Sorting nexin 17 regulates ApoER2 recycling and reelin signaling

SNX17 Mediates Dendritic Spine Maturation via p140Cap

Sorting nexin17 (SNX17) is a member of the sorting nexin family, which plays a crucial role in endosomal trafficking. Previous research has shown that SNX17 is involved in the recycling or degradation of various proteins associated with neurodevelopmental and neurological diseases in cell models. However, the significance of SNX17 in neurological function in the mouse brain has not been thoroughly investigated. In this study, we generated Snx17 knockout mice and observed that the homozygous deletion of Snx17 (Snx17-/-) resulted in lethality. On the other hand, heterozygous mutant mice (Snx17+/-) exhibited anxiety-like behavior with a reduced preference for social novelty. Furthermore, Snx17 haploinsufficiency led to impaired synaptic transmission and reduced maturation of dendritic spines. Through GST pulldown and interactome analysis, we identified the SRC kinase inhibitor, p140Cap, as a potential downstream target of SNX17. We also demonstrated that the interaction between p140Cap and SNX17 is crucial for dendritic spine maturation. Together, this study provides the first in vivo evidence highlighting the important role of SNX17 in maintaining neuronal function, as well as regulating social novelty and anxiety-like behaviors.

The Reelin receptor ApoER2 is a cargo for the adaptor protein complex AP-4: Implications for Hereditary Spastic Paraplegia

Adaptor protein complex 4 (AP-4) is a heterotetrameric complex that promotes export of selected cargo proteins from the trans-Golgi network. Mutations in each of the AP-4 subunits cause a complicated form of Hereditary Spastic Paraplegia (HSP). Herein, we report that ApoER2, a receptor in the Reelin signaling pathway, is a cargo of the AP-4 complex. We identify the motif ISSF/Y within the ApoER2 cytosolic domain as necessary for interaction with the canonical signal-binding pocket of the µ4 (AP4M1) subunit of AP-4. AP4E1- knock-out (KO) HeLa cells and hippocampal neurons from Ap4e1-KO mice display increased co-localization of ApoER2 with Golgi markers. Furthermore, hippocampal neurons from Ap4e1-KO mice and AP4M1-KO human iPSC-derived cortical i3Neurons exhibit reduced ApoER2 protein expression. Analyses of biosynthetic transport of ApoER2 reveal differential post-Golgi trafficking of the receptor, with lower axonal distribution in KO compared to wild-type neurons, indicating a role of AP-4 and the ISSF/Y motif in the axonal localization of ApoER2. Finally, analyses of Reelin signaling in mouse hippocampal and human cortical KO neurons show that AP4 deficiency causes no changes in Reelin-dependent activation of the AKT pathway and only mild changes in Reelin-induced dendritic arborization, but reduces Reelin-induced ERK phosphorylation, CREB activation, and Golgi deployment. This work thus establishes ApoER2 as a novel cargo of the AP-4 complex, suggesting that defects in the trafficking of this receptor and in the Reelin signaling pathway could contribute to the pathogenesis of HSP caused by mutations in AP-4 subunits.

Exploration of Positive and Negative Schizophrenia Symptom Heterogeneity and Establishment of Symptom-Related miRNA-mRNA Regulatory Network: Based on Transcriptome Sequencing Data

Schizophrenia (SCZ) symptoms can be classified as positive and negative ones, each of which has distinct traits and possibly differences in gene expression and regulation. The co-expression networks linked to PANSS (Positive and Negative Syndrome Scale) scores were identified by weighted gene co-expression network analysis (WGCNA) using the expression profiles of miRNA and mRNA in the peripheral blood of first-episode SCZ patients. The heterogeneity between positive and negative symptoms was demonstrated using gene functional enrichment, gene-medication interaction, and immune cell composition analysis. Then, target gene prediction and correlation analysis of miRNA and mRNA constructed a symptom-related miRNA-mRNA regulatory network, screened regulatory pairs, and predicted binding sites. A total of six mRNA co-expression modules, two miRNA co-expression modules, and ten hub genes were screened to be significantly associated with positive symptoms; five mRNA co-expression modules and eight hub genes were correlated with negative symptoms. Positive symptom-related modules were significantly enriched in axon guidance, actin skeleton regulation, and sphingolipid signaling pathway, while negative symptom-related modules were significantly enriched in adaptive immune response, leukocyte migration, dopaminergic synapses, etc. The development of positive symptoms may have been influenced by potential regulatory pairings such as miR-98-5p-EIF3J, miR-98-5p-SOCS4, let-7b-5p-CLUH, miR-454-3p-GTF2H1, and let-7b-5p-SNX17. Additionally, immune cells were substantially connected with several hub genes for symptoms. Positive and negative symptoms in SCZ individuals were heterogeneous to some extent. miRNAs such as let-7b-5p and miR-98-5p might contribute to the incidence of positive symptoms by targeting mRNAs, while the immune system's role in developing negative symptoms may be more nuanced.

