal compartments

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by progressive cognitive decline and neuropathological changes, including the deposition of amyloid beta (Abeta) in senile plaques. The mechanisms causing the disease and Abeta accumulation are not well understood, but important genetic associations with apolipoprotein E genotype and involvement of lipoprotein receptors have become apparent. LR11 (also known as SorLA), a member of the low-density lipoprotein receptor family, has been identified previously as an altered transcript in microarray analyses of samples from human AD cases. Here, we show neuronal expression of the lipoprotein receptor LR11 in control brain in regions vulnerable to AD neuropathology and marked reduction of LR11 expression in these regions in AD brains before cell death. Overexpression of LR11 drastically reduces levels of extracellular Abeta and also lowers levels of total cellular amyloid precursor protein (APP). LR11 colocalizes with APP and regulates its trafficking in endocytic compartments, which are important intracellular sites for APP processing and Abeta generation. Endogenous LR11 localizes to neuronal multivesicular bodies in both rat and human brain. The robust correlation between reduced LR11 expression and AD neuropathology and its potent effects on extracellular Abeta levels suggest that this neuronal lipoprotein receptor could play an important role in AD pathogenesis.

Tumour necrosis factor alpha-converting enzyme mediates ectodomain shedding of Vps10p-domain receptor family members

Several transmembrane molecules are cleaved at juxtamembrane extracellular sites leading to shedding of ectodomains. We analysed shedding of members of the Vps10p-D (Vps10p domain; where Vps is vacuolar protein sorting) family of neuronal type-I receptors with partially overlapping functions, and additional proteolytic events initiated by the shedding. When transfected into CHO (Chinese-hamster ovary) cells (CHO-K1), sorCS1a-sorCS1c isoforms were shed at high rates (approximately 0.61% x min(-1)) that were increased approx. 3-fold upon stimulation with phorbol ester. sorCS1c identified in the cultured neuroblastoma cell line SH-SY5Y was shed similarly. In CHO-K1 transfectants, constitutive and stimulated shedding of sorCS3 also occurred at high rates (0.29% and 1.03% x min(-1)). By comparison, constitutive and stimulated shedding of sorLA occurred at somewhat lower rates (0.07% and 0.48% x min(-1)), whereas sorCS2 and sortilin were shed at very low rates even when stimulated (approximately 0.01% x min(-1)). Except for sorCS2, shedding of the receptors was dramatically reduced in mutant CHO cells (CHO-M2) devoid of active TACE (tumour necrosis factor alpha-converting enzyme), demonstrating that this enzyme accounts for most sheddase activity. The release of sorCS1 and sorLA ectodomains initiated rapid cleavage of the membrane-tethered C-terminal stubs that accumulated only in the presence of gamma-secretase inhibitors. Purified shed sorLA bound several ligands similarly to the entire luminal domain of the receptor, including PDGF-BB (platelet-derived growth factor-BB) and amyloid-beta precursor protein. In addition, PDGF-BB also bound to the luminal domains of sorCS1 and sorCS3. The results suggest that ectodomains shed from a subset of Vps10p-D receptors can function as carrier proteins.

Interaction of the cytosolic domains of sorLA/LR11 with the amyloid precursor protein (APP) and beta-secretase beta-site APP-cleaving enzyme

