Deficiency of sorting nexin 27 (SNX27) leads to growth retardation and elevated levels of N-methyl-D-aspartate receptor 2C (NR2C)

Sorting nexin 27-dependent regulation of Lck and CD4 tunes the initial stages of T-cell activation

Sorting nexin 27 is a unique member of the sorting nexin family of proteins that mediates the endosome-to-plasma membrane trafficking of cargos bearing a PSD95/Dlg1/ZO-1 (PDZ)-binding motif. In brain, sorting nexin 27 regulates synaptic plasticity, and its dysregulation contributes to cognitive impairment and neuronal degeneration. In T lymphocytes, sorting nexin 27 partners with diacylglycerol kinase ζ to facilitate polarized traffic and signaling at the immune synapse. By silencing sorting nexin 27 expression in a human T-cell line, we demonstrate that sorting nexin 27 is a key regulator of the early T-cell tyrosine-based signaling cascade. Sorting nexin 27 transcriptionally controls CD4 abundance in resting conditions and that of its associated molecule, Lck. This guarantees the adequate recruitment of Lck at the immune synapse, which is indispensable for subsequent activation of tyrosine phosphorylation-regulated events. In contrast, reduced sorting nexin 27 expression enhances NF-κB-dependent induction of CXCR4 and triggers production of lytic enzymes and proinflammatory cytokines. These results provide mechanistic explanation to previously described sorting nexin 27 function in the control of immune synapse organization and indicate that impaired sorting nexin 27 expression contributes to CD4 T-cell dysfunction.

Direct modulation of G protein-gated inwardly rectifying potassium (GIRK) channels

Ion channels play a pivotal role in regulating cellular excitability and signal transduction processes. Among the various ion channels, G-protein-coupled inwardly rectifying potassium (GIRK) channels serve as key mediators of neurotransmission and cellular responses to extracellular signals. GIRK channels are members of the larger family of inwardly-rectifying potassium (Kir) channels. Typically, GIRK channels are activated via the direct binding of G-protein βγ subunits upon the activation of G-protein-coupled receptors (GPCRs). GIRK channel activation requires the presence of the lipid signaling molecule, phosphatidylinositol 4,5-bisphosphate (PIP2). GIRK channels are also modulated by endogenous proteins and other molecules, including RGS proteins, cholesterol, and SNX27 as well as exogenous compounds, such as alcohol. In the last decade or so, several groups have developed novel drugs and small molecules, such as ML297, GAT1508 and GiGA1, that activate GIRK channels in a G-protein independent manner. Here, we aim to provide a comprehensive overview focusing on the direct modulation of GIRK channels by G-proteins, PIP2, cholesterol, and novel modulatory compounds. These studies offer valuable insights into the underlying molecular mechanisms of channel function, and have potential implications for both basic research and therapeutic development.

Metabolic reprogramming in astrocytes results in neuronal dysfunction in intellectual disability

Astrocyte aerobic glycolysis provides vital trophic support for central nervous system neurons. However, whether and how astrocytic metabolic dysregulation contributes to neuronal dysfunction in intellectual disability (ID) remain unclear. Here, we demonstrate a causal role for an ID-associated SNX27 mutation (R198W) in cognitive deficits involving reshaping astrocytic metabolism. We generated SNX27R196W (equivalent to human R198W) knock-in mice and found that they displayed deficits in synaptic function and learning behaviors. SNX27R196W resulted in attenuated astrocytic glucose uptake via GLUT1, leading to reduced lactate production and a switch from homeostatic to reactive astrocytes. Importantly, lactate supplementation or a ketogenic diet restored neuronal oxidative phosphorylation and reversed cognitive deficits in SNX27R196W mice. In summary, we illustrate a key role for astrocytic SNX27 in maintaining glucose supply and glycolysis and reveal that altered astrocytic metabolism disrupts the astrocyte-neuron interaction, which contributes to ID. Our work also suggests a feasible strategy for treating ID by restoring astrocytic metabolic function.