The Reelin Receptor ApoER2 is a Cargo for the Adaptor Protein Complex AP-4: Implications for Hereditary Spastic Paraplegia

Adaptor protein complex 4 (AP-4) is a heterotetrameric complex that promotes protein export from the trans -Golgi network. Mutations in each of the AP-4 subunits cause a complicated form of Hereditary Spastic Paraplegia (HSP). Herein, we report that ApoER2, a receptor in the Reelin signaling pathway, is a cargo of the AP-4 complex. We identify the motif ISSF/Y within the ApoER2 cytosolic domain as necessary for interaction with the canonical signal-binding pocket of the µ4 (AP4M1) subunit of AP-4. AP4E1 -knock-out (KO) HeLa cells and hippocampal neurons from Ap4e1 -KO mice display increased Golgi localization of ApoER2. Furthermore, hippocampal neurons from Ap4e1 -KO mice and AP4M1 -KO human iPSC-derived cortical i3Neurons exhibit reduced ApoER2 protein expression. Analyses of biosynthetic transport of ApoER2 reveal differential post-Golgi trafficking of the receptor, with lower axonal distribution in KO compared to wild-type neurons, indicating a role of AP-4 and the ISSF/Y motif in the axonal localization of ApoER2. Finally, analyses of Reelin signaling in mouse hippocampal and human cortical KO neurons show that AP4 deficiency causes no changes in Reelin-dependent activation of the AKT pathway and only mild changes in Reelin-induced dendritic arborization, but reduces Reelin-induced ERK phosphorylation, CREB activation, and Golgi deployment. Altogether, this work establishes ApoER2 as a novel cargo of the AP-4 complex, suggesting that defects in the trafficking of this receptor and in the Reelin signaling pathway could contribute to the pathogenesis of HSP caused by mutations in AP-4 subunits.

Extracellular molecular signals shaping dendrite architecture during brain development

Proper growth and branching of dendrites are crucial for adequate central nervous system (CNS) functioning. The neuronal dendritic geometry determines the mode and quality of information processing. Any defects in dendrite development will disrupt neuronal circuit formation, affecting brain function. Besides cell-intrinsic programmes, extrinsic factors regulate various aspects of dendritic development. Among these extrinsic factors are extracellular molecular signals which can shape the dendrite architecture during early development. This review will focus on extrinsic factors regulating dendritic growth during early neuronal development, including neurotransmitters, neurotrophins, extracellular matrix proteins, contact-mediated ligands, and secreted and diffusible cues. How these extracellular molecular signals contribute to dendritic growth has been investigated in developing nervous systems using different species, different areas within the CNS, and different neuronal types. The response of the dendritic tree to these extracellular molecular signals can result in growth-promoting or growth-limiting effects, and it depends on the receptor subtype, receptor quantity, receptor efficiency, the animal model used, the developmental time windows, and finally, the targeted signal cascade. This article reviews our current understanding of the role of various extracellular signals in the establishment of the architecture of the dendrites.

Recruitment of the SNX17-Retriever recycling pathway regulates synaptic function and plasticity

Trafficking of cell-surface proteins from endosomes to the plasma membrane is a key mechanism to regulate synaptic function. In non-neuronal cells, proteins recycle to the plasma membrane either via the SNX27-Retromer-WASH pathway or via the recently discovered SNX17-Retriever-CCC-WASH pathway. While SNX27 is responsible for the recycling of key neuronal receptors, the roles of SNX17 in neurons are less understood. Here, using cultured hippocampal neurons, we demonstrate that the SNX17 pathway regulates synaptic function and plasticity. Disruption of this pathway results in a loss of excitatory synapses and prevents structural plasticity during chemical long-term potentiation (cLTP). cLTP drives SNX17 recruitment to synapses, where its roles are in part mediated by regulating the surface expression of β1-integrin. SNX17 recruitment relies on NMDAR activation, CaMKII signaling, and requires binding to the Retriever and PI(3)P. Together, these findings provide molecular insights into the regulation of SNX17 at synapses and define key roles for SNX17 in synaptic maintenance and in regulating enduring forms of synaptic plasticity.

Neuroprotective Effects of Carnosic Acid: Insight into Its Mechanisms of Action

Carnosic acid is a diterpenoid abundantly present in plants belonging to the genus Rosmarinus and Salvia of the family Lamiaceae, accounting for their application in traditional medicine. The diverse biological properties of carnosic acid that include antioxidant, anti-inflammatory, and anticarcinogenic activities have instigated studies on its mechanistic role, providing further insights into its potential as a therapeutic agent. Accumulating evidence has established the relevance of carnosic acid as a neuroprotective agent exhibiting therapeutic efficacy in combatting neuronal-injury-induced disorders. The physiological importance of carnosic acid in the mitigation of neurodegenerative disorders is just beginning to be understood. This review summarizes the current data on the mode of action through which carnosic acid exerts its neuroprotective role that may serve to strategize novel therapeutic approaches for these debilitating neurodegenerative disorders.

Participation of OCRL1, and APPL1, in the expression, proteolysis, phosphorylation and endosomal trafficking of megalin: Implications for Lowe Syndrome