sorLA is a recently identified neuronal receptor for amyloid precursor protein (APP) that is known to interact with APP and affect its intracellular transport and processing. Decreased levels of sorLA in the brain of Alzheimer's disease (AD) patients and elevated levels of amyloid-beta peptide (Abeta) in sorLA-deficient mice point to the importance of the receptor in this neurodegenerative disorder. We analyzed APP cleavage in an APP-shedding assay and found that both sorLA and, surprisingly, a sorLA tail construct inhibited APP cleavage in a beta-site APP-cleaving enzyme (BACE)-dependent manner. In line with this finding, sorLA and the sorLA tail significantly reduced secreted Abeta levels when BACE was overexpressed, suggesting that sorLA influences beta-cleavage. To understand the effect of sorLA on APP cleavage by BACE, we analyzed whether sorLA interacts with APP and/or BACE. Because both full-length sorLA and sorLA C-terminal tail constructs were functionally relevant for APP processing, we analyzed sorLA-APP for a potential cytoplasmatic interaction domain. sorLA and C99 coimmunoprecipitated, pointing toward the existence of a new cytoplasmatic interaction site between sorLA and APP. Moreover, sorLA and BACE also coimmunoprecipitate. Thus, sorLA interacts both with BACE and APP and might therefore directly affect BACE-APP complex formation. To test whether sorLA impacts BACE-APP interactions, we used a fluorescence resonance energy transfer assay to evaluate BACE-APP interactions in cells. We discovered that sorLA significantly reduced BACE-APP interactions in Golgi. We postulate that sorLA acts as a trafficking receptor that prevents BACE-APP interactions and hence BACE cleavage of APP.

Molecular Dissection of the Interaction between Amyloid Precursor Protein and Its Neuronal Trafficking Receptor SorLA/LR11

SorLA/LR11 is a sorting receptor that regulates the intracellular transport and processing of the amyloid precursor protein (APP) in neurons. SorLA/LR11-mediated binding results in sequestration of APP in the Golgi and in protection from processing into the amyloid-beta peptide (Abeta), the principal component of senile plaques in Alzheimer's disease (AD). To gain insight into the molecular mechanisms governing sorLA and APP interaction, we have dissected the respective protein interacting domains. Using a fluorescence resonance energy transfer (FRET) based assay of protein proximity, we identified binding sites in the extracellular regions of both proteins. Fine mapping by surface plasmon resonance analysis and analytical ultracentrifugation of recombinant APP and sorLA fragments further narrowed down the binding domains to the cluster of complement-type repeats in sorLA that forms a 1:1 stoichiometric complex with the carbohydrate-linked domain of APP. These data shed new light on the molecular determinants of neuronal APP trafficking and processing and on possible targets for intervention with senile plaque formation in patients with AD.

Gene expression profiling of the preclinical scrapie-infected hippocampus

The molecular events that underlie prion disease neuropathology remain poorly defined. Within the hippocampus of the ME7/CV mouse scrapie model, profound CA1 neuronal loss occurs between 160 and 180 days post-infection (dpi). To elucidate the molecular events that may contribute to this neuronal loss, we have applied Affymetrix high-density oligonucleotide probe arrays to the study of ME7-infected hippocampal gene expression at 170 dpi. The study has identified 78 genes that are differentially expressed greater than 1.5-fold within the preclinical ME7-infected hippocampus prior to the profound late stage glial cell activation. The results indicate oxidative and endoplasmic reticulum (ER) stress, activated ER and mitochondrial apoptosis pathways, and activated cholesterol biosynthesis within the scrapie-infected hippocampus, and offer insight into the molecular events which underlie the neuropathology.

SorCS3 does not require propeptide cleavage to bind nerve growth factor

The functional properties of the Vps10p-domain receptor SorCS3 are undescribed. Here, we examine its processing and sorting in cellular transfectants, and analyze the binding of potential ligands to the purified receptor. We show that SorCS3 is synthesized as a proprotein and converted to its mature form by N-terminal propeptide cleavage in distal Golgi compartments. The propeptide is not a requirement for normal processing of the receptor and does not prevent ligands from binding to the SorCS3 precursor form. Expression of wt and chimeric receptors further suggests that SorCS3 predominates on the plasma membrane, exhibits slow internalization and does not engage in intracellular trafficking. SorCS3 emerges as a new neurotrophin binding Vps10p-domain receptor functionally distinct from its relatives Sortilin and SorLA.