SNX27: A trans-species cognitive modulator with implications for anxiety and stress susceptibility

Sorting Nexin 27 (SNX27) is a brain-enriched endosome-associated cargo adaptor that shapes excitatory control, being relevant for cognitive and reward processing, and for several neurological conditions. Despite this, SNX27's role in the nervous system remains poorly explored. To further understand SNX27 function, we performed an extensive behavioral characterization comprising motor, cognitive and emotional dimensions of SNX27+/- mice. Furthermore, attending on the recently described association between SNX27 function and cellular stress signaling mechanisms in vitro, we explored SNX27-stress interplay using a Caenorhabditis elegans Δsnx-27 mutant and wild-type (WT) rodents after stress exposure. SNX27+/- mice, as C. elegans Δsnx-27 mutants, present cognitive impairments, highlighting a conserved role for SNX27 in cognitive modulation across species. Interestingly, SNX27 downmodulation leads to anxiety-like behavior in mice evaluated in the Elevated Plus Maze (EPM). This anxious phenotype is associated with increased dendritic complexity of the bed nucleus of the stria terminalis (BNST) neurons, and increased complexity of the basolateral amygdala (BLA) pyramidal neurons. These findings highlight the still unknown role of SNX27 in anxiety regulation. Moreover, we uncovered a direct link between SNX27 dysfunction and stress susceptibility in C. elegans and found that stress-exposed rodents display decreased SNX27 levels in stress-susceptible brain regions. Altogether, we provided new insights on SNX27's relevance in anxiety-related behaviors and neuronal structure in stress-associated brain regions.

Partial loss of Sorting Nexin 27 resembles age- and Down syndrome-associated T cell dysfunctions

BACKGROUND

Sorting Nexin 27 (SNX27)-retromer complex facilitates cargo recycling from endosomes to the plasma membrane. SNX27 downregulation in neurons, as the result of Trisomy 21 (T21), has been linked with cognitive deficits due to impairment of AMPA and NMDA receptor recycling. Studies in human T cell lines likewise demonstrated that SNX27 regulates the correct delivery of cargoes to the immune synapse limiting the activation of pro-inflammatory pathways. Nevertheless, the physiological consequences of partial SNX27 loss in T cell homeostasis are still unclear.

RESULTS

In this study, we have explored the consequences of T cell specific partial SNX27 downregulation in mice. T cells with partial SNX27 deficiency show a marked deficit in the CD4+ T cell pool, a hallmark of aging in mice and humans, and a well-characterized comorbidity of individuals with Down syndrome (DS). When analyzed ex vivo, CD4+ T cells with partial SNX27 deletion demonstrate enhanced proliferation but diminished IL-2 production. In contrast, the CD8+ population show enhanced expression of pro-inflammatory cytokines and lytic enzymes.

CONCLUSIONS

This mouse model supports the relevance of SNX27 in the organization of the immune synapse, previously described in cell lines, as well as in the control of T cell homeostasis. Individuals with DS experiment an acceleration of the aging process, which particularly affects the immune and central nervous systems. Thus, we hypothesize that reduced SNX27 expression in DS could contribute to the dysregulation of these systems and further research in SNX27 will shed light on the molecular factors underlying the phenotypes observed in people with DS and its contribution to aging.

Understanding the genetic mechanisms and cognitive impairments in Down syndrome: towards a holistic approach

The most common genetic cause of intellectual disability is Down syndrome (DS), trisomy 21. It commonly results from three copies of human chromosome 21 (HC21). There are no mutations or deletions involved in DS. Instead, the phenotype is caused by altered transcription of the genes on HC21. These transcriptional variations are responsible for a myriad of symptoms affecting every organ system. A very debilitating aspect of DS is intellectual disability (ID). Although tremendous advances have been made to try and understand the underlying mechanisms of ID, there is a lack of a unified, holistic view to defining the cause and managing the cognitive impairments. In this literature review, we discuss the mechanisms of neuronal over-inhibition, abnormal morphology, and other genetic factors in contributing to the development of ID in DS patients and to gain a holistic understanding of ID in DS patients. We also highlight potential therapeutic approaches to improve the quality of life of DS patients.

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.