Megalin/LRP2 is the primary multiligand receptor for the re-absorption of low molecular weight proteins in the proximal renal tubule. Its function is significantly dependent on its endosomal trafficking. Megalin recycling from endosomal compartments is altered in an X-linked disease called Lowe Syndrome (LS), caused by mutations in the gene encoding for the phosphatidylinositol 5-phosphatase OCRL1. LS patients show increased low-molecular-weight proteins with reduced levels of megalin ectodomain in the urine and accumulation of the receptor in endosomal compartments of the proximal tubule cells. To gain insight into the deregulation of megalin in the LS condition, we silenced OCRL1 in different cell lines to evaluate megalin expression finding that it is post-transcriptionally regulated. As an indication of megalin proteolysis, we detect the ectodomain of the receptor in the culture media. Remarkably, in OCRL1 silenced cells, megalin ectodomain secretion appeared significantly reduced, according to the observation in the urine of LS patients. Besides, the silencing of APPL1, a Rab5 effector associated with OCRL1 in endocytic vesicles, also reduced the presence of megalin’s ectodomain in the culture media. In both silencing conditions, megalin cell surface levels were significantly decreased. Considering that GSK3ß-mediated megalin phosphorylation reduces receptor recycling, we determined that the endosomal distribution of megalin depends on its phosphorylation status and OCRL1 function. As a physiologic regulator of GSK3ß, we focused on insulin signaling that reduces kinase activity. Accordingly, megalin phosphorylation was significantly reduced by insulin in wild-type cells. Moreover, even though in cells with low activity of OCRL1 the insulin response was reduced, the phosphorylation of megalin was significantly decreased and the receptor at the cell surface increased, suggesting a protective role of insulin in a LS cellular model.

Lipophorin receptors regulate mushroom body development and complex behaviors in Drosophila

BACKGROUND

Drosophila melanogaster lipophorin receptors (LpRs), LpR1 and LpR2, are members of the LDLR family known to mediate lipid uptake in a range of organisms from Drosophila to humans. The vertebrate orthologs of LpRs, ApoER2 and VLDL-R, function as receptors of a glycoprotein involved in development of the central nervous system, Reelin, which is not present in flies. ApoER2 and VLDL-R are associated with the development and function of the hippocampus and cerebral cortex, important association areas in the mammalian brain, as well as with neurodevelopmental and neurodegenerative disorders linked to those regions. It is currently unknown whether LpRs play similar roles in the Drosophila brain.

RESULTS

We report that LpR-deficient flies exhibit impaired olfactory memory and sleep patterns, which seem to reflect anatomical defects found in a critical brain association area, the mushroom bodies (MB). Moreover, cultured MB neurons respond to mammalian Reelin by increasing the complexity of their neurite arborization. This effect depends on LpRs and Dab, the Drosophila ortholog of the Reelin signaling adaptor protein Dab1. In vitro, two of the long isoforms of LpRs allow the internalization of Reelin, suggesting that Drosophila LpRs interact with human Reelin to induce downstream cellular events.

CONCLUSIONS

These findings demonstrate that LpRs contribute to MB development and function, supporting the existence of a LpR-dependent signaling in Drosophila, and advance our understanding of the molecular factors functioning in neural systems to generate complex behaviors in this model. Our results further emphasize the importance of Drosophila as a model to investigate the alterations in specific genes contributing to neural disorders.

Spatial regulation of endosomes in growing dendrites

Many membrane proteins are highly enriched in either dendrites or axons. This non-uniform distribution is a critical feature of neuronal polarity and underlies neuronal function. The molecular mechanisms responsible for polarized distribution of membrane proteins has been studied for some time and many answers have emerged. A less well studied feature of neurons is that organelles are also frequently non-uniformly distributed. For instance, EEA1-positive early endosomes are somatodendritic whereas synaptic vesicles are axonal. In addition, some organelles are present in both axons and dendrites, but not distributed uniformly along the processes. One well known example are lysosomes which are abundant in the soma and proximal dendrite, but sparse in the distal dendrite and the distal axon. The mechanisms that determine the spatial distribution of organelles along dendrites are only starting to be studied. In this review, we will discuss the cell biological mechanisms of how the distribution of diverse sets of endosomes along the proximal-distal axis of dendrites might be regulated. In particular, we will focus on the regulation of bulk homeostatic mechanisms as opposed to local regulation. We posit that immature dendrites regulate organelle motility differently from mature dendrites in order to spatially organize dendrite growth, branching and sculpting.

The Role of Sorting Nexin 17 in Cardiac Development

Sorting nexin 17 (SNX17), a member of sorting nexin (SNX) family, acts as a modulator for endocytic recycling of membrane proteins. Results from our previous study demonstrated the embryonic lethality of homozygous defect of SNX17. In this study, we investigated the role of SNX17 in rat fetal development. Specifically, we analyzed patterns of SNX17 messenger RNA (mRNA) expression in multiple rat tissues and found high expression in the cardiac outflow tract (OFT). This expression was gradually elevated during the cardiac OFT morphogenesis. Homozygous deletion of the SNX17 gene in rats resulted in mid-gestational embryonic lethality, which was accompanied by congenital heart defects, including the double-outlet right ventricle and atrioventricular and ventricular septal defects, whereas heterozygotes exhibited normal fetal development. Moreover, we found normal migration distance and the number of cardiac neural crest cells during the OFT morphogenesis. Although cellular proliferation in the cardiac OFT endocardial cushion was not affected, cellular apoptosis was significantly suppressed. Transcriptomic profiles and quantitative real-time PCR data in the cardiac OFT showed that SNX17 deletion resulted in abnormal expression of genes associated with cardiac development. Overall, these findings suggest that SNX17 plays a crucial role in cardiac development.

A common variant in CCDC93 protects against myocardial infarction and cardiovascular mortality by regulating endosomal trafficking of low-density lipoprotein receptor

AIMS

Genome-wide association studies have previously identified INSIG2 as a candidate gene for plasma low-density lipoprotein cholesterol (LDL-c). However, we suspect a role for CCDC93 in the same locus because of its involvement in the recycling of the LDL-receptor (LDLR).