The apoE receptor apoER2 is involved in the maintenance of efficient synaptic plasticity

ApoER2 is one of the major receptors for ApoE in the brain, and has been shown to be involved not only in lipoprotein endocytosis, as other members of the LDL receptor family of receptors, but also in various cellular functions such as signalling and cellular guidance. By using a model of synaptic plasticity in mice lacking none, one or two alleles of the apoER2 gene, we investigated the implication of such a receptor deficiency on the remodelling process. Our results indicate that animals lacking apoER2 express higher levels of brain APP, as well as both key amyloid peptides, while apoE levels are slightly lower. Following entorhinal cortex lesioning, apoE levels increase in the deafferented hippocampus, while a delay in the increase of APP was observed. Hippocampal amyloid levels are also increased in response to the lesion, and highly potentiated by the complete absence of apoER2 gene. The results suggest a significant role for apoER2 in signalling various proteins in response to massive deafferentation and may participate in maintaining efficient synaptic plasticity and dendritic remodelling.

Multi-tasking by the p75 neurotrophin receptor: sortilin things out?

Signalling by the p75 neurotrophin receptor has been implicated in diverse neuronal responses, including increased differentiation or survival, inhibition of regeneration, and initiation of apoptotic cell death. These numerous roles are matched by, but are not yet correlated with, a multiplicity of extracellular ligands and intracellular interactors. Membrane proteins such as sortilin, a member of the Vps10p family of sorting receptors, and the glycosylphosphatidylinositol-linked Nogo receptor (NgR) and the associated adaptor lingo 1 have recently been added to the list of p75-interacting modulators. Other studies have described intramembranal cleavage of p75 and the potential nuclear targeting of cleavage fragments or of the complete receptor after it has been internalized into a putative signalling endosome. These findings suggest that some of the diversity in p75 activities might be due to differential subcellular localization and transport of p75 receptor complexes. We therefore argue that cell-biology-driven approaches are now required to make sense of p75 signalling.

Stabilin-1 localizes to endosomes and the trans-Golgi network in human macrophages and interacts with GGA adaptors

Stabilin-1 and stabilin-2 constitute a novel family of fasciclin domain-containing hyaluronan receptor homologues recently described by us. Whereas stabilin-1 is expressed in sinusoidal endothelial cells and in macrophages in vivo, stabilin-2 is absent from the latter. In the present study, we analyzed the subcellular distribution of stabilin-1 in primary human macrophages. Using flow cytometry, expression of stabilin-1 was demonstrated on the surface of interleukin-4/dexamethasone-stimulated macrophages (MPhi2). By immunofluorescence and confocal microscopy, we established that stabilin-1 is preferentially localized in early endosome antigen-1-positive early/sorting endosomes and in recycling endosomes identified by transferrin endocytosis. Association of stabilin-1 was infrequently seen with p62 lck ligand-positive late endosomes and with CD63-positive lysosomes but not in lysosome-associated membrane protein-1-positive lysosomes. Stabilin-1 was also found in the trans-Golgi network (TGN) but not in Golgi stack structures. Glutathione S-transferase pull-down assay revealed that the cytoplasmic tail of stabilin-1 but not stabilin-2 binds to recently discovered Golgi-localized, gamma-ear-containing, adenosine 5'-diphosphate-ribosylation factor-binding (GGA) adaptors GGA1, GGA2, and GGA3 long, mediating traffic between Golgi and endosomal/lysosomal compartments. Stabilin-1 did not bind to GGA3 short, which lacks a part of the Vps27p/Hrs/STAM domain. Deletion of DDSLL and LL amino acid motifs resulted in decreased binding of stabilin-1 with GGAs. A small portion of stabilin-1 colocalized with GGA2 and GGA3 in the TGN in MPhi2. Treatment with brefeldin A resulted in accumulation of stabilin-1 in the TGN. Our results suggest that stabilin-1 is involved in the GGA-mediated sorting processes at the interface of the biosynthetic and endosomal pathways; similarly to other GGA-interacting proteins, stabilin-1 may thus function in endocytic and secretory processes of human macrophages.