In the fast lane: Receptor trafficking during status epilepticus

Status epilepticus (SE) remains a significant cause of morbidity and mortality and often is refractory to standard first-line treatments. A rapid loss of synaptic inhibition and development of pharmacoresistance to benzodiazepines (BZDs) occurs early during SE, while NMDA and AMPA receptor antagonists remain effective treatments after BZDs have failed. Multimodal and subunit-selective receptor trafficking within minutes to an hour of SE involves GABA-A, NMDA, and AMPA receptors and contributes to shifts in the number and subunit composition of surface receptors with differential impacts on the physiology, pharmacology, and strength of GABAergic and glutamatergic currents at synaptic and extrasynaptic sites. During the first hour of SE, synaptic GABA-A receptors containing γ2 subunits move to the cell interior while extrasynaptic GABA-A receptors with δ subunits are preserved. Conversely, NMDA receptors containing N2B subunits are increased at synaptic and extrasynaptic sites, and homomeric GluA1 ("GluA2-lacking") calcium permeant AMPA receptor surface expression also is increased. Molecular mechanisms, largely driven by NMDA receptor or calcium permeant AMPA receptor activation early during circuit hyperactivity, regulate subunit-specific interactions with proteins involved with synaptic scaffolding, adaptin-AP2/clathrin-dependent endocytosis, endoplasmic reticulum (ER) retention, and endosomal recycling. Reviewed here is how SE-induced shifts in receptor subunit composition and surface representation increase the excitatory to inhibitory imbalance that sustains seizures and fuels excitotoxicity contributing to chronic sequela such as "spontaneous recurrent seizures" (SRS). A role for early multimodal therapy is suggested both for treatment of SE and for prevention of long-term comorbidities.

Endosomal Sorting Protein SNX27 and Its Emerging Roles in Human Cancers

SNX27 belongs to the sorting nexin (SNX) family of proteins that play a critical role in protein sorting and trafficking in the endocytosis pathway. This protein family is characterized by the presence of a Phox (PX) domain; however, SNX27 is unique in containing an additional PDZ domain. Recently, SNX27 has gained popularity as an important sorting protein that is associated with the retromer complex and mediates the recycling of internalized proteins from endosomes to the plasma membrane in a PDZ domain-dependent manner. Over 100 cell surface proteins have been identified as binding partners of the SNX27-retromer complex. However, the roles and underlying mechanisms governed by SNX27 in tumorigenesis remains to be poorly understood. Many of its known binding partners include several G-protein coupled receptors, such as β2-andrenergic receptor and parathyroid hormone receptor, are associated with multiple pathways implicated in oncogenic signaling and tumorigenesis. Additionally, SNX27 mediates the recycling of GLUT1 and the activation of mTORC1, both of which can regulate intracellular energy balance and promote cell survival and proliferation under conditions of nutrient deprivation. In this review, we summarize the structure and fundamental roles of SNX proteins, with a focus on SNX27, and provide the current evidence indicating towards the role of SNX27 in human cancers. We also discuss the gap in the field and future direction of SNX27 research. Insights into the emerging roles and mechanism of SNX27 in cancers will provide better development strategies to prevent and treat tumorigenesis.

The emerging roles of retromer and sorting nexins in the life cycle of viruses

Retromer and sorting nexins (SNXs) transport cargoes from endosomes to the trans-Golgi network or plasma membrane. Recent studies have unveiled the emerging roles for retromer and SNXs in the life cycle of viruses, including members of Coronaviridae, Flaviviridae and Retroviridae. Key components of retromer/SNXs, such as Vps35, Vps26, SNX5 and SNX27, can affect multiple steps of the viral life cycle, including facilitating the entry of viruses into cells, participating in viral replication, and promoting the assembly of virions. Here we present a comprehensive updated review on the interplay between retromer/SNXs and virus, which will shed mechanistic insights into controlling virus infection.

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Retromer/SNXs could regulate viral infection directly or indirectly.

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Retromer/SNXs plays an important role for SARS-CoV-2 infection.

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HPV entry is mediated by retromer/SNXs.

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Retromer is required for HCV replication.

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Retromer affects the late step of HIV replication.

SNX27-Retromer directly binds ESCPE-1 to transfer cargo proteins during endosomal recycling

Coat complexes coordinate cargo recognition through cargo adaptors with biogenesis of transport carriers during integral membrane protein trafficking. Here, we combine biochemical, structural, and cellular analyses to establish the mechanistic basis through which SNX27-Retromer, a major endosomal cargo adaptor, couples to the membrane remodeling endosomal SNX-BAR sorting complex for promoting exit 1 (ESCPE-1). In showing that the SNX27 FERM (4.1/ezrin/radixin/moesin) domain directly binds acidic-Asp-Leu-Phe (aDLF) motifs in the SNX1/SNX2 subunits of ESCPE-1, we propose a handover model where SNX27-Retromer captured cargo proteins are transferred into ESCPE-1 transport carriers to promote endosome-to-plasma membrane recycling. By revealing that assembly of the SNX27:Retromer:ESCPE-1 coat evolved in a stepwise manner during early metazoan evolution, likely reflecting the increasing complexity of endosome-to-plasma membrane recycling from the ancestral opisthokont to modern animals, we provide further evidence of the functional diversification of yeast pentameric Retromer in the recycling of hundreds of integral membrane proteins in metazoans.