METHODS AND RESULTS

Characterization of the INSIG2 locus was followed by studies in over 107 000 individuals from the general population, the Copenhagen General Population Study and the Copenhagen City Heart Study, for associations of genetic variants with plasma lipids levels, with risk of myocardial infarction (MI) and with cardiovascular mortality. CCDC93 was furthermore studied in cells and mice. The lead variant of the INSIG2 locus (rs10490626) is not associated with changes in the expression of nearby genes but is a part of a genetic block, which excludes INSIG2. This block includes a coding variant in CCDC93 p.Pro228Leu, which is in strong linkage disequilibrium with rs10490626 (r2 > 0.96). In the general population, separately and combined, CCDC93 p.Pro228Leu is dose-dependently associated with lower LDL-c (P-trend 2.5 × 10-6 to 8.0 × 10-9), with lower risk of MI (P-trend 0.04-0.002) and lower risk of cardiovascular mortality (P-trend 0.005-0.004). These results were validated for LDL-c, risk of both coronary artery disease and MI in meta-analyses including from 194 000 to >700 000 participants. The variant is shown to increase CCDC93 protein stability, while overexpression of human CCDC93 decreases plasma LDL-c in mice. Conversely, CCDC93 ablation reduces LDL uptake as a result of reduced LDLR levels at the cell membrane.

CONCLUSION

This study provides evidence that a common variant in CCDC93, encoding a protein involved in recycling of the LDLR, is associated with lower LDL-c levels, lower risk of MI and cardiovascular mortality.

Sorting Out Sorting Nexins Functions in the Nervous System in Health and Disease

Endocytosis is a fundamental process that controls protein/lipid composition of the plasma membrane, thereby shaping cellular metabolism, sensing, adhesion, signaling, and nutrient uptake. Endocytosis is essential for the cell to adapt to its surrounding environment, and a tight regulation of the endocytic mechanisms is required to maintain cell function and survival. This is particularly significant in the central nervous system (CNS), where composition of neuronal cell surface is crucial for synaptic functioning. In fact, distinct pathologies of the CNS are tightly linked to abnormal endolysosomal function, and several genome wide association analysis (GWAS) and biochemical studies have identified intracellular trafficking regulators as genetic risk factors for such pathologies. The sorting nexins (SNXs) are a family of proteins involved in protein trafficking regulation and signaling. SNXs dysregulation occurs in patients with Alzheimer’s disease (AD), Down’s syndrome (DS), schizophrenia, ataxia and epilepsy, among others, establishing clear roles for this protein family in pathology. Interestingly, restoration of SNXs levels has been shown to trigger synaptic plasticity recovery in a DS mouse model. This review encompasses an historical and evolutionary overview of SNXs protein family, focusing on its organization, phyla conservation, and evolution throughout the development of the nervous system during speciation. We will also survey SNXs molecular interactions and highlight how defects on SNXs underlie distinct pathologies of the CNS. Ultimately, we discuss possible strategies of intervention, surveying how our knowledge about the fundamental processes regulated by SNXs can be applied to the identification of novel therapeutic avenues for SNXs-related disorders.

SNX17 protects the heart from doxorubicin-induced cardiotoxicity by modulating LMOD2 degradation

Anthracyclines including doxorubicin (DOX) are still the most widely used and efficacious antitumor drugs, although their cardiotoxicity is a significant cause of heart failure. Despite considerable efforts being made to minimize anthracycline-induced cardiac adverse effects, little progress has been achieved. In this study, we aimed to explore the role and underlying mechanism of SNX17 in DOX-induced cardiotoxicity. We found that SNX17 was downregulated in cardiomyocytes treated with DOX both in vitro and in vivo. DOX treatment combined with SNX17 interference worsened the damage to neonatal rat ventricular myocytes (NRVMs). Furthermore, the rats with SNX17 deficiency manifested increased susceptibility to DOX-induced cardiotoxicity (myocardial damage and fibrosis, impaired contractility and cardiac death). Mechanistic investigation revealed that SNX17 interacted with leiomodin-2 (LMOD2), a key regulator of the thin filament length in muscles, via its C-TERM domain and SNX17 deficiency exacerbated DOX-induced cardiac systolic dysfunction by promoting aberrant LMOD2 degradation through lysosomal pathway. In conclusion, these findings highlight that SNX17 plays a protective role in DOX-induced cardiotoxicity, which provides an attractive target for the prevention and treatment of anthracycline induced cardiotoxicity.

Reelin activates the small GTPase TC10 and VAMP7 to promote neurite outgrowth and regeneration of dorsal root ganglia (DRG) neurons

Axonal outgrowth is a fundamental process during the development of central (CNS) and peripheral (PNS) nervous system as well as in nerve regeneration and requires accurate axonal navigation and extension to the correct target. These events need proper coordination between membrane trafficking and cytoskeletal rearrangements and are under the control of the small GTPases of the Rho family, among other molecules. Reelin, a relevant protein for CNS development and synaptic function in the adult, is also present in the PNS. Upon sciatic nerve damage, Reelin expression increases and, on the other hand, mice deficient in Reelin exhibit an impaired nerve regeneration. However, the mechanism(s) involved the Reelin-dependent axonal growth is still poorly understood. In this work, we present evidence showing that Reelin stimulates dorsal root ganglia (DRG) regeneration after axotomy. Moreover, dissociated DRG neurons express the Reelin receptor Apolipoprotein E-receptor 2 and also require the presence of TC10 to develop their axons. TC10 is a Rho GTPase that promotes neurite outgrowth through the exocytic fusion of vesicles at the growth cone. Here, we demonstrate for the first time that Reelin controls TC10 activation in DRG neurons. Besides, we confirmed that the known CNS Reelin target Cdc42 is also activated in DRG and controls TC10 activity. Finally, in the process of membrane addition, we found that Reelin stimulates the fusion of membrane carriers containing the v-SNARE protein VAMP7 in vesicles that contain TC10. Altogether, our work shows a new role of Reelin in PNS, opening the option of therapeutic interventions to improve the regeneration process.