The mosaic receptor sorLA/LR11 binds components of the plasminogen-activating system and platelet-derived growth factor-BB similarly to LRP1 (low-density lipoprotein receptor-related protein), but mediates slow internalization of bound ligand

The type-1 receptor sorLA/LR11, a member of the Vps10p-domain receptor family that also contains domains characterizing members of the LDL (low-density lipoprotein) receptor family, has been shown to induce increased uPAR (urokinase receptor) expression as well as enhanced migration and invasion activities in smooth muscle cells in the presence of PDGF-BB (platelet-derived growth factor-BB). Here we show that sorLA interacts with both components of the plasminogen activating system and PDGF-BB similarly to LRP1 (LDL receptor-related protein/alpha2-macroglobulin receptor), which is an important clearance receptor with established functions in controlling uPAR expression as well as PDGF-BB signalling. In contrast with LRP1, sorLA does not interact with alpha2-macroglobulin, which is a binding protein for several growth factors, including PDGF-BB. By using LRP1-deficient cells transfected with sorLA, we demonstrate that sorLA-bound ligand is internalized at a much lower rate than LRP1-bound ligand, and that sorLA is inefficient in regulating cell surface uPAR expression, which depends on rapid internalization of the ternary complex between urokinase-type plasminogen activator, its type-1 inhibitor, and uPAR. Thus, although overlapping with regard to binding profiles, sorLA is substantially less efficient as a clearance receptor than LRP1. We propose that sorLA can divert ligands away from LRP1 and thereby inhibit both their clearance and signalling events mediated by LRP1.

Functional organization of the sortilin Vps10p domain

A Vps10p domain makes up the entire luminal part of Sortilin, and this type of domain is the hallmark of a new family of neuronal receptors that target a variety of ligands, including neurotrophins and neuropeptides. We have shown that two structural features of the Vps10p domain, the N-terminal propeptide and the C-terminal segment of ten conserved cysteines (10CC), are key elements in the function of Sortilin. The propeptide has two functions. (i) It binds the mature part of Sortilin and prevents ligands in the biosynthetic pathway from binding to the uncleaved proreceptor, and (ii) it facilitates receptor transport in early Golgi compartments by a mechanism that does not depend on its ability to prevent ligand binding. In contrast, other Vps10p domain receptors, such as SorLA and SorCS3, do not need their propeptide for normal and swift processing. The 10CC segment constitutes an exchangeable module containing five conserved disulfide bridges, and using module-shuffling and truncations, we have shown that the 10CC segment is a major ligand-binding region in Sortilin.

The three sorCS genes are differentially expressed and regulated by synaptic activity

We have isolated the murine sorCS3 gene, a new member of the family of receptors containing a Vps10p-domain. Receptors of this family facilitate rapid endocytosis and are thought to be involved in intracellular sorting. SorCS3 and the highly homologous sorCS1 and sorCS2 genes were expressed in a combinatorial, mostly non-overlapping pattern in both the developing and mature central nervous system. During development, distribution and abundancy of their transcripts was regulated. Moreover, their expression was differentially influenced by neuronal activity in the hippocampus of adult mice. Although kainic acid-induced seizures had no effect on sorCS2 mRNA levels, they dramatically increased the expression of sorCS1 and sorCS3. The activity-dependent induction of sorCS1 expression required de novo protein synthesis, whereas that of sorCS3 did not. Our results imply that the three sorCS genes have diverse, but partly overlapping functions in the developing and mature central nervous system.

Sortilin is essential for proNGF-induced neuronal cell death

Sortilin (approximately 95 kDa) is a member of the recently discovered family of Vps10p-domain receptors, and is expressed in a variety of tissues, notably brain, spinal cord and muscle. It acts as a receptor for neurotensin, but predominates in regions of the nervous system that neither synthesize nor respond to this neuropeptide, suggesting that sortilin has additional roles. Sortilin is expressed during embryogenesis in areas where nerve growth factor (NGF) and its precursor, proNGF, have well-characterized effects. These neurotrophins can be released by neuronal tissues, and they regulate neuronal development through cell survival and cell death signalling. NGF regulates cell survival and cell death via binding to two different receptors, TrkA and p75NTR (ref. 10). In contrast, proNGF selectively induces apoptosis through p75NTR but not TrkA. However, not all p75NTR-expressing cells respond to proNGF, suggesting that additional membrane proteins are required for the induction of cell death. Here we report that proNGF creates a signalling complex by simultaneously binding to p75NTR and sortilin. Thus sortilin acts as a co-receptor and molecular switch governing the p75NTR-mediated pro-apoptotic signal induced by proNGF.