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

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

Regulation of NMDA receptor trafficking and gating by activity-dependent CaMKIIα phosphorylation of the GluN2A subunit

NMDA receptor (NMDAR)-dependent Ca2+ influx underpins multiple forms of synaptic plasticity. Most synaptic NMDAR currents in the adult forebrain are mediated by GluN2A-containing receptors, which are rapidly inserted into synapses during long-term potentiation (LTP); however, the underlying molecular mechanisms remain poorly understood. In this study, we show that GluN2A is phosphorylated at Ser-1459 by Ca2+/calmodulin-dependent kinase IIα (CaMKIIα) in response to glycine stimulation that mimics LTP in primary neurons. Phosphorylation of Ser-1459 promotes GluN2A interaction with the sorting nexin 27 (SNX27)-retromer complex, thereby enhancing the endosomal recycling of NMDARs. Loss of SNX27 or CaMKIIα function blocks the glycine-induced increase in GluN2A-NMDARs on the neuronal membrane. Interestingly, mutations of Ser-1459, including the rare S1459G human epilepsy variant, prolong the decay times of NMDAR-mediated synaptic currents in heterosynapses by increasing the duration of channel opening. These findings not only identify a critical role of Ser-1459 phosphorylation in regulating the function of NMDARs, but they also explain how the S1459G variant dysregulates NMDAR function.

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.

Toward Understanding the Molecular Role of SNX27/Retromer in Human Health and Disease

Aberrations in membrane trafficking pathways have profound effects in cellular dynamics of cellular sorting processes and can drive severe physiological outcomes. Sorting nexin 27 (SNX27) is a metazoan-specific sorting nexin protein from the PX-FERM domain family and is required for endosomal recycling of many important transmembrane receptors. Multiple studies have shown SNX27-mediated recycling requires association with retromer, one of the best-known regulators of endosomal trafficking. SNX27/retromer downregulation is strongly linked to Down's Syndrome (DS) via glutamate receptor dysfunction and to Alzheimer's Disease (AD) through increased intracellular production of amyloid peptides from amyloid precursor protein (APP) breakdown. SNX27 is further linked to addiction via its role in potassium channel trafficking, and its over-expression is linked to tumorigenesis, cancer progression, and metastasis. Thus, the correct sorting of multiple receptors by SNX27/retromer is vital for normal cellular function to prevent human diseases. The role of SNX27 in regulating cargo recycling from endosomes to the cell surface is firmly established, but how SNX27 assembles with retromer to generate tubulovesicular carriers remains elusive. Whether SNX27/retromer may be a putative therapeutic target to prevent neurodegenerative disease is now an emerging area of study. This review will provide an update on our molecular understanding of endosomal trafficking events mediated by the SNX27/retromer complex on endosomes.

Sorting nexin-27 regulates AMPA receptor trafficking through the synaptic adhesion protein LRFN2

The endosome-associated cargo adaptor sorting nexin-27 (SNX27) is linked to various neuropathologies through sorting of integral proteins to the synaptic surface, most notably AMPA receptors. To provide a broader view of SNX27-associated pathologies, we performed proteomics in rat primary neurons to identify SNX27-dependent cargoes, and identified proteins linked to excitotoxicity, epilepsy, intellectual disabilities, and working memory deficits. Focusing on the synaptic adhesion molecule LRFN2, we established that SNX27 binds to LRFN2 and regulates its endosomal sorting. Furthermore, LRFN2 associates with AMPA receptors and knockdown of LRFN2 results in decreased surface AMPA receptor expression, reduced synaptic activity, and attenuated hippocampal long-term potentiation. Overall, our study provides an additional mechanism by which SNX27 can control AMPA receptor-mediated synaptic transmission and plasticity indirectly through the sorting of LRFN2 and offers molecular insight into the perturbed function of SNX27 and LRFN2 in a range of neurological conditions.

Overexpression of Human SNX27 Enhances Learning and Memory Through Modulating Synaptic Plasticity in Mice

Abnormal synaptic transmission leads to learning and memory disorders and is the main feature of neurological diseases. Sorting nexin 27 (SNX27) is an endosomal adaptor protein associated with a variety of nervous system diseases, and it is mainly responsible for the trafficking of postsynaptic membrane receptors. However, the roles of SNX27 in regulating synaptic and cognitive function are not fully understood. Here, we first generated a neuron-specific human-SNX27 transgenic mouse model (hSNX27 Tg) that exhibited enhanced excitatory synaptic transmission and long-term potentiation (LTP). In addition, we found that the hSNX27 Tg mice displayed enhanced learning and memory, lower-level anxiety-like behavior, and increased social interaction. Furthermore, we found that SNX27 overexpression upregulated the expression of glutamate receptors in the cortex and hippocampus of hSNX27 Tg mice. Together, these results indicate that SNX27 overexpression promotes synaptic function and cognition through modulating glutamate receptors.