The emerging role of sorting nexins in cardiovascular diseases

The sorting nexin (SNX) family consists of a diverse group of cytoplasmic- and membrane-associated phosphoinositide-binding proteins that play pivotal roles in the regulation of protein trafficking. This includes the entire endocytic pathway, such as endocytosis, endosomal sorting, and endosomal signaling. Dysfunctions of SNX pathway are involved in several forms of cardiovascular disease (CVD). Moreover, SNX gene variants are associated with CVDs. In this review, we discuss the current knowledge on SNX-mediated regulatory mechanisms and their roles in the pathogenesis and treatment of CVDs.

Differential Action of Reelin on Oligomerization of ApoER2 and VLDL Receptor in HEK293 Cells Assessed by Time-Resolved Anisotropy and Fluorescence Lifetime Imaging Microscopy

The canonical Reelin signaling cascade regulates correct neuronal layering during embryonic brain development. Details of this pathway are still not fully understood since the participating components are highly variable and create a complex mixture of interacting molecules. Reelin is proteolytically processed resulting in five different fragments some of which carrying the binding site for two different but highly homologous receptors, apolipoprotein E receptor 2 (ApoER2) and very low density lipoprotein receptor (VLDLR). The receptors are expressed in different variants in different areas of the developing brain. Binding of Reelin and its central fragment to the receptors results in phosphorylation of the intracellular adapter disabled-1 (Dab1) in neurons. Here, we studied the changes of the arrangement of the receptors upon Reelin binding and its central fragment at the molecular level in human embryonic kidney 293 (HEK293) cells by time-resolved anisotropy and fluorescence lifetime imaging microscopy (FLIM). In the off-state of the pathway ApoER2 and VLDLR form homo or hetero-di/oligomers. Upon binding of full length Reelin ApoER2 and VLDLR homo-oligomers are rearranged to higher order receptor clusters which leads to Dab1 phosphorylation. When the central fragment of Reelin binds to the receptors the cluster size of homo-oligomers is not affected and Dab1 is not phosphorylated. Hetero-oligomerization, however, can be induced, but does not lead to Dab1 phosphorylation. Cells expressing only ApoER2 or VLDLR change their shape when stimulated with the central fragment. Cells expressing ApoER2 produce filopodia/lamellipodia and cell size increases, whereas VLDLR-expressing cells decrease in size. These findings demonstrate that the primary event in the canonical Reelin pathway is the rearrangement of preformed receptor homo-oligomers to higher order clusters. In addition the possibility of yet another signaling mechanism which is mediated by the central Reelin fragment independent of Dab1 phosphorylation became apparent.

The Reelin Receptors Apolipoprotein E receptor 2 (ApoER2) and VLDL Receptor

Apolipoprotein E receptor 2 (ApoER2) and VLDL receptor belong to the low density lipoprotein receptor family and bind apolipoprotein E. These receptors interact with the clathrin machinery to mediate endocytosis of macromolecules but also interact with other adapter proteins to perform as signal transduction receptors. The best characterized signaling pathway in which ApoER2 and VLDL receptor (VLDLR) are involved is the Reelin pathway. This pathway plays a pivotal role in the development of laminated structures of the brain and in synaptic plasticity of the adult brain. Since Reelin and apolipoprotein E, are ligands of ApoER2 and VLDLR, these receptors are of interest with respect to Alzheimer’s disease. We will focus this review on the complex structure of ApoER2 and VLDLR and a recently characterized ligand, namely clusterin.

SNX17 produces anti-arrhythmic effects by preserving functional SERCA2a protein in myocardial infarction

BACKGROUND

Sorting nexin 17 (SNX17) is a critical cytoplasmic adaptor protein that regulates endosomal trafficking of membrane proteins to determine their recycling and/or degradation. The potential role of SNX17 in cardiovascular pathophysiology has not been reported.

METHODS AND RESULTS

Cardiac arrhythmias were monitored using standard limb lead II electrocardiograph, and cardiac performances were determined by echocardiography in a rat model of myocardial infarction (MI) created by left anterior descending coronary artery ligation. We found that SNX17 was substantially downregulated in ischemic myocardium. The downregulation contributed to the cardiac electrical disturbances and contractile dysfunction as SNX17 replacement mitigated the detrimental alterations of MI hearts. Specifically, silence of SNX17 expression using RNA interference caused intracellular Ca²⁺ overload as revealed by the abnormal rise of resting [Ca²⁺]_i and deceleration of Ca²⁺ decay, whereas SNX17 overexpression using vectors elicited the opposite effects. Moreover, the protein level of SERCA2a was significantly decreased by silencing SNX17. Immunohistochemistry indicated that SNX17 and SERCA2a were co-localized, and co-immunoprecipitation revealed the binding between the phox-homology domain of SNX17 and SERCA2a protein. Furthermore, lysosome inhibitor chloroquine prevented SNX17 silencing-induced reduction of SERCA2a protein level.