Neuroanatomical distribution of huntingtin-associated protein 1-mRNA in the male mouse brain

Huntingtin-associated protein 1 (HAP1) was identified as an interactor of the gene product (Huntingtin) responsible for Huntington's disease and found to be a core component of the stigmoid body. Even though HAP1 is highly expressed in the brain, detailed information on HAP1 distribution has not been fully described. Focusing on the neuroanatomical analysis of HAP1-mRNA expression using in situ hybridization histochemistry, the present study clarified its detailed regional distribution in the entire mouse brain. Mouse HAP1 (Hap1)-mRNAs were abundantly expressed in the limbic-related forebrain regions and midline/periventricular brainstem regions including the olfactory bulb, limbic-associated cortices, hippocampus, septum, amygdala, bed nucleus of the stria terminalis, preoptico-hypothalamic regions, central gray, raphe nuclei, locus coeruleus, parabrachial nuclei, nucleus of the solitary tract, and area postrema. In contrast, little expression was detected in the striatum and thalamus, implying that Hap1 is associated with neurodegeneration-sparing regions rather than target lesions in Huntington's disease. The distribution pattern, resembling that of the stigmoid body, suggests that HAP1 and the stigmoid body are implicated in protection from neuronal death rather than induction of neurodegeneration in Huntington's disease, and that they play an important role in integrating instinct behaviors and underlying autonomic, visceral, arousal, drive, memory, and neuroendocrinergic functions, particularly during extensive homeostatic or emotional processes. These data will provide an important morphological base for a future understanding of functions of HAP1 and the stigmoid body in the brain.

The prodomain of a secreted hydrophobic mini-protein facilitates its export from the endoplasmic reticulum by hitchhiking on sorting receptors

Misfolded secretory proteins are retained in the endoplasmic reticulum (ER) by quality control mechanisms targeted to exposed hydrophobic surfaces. Paradoxically, certain conotoxins expose extensive hydrophobic surfaces upon folding to their bioactive structures. How then can such secreted mini-proteins traverse the secretory pathway? Here we show that secretion of the hydrophobic conotoxin-TxVI is strongly dependent on its propeptide domain, which enhances TxVI export from the ER. The propeptide domain interacts with sorting receptors from the sortilin Vps10p domain family. The sortilin-TxVI interaction occurs in the ER, and sortilin facilitates export of TxVI from the ER to the Golgi. Thus, the prodomain in a secreted hydrophobic protein acts as a tag that can facilitate its ER export by a hitchhiking mechanism.

Characterization of sorCS1, an alternatively spliced receptor with completely different cytoplasmic domains that mediate different trafficking in cells

We previously isolated and sequenced murine sorCS1, a type 1 receptor containing a Vps10p-domain and a leucine-rich domain. We now show that human sorCS1 has three isoforms, sorCS1a-c, with completely different cytoplasmic tails and differential expression in tissues. The b tail shows high identity with that of murine sorCS1b, whereas the a and c tails have no reported counterparts. Like the Vps10p-domain receptor family members sortilin and sorLA, sorCS1 is synthesized as a proreceptor that is converted in late Golgi compartments by furin-mediated cleavage. Mature sorCS1 bound its own propeptide with low affinity but none of the ligands previously shown to interact with sortilin and sorLA. In transfected cells, about 10% of sorCS1a was expressed on the cell surface and proved capable of rapid endocytosis in complex with specific antibody, whereas sorCS1b presented a high cell surface expression but essentially no endocytosis, and sorCS1c was intermediate. This is an unusual example of an alternatively spliced single transmembrane receptor with completely different cytoplasmic domains that mediate different trafficking in cells.