Characterization of the interaction between β-catenin and sorting nexin 27: contribution of the type I PDZ-binding motif to Wnt signaling

Background: Sorting Nexin 27 (SNX27) is a 62-kDa protein localized to early endosomes and known to regulate the intracellular trafficking of ion channels and receptors. In addition to a PX domain common among all of the sorting nexin family, SNX27 is the only sorting family member that contains a PDZ domain. To identify novel SNX27–PDZ binding partners, we performed a proteomic screen in mouse principal kidney cortical collecting duct cells (mpkCCD) using a GST-SNX27 fusion construct as bait. We found that the C-terminal type I PDZ binding motif (DTDL) of β-catenin, an adherens junction scaffolding protein and transcriptional co-activator, interacts directly with SNX27. Using biochemical and immunofluorescent techniques, β-catenin was identified in endosomal compartments where co-localization with SNX27 was observed. Furthermore, E-cadherin, but not Axin, GSK3 or Lef-1 was located in SNX27 protein complexes. While overexpression of wild-type β-catenin protein increased TCF-LEF dependent transcriptional activity, an enhanced transcriptional activity was not observed in cells expressing β-Catenin ΔFDTDL or diminished SNX27 expression.

These results imply importance of the C-terminal PDZ binding motif for the transcriptional activity of β-catenin and propose that SNX27 might be involved in the assembly of β-catenin complexes in the endosome.

An update on cellular and molecular determinants of Parkinson's disease with emphasis on the role of the retromer complex

Parkinson's disease (PD) is a highly prevalent neurodegenerative condition. The disease involves the progressive degeneration of dopaminergic neurons located in the substantia nigra pars compacta. Among late-onset, familial forms of Parkinson are cases with mutations in the PARK17 locus encoding the vacuolar protein sorting 35 (Vps35), a subunit of the retromer complex. The retromer complex is composed of a heterotrimeric protein core (Vps26-Vps35-Vps29). The best-known role of retromer is the retrieval of cargoes from endosomes to the Golgi complex or the plasma membrane. However, recent literature indicates that retromer performs roles associated with lysosomal and mitochondrial functions and degradative pathways such as autophagy. A common point mutation affecting the retromer subunit Vps35 is D620N, which has been linked to the alterations in the aforementioned cellular processes as well as with neurodegeneration. Here, we review the main aspects of the malfunction of the retromer complex and its implications for PD pathology. Besides, we highlight several controversies still awaiting clarification.

SNX27 regulates DRA activity and mediates its direct recycling by PDZ-interaction in early endosomes at the apical pole of Caco2 cells

DRA (downregulated in adenoma, SLC26A3) and NHE3 (Na⁺/H⁺ exchanger 3, SLC9A3) together mediate intestinal electroneutral NaCl absorption. Both transporters contain PDZ (postsynaptic density 95, disc large, zonula occludens 1) binding motifs and interact with PDZ adaptor proteins regulating their activity and recycling. SNX27 (sorting nexin 27) contains a PDZ domain and is involved in the recycling of cargo proteins including NHE3. The interaction of SNX27 with DRA and its potential role for the activity and recycling of DRA have been evaluated in this study. SNX27 specifically interacts with DRA via its PDZ domain. The knockdown (KD) of SNX27 reduced DRA activity by 50% but was not accompanied by a decrease of DRA surface expression. This indicates that DRA is trafficked to specific functional domains in the plasma membrane in which DRA is particularly active. Consistently, the disruption of lipid raft integrity by methyl-β-cyclodextrin has an inhibitory effect on DRA activity that was strongly reduced after SNX27 KD. In differentiated intestinal Caco2 cells, superresolution microscopy and a novel quantitative axial approach revealed that DRA and SNX27 colocalize in rab5-positive early endosomes at the apical pole. SNX27 regulates the activity of DRA in the apical plasma membrane through binding with its PDZ domain. This interaction occurs in rab5-positive early endosomes at the apical pole of differentiated intestinal Caco2 cells. SNX27 is involved in the direct recycling of DRA to the plasma membrane where it is inserted into lipid rafts facilitating increased activity.NEW & NOTEWORTHY SNX27 has a PDZ domain and is involved in the regulation and recycling of transmembrane proteins. The role of SNX27 on the activity and recycling of the intestinal Cl^-/HCO3- exchanger DRA has not yet been studied. This study shows that SNX27 directly interacts with DRA in early endosomes at the apical pole of intestinal Caco2 cells and mediates its direct recycling to facilitate high activity in lipid rafts in the apical plasma membrane.