CONCLUSION

Abnormal downregulation of SNX17 contributes to ischemic damages of cardiac electrophysiology and contractile function. SNX17 is an endogenous anti-arrhythmic factor acting by preserving functional SERCA2a protein in MI thereby offering a new strategy for the management of MI to alleviate ischemic myocardial injuries.

Microarray expression profile of lncRNAs and mRNAs in the placenta of non-diabetic macrosomia

Macrosomia, not only is closely associated with short-term, birth-related problems, but also has long-term consequences for the offspring. We investigated the expression of long non-coding RNAs (lncRNAs) and messenger RNAs (mRNAs) in the placenta of macrosomia births using a microarray profile. The data showed that 2929 lncRNAs and 4574 mRNAs were upregulated in the placenta of macrosomia births compared with the normal birth weight group (fold change ⩾2.0, P<0.05), and 2127 lncRNAs and 2511 mRNAs were downregulated (fold change ⩾2.0, P<0.05). To detect the function of the differentially expressed lncRNAs and their possible relationship with the differentially expressed mRNAs, we also performed gene ontology analysis and pathway analysis. The results demonstrated that the PI3K-AKT signalling pathway, the mitogen-activated protein kinase (MAPK) signalling pathway, the focal adhesion pathway, the B cell receptor signalling pathway, and the protein processing in endoplasmic reticulum and lysosome pathway were significantly differentially expressed in the macrosomia placenta. Four lncRNAs were randomly chosen from the differentially expressed lncRNAs to validate the microarray data by quantitative polymerase chain reaction (qPCR). The qPCR results were consistent with the microarray data. In conclusion, lncRNAs were significantly differentially expressed in the placenta of macrosomia patients, and may contribute to the pathogenesis of macrosomia.

The functions of Reelin in membrane trafficking and cytoskeletal dynamics: implications for neuronal migration, polarization and differentiation

Reelin is a large extracellular matrix protein with relevant roles in mammalian central nervous system including neurogenesis, neuronal polarization and migration during development; and synaptic plasticity with its implications in learning and memory, in the adult. Dysfunctions in reelin signaling are associated with brain lamination defects such as lissencephaly, but also with neuropsychiatric diseases like autism, schizophrenia and depression as well with neurodegeneration. Reelin signaling involves a core pathway that activates upon reelin binding to its receptors, particularly ApoER2 (apolipoprotein E receptor 2)/LRP8 (low-density lipoprotein receptor-related protein 8) and very low-density lipoprotein receptor, followed by Src/Fyn-mediated phosphorylation of the adaptor protein Dab1 (Disabled-1). Phosphorylated Dab1 (pDab1) is a hub in the signaling cascade, from which several other downstream pathways diverge reflecting the different roles of reelin. Many of these pathways affect the dynamics of the actin and microtubular cytoskeleton, as well as membrane trafficking through the regulation of the activity of small GTPases, including the Rho and Rap families and molecules involved in cell polarity. The complexity of reelin functions is reflected by the fact that, even now, the precise mode of action of this signaling cascade in vivo at the cellular and molecular levels remains unclear. This review addresses and discusses in detail the participation of reelin in the processes underlying neurogenesis, neuronal migration in the cerebral cortex and the hippocampus; and the polarization, differentiation and maturation processes that neurons experiment in order to be functional in the adult brain. In vivo and in vitro evidence is presented in order to facilitate a better understanding of this fascinating system.

News on the molecular regulation and function of hepatic low-density lipoprotein receptor and LDLR-related protein 1

PURPOSE OF REVIEW

Clearing of atherogenic lipoprotein particles by the liver requires hepatic low-density lipoprotein receptor (LDLR) and LDLR-related protein 1 (LRP1). This review highlights recent studies that have expanded our understanding of the molecular regulation and metabolic functions of LDLR and LRP1 in the liver.

RECENT FINDINGS

Various proteins orchestrate the intracellular trafficking of LDLR and LRP1. After internalization, the receptors are redirected via recycling endosomes to the cell surface. Several new endocytic proteins that facilitate the endosomal trafficking of LDLR and consequently the clearance of circulating LDL cholesterol have recently been reported. Mutations in some of these proteins cause hypercholesterolemia in human. In addition, LRP1 controls cellular cholesterol efflux by modulating the expression of ABCA1 and ABCG1, and hepatic LRP1 protects against diet-induced hepatic insulin resistance and steatosis through the regulation of insulin receptor trafficking.

SUMMARY

LDLR and LRP1 have prominent roles in cellular and organismal cholesterol homeostasis. Their functioning, including their trafficking in the cell, is controlled by numerous proteins. Comprehensive studies into the molecular regulation of LDLR and LRP1 trafficking have advanced our fundamental understanding of cholesterol homeostasis, and these insights may lead to novel therapeutic strategies for atherosclerosis, hyperlipidemia and insulin resistance in the future.