Survey of the year 2001 commercial optical biosensor literature

We have assembled references of 700 articles published in 2001 that describe work performed using commercially available optical biosensors. To illustrate the technology's diversity, the citation list is divided into reviews, methods and specific applications, as well as instrument type. We noted marked improvements in the utilization of biosensors and the presentation of kinetic data over previous years. These advances reflect a maturing of the technology, which has become a standard method for characterizing biomolecular interactions.

Characterization of the VPS10 domain of SorLA/LR11 as binding site for the neuropeptide HA

The single transmembrane receptor sorLA/LR11 contains binding domains typical for the low-density lipoprotein receptors and a VPS10 domain which, in the related receptor sortilin, binds the neuropeptide neurotensin. SorLA is synthesized as a proreceptor which is processed to the mature form by a furin-like propeptidase. Endogenous sorLA and its hydra homologue HAB bind the neuropeptide head activator (HA). Transiently expressed partial sorLA constructs were investigated for ligand binding. We found that HA binds with nanomolar affinity to the VPS10 domain. The sorLA propeptide also bound to the VPS10 domain, whereas the receptor-associated protein RAP interacted both with the class A repeats and the VPS10 domain. The sorLA propeptide inhibited binding of HA to full-length sorLA and to the VPS10 domain. It also interfered with binding of HA to hydra HAB, which is taken as evidence for a highly conserved tertiary structure of the VPS10 domains of this receptor in hydra and mammals. The propeptide inhibited HA-stimulated mitosis and proliferation in the human neuroendocrine cell line BON and the neuronal precursor cell line NT2. We conclude that sorLA is necessary for HA signaling and function.

Developmental signaling in Hydra: what does it take to build a "simple" animal?

Developmental processes in multicellular animals depend on an array of signal transduction pathways. Studies of model organisms have identified a number of such pathways and dissected them in detail. However, these model organisms are all bilaterians. Investigations of the roles of signal transduction pathways in the early-diverging metazoan Hydra have revealed that a number of the well-known developmental signaling pathways were already in place in the last common ancestor of Hydra and bilaterians. In addition to these shared pathways, it appears that developmental processes in Hydra make use of pathways involving a variety of peptides. Such pathways have not yet been identified as developmental regulators in more recently diverged animals. In this review I will summarize work to date on developmental signaling pathways in Hydra and discuss the future directions in which such work will need to proceed to realize the potential that lies in this simple animal.

The sorLA cytoplasmic domain interacts with GGA1 and -2 and defines minimum requirements for GGA binding

We report that the Vps10p domain receptor sorLA binds the adaptor proteins GGA1 and -2, which take part in Golgi-endosome sorting. The GGAs bind with differential requirements via three critical residues in the C-terminal segment of the sorLA cytoplasmic tail. Unlike in sortilin and the mannose 6-phosphate receptors, the GGA-binding segment in sorLA contains neither an acidic cluster nor a dileucine. Our results support the concept of sorLA as a potential sorting receptor and suggest that key residues in sorLA and sortilin conform to a new type of motif (psi-psi-X-X-phi) defining minimum requirements for GGA binding to cytoplasmic receptor domains.

SorCS1, a member of the novel sorting receptor family, is localized in somata and dendrites of neurons throughout the murine brain

The expression of the sorCS1 protein in the central nervous system of adult mice was studied by immunohistochemistry. A detailed mapping revealed a distribution of sorCS1 immunoreactivity in a widespread population of neurons throughout the brain. Two different types of cellular localization were observed. Many neurons exhibited a punctate cytoplasmic staining which extended into the dendrites, in other neurons sorCS1 immunoreactivity was associated with the plasma membrane. This suggests variable functions for sorCS1 in the neurons of the brain.