Genome-wide association study provides strong evidence of genes affecting the reproductive performance of Nellore beef cows

Reproductive traits are economically important for beef cattle production; however, these traits are still a bottleneck in indicine cattle since these animals typically reach puberty at older ages when compared to taurine breeds. In addition, reproductive traits are complex phenotypes, i.e., they are controlled by both the environment and many small-effect genes involved in different pathways. In this study, we conducted genome-wide association study (GWAS) and functional analyses to identify important genes and pathways associated with heifer rebreeding (HR) and with the number of calvings at 53 months of age (NC53) in Nellore cows. A total of 142,878 and 244,311 phenotypes for HR and NC53, respectively, and 2,925 animals genotyped with the Illumina Bovine HD panel (Illumina^®, San Diego, CA, USA) were used in GWAS applying the weighted single-step GBLUP (WssGBLUP) method. Several genes associated with reproductive events were detected in the 20 most important 1Mb windows for both traits. Significant pathways for HR and NC53 were associated with lipid metabolism and immune processes, respectively. MHC class II genes, detected on chromosome 23 (window 25-26Mb) for NC53, were significantly associated with pregnancy success of Nellore cows. These genes have been proved previously to be associated with reproductive traits such as mate choice in other breeds and species. Our results suggest that genes associated with the reproductive traits HR and NC53 may be involved in embryo development in mammalian species. Furthermore, some genes associated with mate choice may affect pregnancy success in Nellore cattle.

Functional Roles of the Interaction of APP and Lipoprotein Receptors

The biological fates of the key initiator of Alzheimer’s disease (AD), the amyloid precursor protein (APP), and a family of lipoprotein receptors, the low-density lipoprotein (LDL) receptor-related proteins (LRPs) and their molecular roles in the neurodegenerative disease process are inseparably interwoven. Not only does APP bind tightly to the extracellular domains (ECDs) of several members of the LRP group, their intracellular portions are also connected through scaffolds like the one established by FE65 proteins and through interactions with adaptor proteins such as X11/Mint and Dab1. Moreover, the ECDs of APP and LRPs share common ligands, most notably Reelin, a regulator of neuronal migration during embryonic development and modulator of synaptic transmission in the adult brain, and Agrin, another signaling protein which is essential for the formation and maintenance of the neuromuscular junction (NMJ) and which likely also has critical, though at this time less well defined, roles for the regulation of central synapses. Furthermore, the major independent risk factors for AD, Apolipoprotein (Apo) E and ApoJ/Clusterin, are lipoprotein ligands for LRPs. Receptors and ligands mutually influence their intracellular trafficking and thereby the functions and abilities of neurons and the blood-brain-barrier to turn over and remove the pathological product of APP, the amyloid-β peptide. This article will review and summarize the molecular mechanisms that are shared by APP and LRPs and discuss their relative contributions to AD.

Reelin: Neurodevelopmental Architect and Homeostatic Regulator of Excitatory Synapses

Over half a century ago, D. S. Falconer first reported a mouse with a reeling gate. Four decades later, the Reln gene was isolated and identified as the cause of the reeler phenotype. Initial studies found that loss of Reelin, a large, secreted glycoprotein encoded by the Reln gene, results in abnormal neuronal layering throughout several regions of the brain. In the years since, the known functions of Reelin signaling in the brain have expanded to include multiple postdevelopmental neuromodulatory roles, revealing an ever increasing body of evidence to suggest that Reelin signaling is a critical player in the modulation of synaptic function. In writing this review, we intend to highlight the most fundamental aspects of Reelin signaling and integrate how these various neuromodulatory effects shape and protect synapses.

Canonical and Non-canonical Reelin Signaling

Reelin is a large secreted glycoprotein that is essential for correct neuronal positioning during neurodevelopment and is important for synaptic plasticity in the mature brain. Moreover, Reelin is expressed in many extraneuronal tissues; yet the roles of peripheral Reelin are largely unknown. In the brain, many of Reelin's functions are mediated by a molecular signaling cascade that involves two lipoprotein receptors, apolipoprotein E receptor-2 (Apoer2) and very low density-lipoprotein receptor (Vldlr), the neuronal phosphoprotein Disabled-1 (Dab1), and members of the Src family of protein tyrosine kinases as crucial elements. This core signaling pathway in turn modulates the activity of adaptor proteins and downstream protein kinase cascades, many of which target the neuronal cytoskeleton. However, additional Reelin-binding receptors have been postulated or described, either as coreceptors that are essential for the activation of the "canonical" Reelin signaling cascade involving Apoer2/Vldlr and Dab1, or as receptors that activate alternative or additional signaling pathways. Here we will give an overview of canonical and alternative Reelin signaling pathways, molecular mechanisms involved, and their potential physiological roles in the context of different biological settings.

ApoER2 and Reelin are expressed in regenerating peripheral nerve and regulate Schwann cell migration by activating the Rac1 GEF protein, Tiam1

ApoER2 and its ligand Reelin participate in neuronal migration during development. Upon receptor binding, Reelin induces the proteolytic processing of ApoER2 as well as the activation of signaling pathway, including small Rho GTPases. Besides its presence in the central nervous system (CNS), Reelin is also secreted by Schwann cells (SCs), the glial cells of the peripheral nervous system (PNS). Reelin deficient mice (reeler) show decreased axonal regeneration in the PNS; however neither the presence of ApoER2 nor the role of the Reelin signaling pathway in the PNS have been evaluated. Interestingly SC migration occurs during PNS development and during injury-induced regeneration and involves activation of small Rho GTPases. Thus, Reelin-ApoER2 might regulate SC migration during axon regeneration in the PNS. Here we demonstrate the presence of ApoER2 in PNS. After sciatic nerve injury Reelin was induced and its receptor ApoER2 was proteolytically processed. In vitro, SCs express both Reelin and ApoER2 and Reelin induces SC migration. To elucidate the molecular mechanism underlying Reelin-dependent SC migration, we examined the involvement of Rac1, a conspicuous small GTPase family member. FRET experiments revealed that Reelin activates Rac1 at the leading edge of SCs. In addition, Tiam1, a major Rac1-specific GEF was required for Reelin-induced SC migration. Moreover, Reelin-induced SC migration was decreased after suppression of the polarity protein PAR3, consistent with its association to Tiam1. Even more interesting, we demonstrated that PAR3 binds preferentially to the full-length cytoplasmic tail of ApoER2 corresponding to the splice-variant containing the exon 19 that encodes a proline-rich insert and that ApoER2 was required for SC migration. Our study reveals a novel function for Reelin/ApoER2 in PNS, inducing cell migration of SCs, a process relevant for PNS development and regeneration.

Biogenesis of endosome-derived transport carriers

Sorting of macromolecules within the endosomal system is vital for physiological control of nutrient homeostasis, cell motility, and proteostasis. Trafficking routes that export macromolecules from the endosome via vesicle and tubule transport carriers constitute plasma membrane recycling and retrograde endosome-to-Golgi pathways. Proteins of the sorting nexin family have been discovered to function at nearly every step of endosomal transport carrier biogenesis and it is becoming increasingly clear that they form the core machineries of cargo-specific transport pathways that are closely integrated with cellular physiology. Here, we summarize recent progress in elucidating the pathways that mediate the biogenesis of endosome-derived transport carriers.

Adapting for endocytosis: roles for endocytic sorting adaptors in directing neural development

Proper cortical development depends on the orchestrated actions of a multitude of guidance receptors and adhesion molecules and their downstream signaling. The levels of these receptors on the surface and their precise locations can greatly affect guidance outcomes. Trafficking of receptors to a particular surface locale and removal by endocytosis thus feed crucially into the final guidance outcomes. In addition, endocytosis of receptors can affect downstream signaling (both quantitatively and qualitatively) and regulated endocytosis of guidance receptors is thus an important component of ensuring proper neural development. We will discuss the cell biology of regulated endocytosis and the impact on neural development. We focus our discussion on endocytic accessory proteins (EAPs) (such as numb and disabled) and how they regulate endocytosis and subsequent post-endocytic trafficking of their cognate receptors (such as Notch, TrkB, β-APP, VLDLR, and ApoER2).

Neurotrophins regulate ApoER2 proteolysis through activation of the Trk signaling pathway

Background

ApoER2 and the neurotrophin receptors Trk and p75^NTR are expressed in the CNS and regulate key functional aspects of neurons, including development, survival, and neuronal function. It is known that both ApoER2 and p75^NTR are processed by metalloproteinases, followed by regulated intramembrane proteolysis. TrkA activation by nerve growth factor (NGF) increases the proteolytic processing of p75^NTR mediated by ADAM17. Reelin induces the sheeding of ApoER2 ectodomain depending on metalloproteinase activity. However, it is not known if there is a common regulation mechanism for processing these receptors.

Results

We found that TrkA activation by NGF in PC12 cells induced ApoER2 processing, which was dependent on TrkA activation and metalloproteinases. NGF-induced ApoER2 proteolysis was independent of mitogen activated protein kinase activity and of phosphatidylinositol-3 kinase activity. In contrast, the basal proteolysis of ApoER2 increased when both kinases were pharmacologically inhibited. The ApoER2 ligand reelin regulated the proteolytic processing of its own receptor but not of p75^NTR. Finally, in primary cortical neurons, which express both ApoER2 and TrkB, we found that the proteolysis of ApoER2 was also regulated by brain-derived growth factor (BDNF).

Conclusions

Our results highlight a novel relationship between neurotrophins and the reelin-ApoER2 system, suggesting that these two pathways might be linked to regulate brain development, neuronal survival, and some pathological conditions.

Structural determinants for binding of sorting nexin 17 (SNX17) to the cytoplasmic adaptor protein Krev interaction trapped 1 (KRIT1)

Sorting nexin 17 (SNX17) is a member of the family of cytoplasmic sorting nexin adaptor proteins that regulate endosomal trafficking of cell surface proteins. SNX17 localizes to early endosomes where it directly binds NPX(Y/F) motifs in the cytoplasmic tails of its target receptors to mediate their rates of endocytic internalization, recycling, and/or degradation. SNX17 has also been implicated in mediating cell signaling and can interact with cytoplasmic proteins. KRIT1 (Krev interaction trapped 1), a cytoplasmic adaptor protein associated with cerebral cavernous malformations, has previously been shown to interact with SNX17. Here, we demonstrate that SNX17 indeed binds directly to KRIT1 and map the binding to the second Asn-Pro-Xaa-Tyr/Phe (NPX(Y/F)) motif in KRIT1. We further characterize the interaction as being mediated by the FERM domain of SNX17. We present the co-crystal structure of SNX17-FERM with the KRIT1-NPXF2 peptide to 3.0 Å resolution and demonstrate that the interaction is highly similar in structure and binding affinity to that between SNX17 and P-selectin. We verify the molecular details of the interaction by site-directed mutagenesis and pulldown assay and thereby confirm that the major binding site for SNX17 is confined to the NPXF2 motif in KRIT1. Taken together, our results verify a direct interaction between SNX17 and KRIT1 and classify KRIT1 as a SNX17 binding partner.