Regulation of NMDA glutamate receptor functions by the GluN2 subunits

The N-methyl-D-aspartate receptors (NMDARs) are ionotropic glutamate receptors that mediate the flux of calcium (Ca²⁺ ) into the post-synaptic compartment. Ca²⁺ influx subsequently triggers the activation of various intracellular signalling cascades that underpin multiple forms of synaptic plasticity. Functional NMDARs are assembled as heterotetramers composed of two obligatory GluN1 subunits and two GluN2 or GluN3 subunits. Four different GluN2 subunits (GluN2A-D) are present throughout the central nervous system; however, they are differentially expressed, both developmentally and spatially, in a cell- and synapse-specific manner. Each GluN2 subunit confers NMDARs with distinct ion channel properties and intracellular trafficking pathways. Regulated membrane trafficking of NMDARs is a dynamic process that ultimately determines the number of NMDARs at synapses, and is controlled by subunit-specific interactions with various intracellular regulatory proteins. Here we review recent progress made towards understanding the molecular mechanisms that regulate the trafficking of GluN2-containing NMDARs, focusing on the roles of several key synaptic proteins that interact with NMDARs via their carboxyl termini.

Sorting nexin 27 (SNX27) variants associated with seizures, developmental delay, behavioral disturbance, and subcortical brain abnormalities

Sorting nexin 27 (SNX27) influences the composition of the cellular membrane via regulation of selective endosomal recycling. Molecular analysis indicates that SNX27 regulates numerous cellular processes through promiscuous interactions with its receptor cargos. SNX27 deficient (Snx27 ^-/- ) mice exhibit reduced embryonic survival, marked postnatal growth restriction and lethality. Haploinsufficient mice (Snx27 ^+/- ) show a less severe phenotype, with deficits in learning, memory, synaptic transmission and neuronal plasticity. One family previously reported with a homozygous SNX27 frameshift variant (c.515_516del;p.His172Argfs*6), exhibited infantile intractable myoclonic epilepsy, axial hypotonia, startle-like movements, cardiac septal defects, global developmental delay, failure to thrive, recurrent chest infections, persistent hypoxemia and early death secondary to respiratory failure. Here, we report two additional patients with compound heterozygous SNX27 variants, that are predicted to be damaging: (a) c.510C>G;p.Tyr170* and c.1295G>A;p.Cys432Tyr, and (b) c.782dupT;p.Leu262Profs*6 and c.989G>A;p.Arg330His. They exhibit global developmental delay, behavioral disturbance, epilepsy, some dysmorphic features and subcortical white matter abnormalities. In addition, possible connective tissue involvement was noted. Epilepsy, developmental delays and subcortical white matter abnormalities appear to be core features of SNX27-related disorders. We correlate the observed phenotype with available in vitro, in vivo and proteomic data and suggest additional possible molecular mediators of SNX27-related pathology.

Regulation of the endosomal SNX27-retromer by OTULIN

OTULIN (OTU Deubiquitinase With Linear Linkage Specificity) specifically hydrolyzes methionine1 (Met1)-linked ubiquitin chains conjugated by LUBAC (linear ubiquitin chain assembly complex). Here we report on the mass spectrometric identification of the OTULIN interactor SNX27 (sorting nexin 27), an adaptor of the endosomal retromer complex responsible for protein recycling to the cell surface. The C-terminal PDZ-binding motif (PDZbm) in OTULIN associates with the cargo-binding site in the PDZ domain of SNX27. By solving the structure of the OTU domain in complex with the PDZ domain, we demonstrate that a second interface contributes to the selective, high affinity interaction of OTULIN and SNX27. SNX27 does not affect OTULIN catalytic activity, OTULIN-LUBAC binding or Met1-linked ubiquitin chain homeostasis. However, via association, OTULIN antagonizes SNX27-dependent cargo loading, binding of SNX27 to the VPS26A-retromer subunit and endosome-to-plasma membrane trafficking. Thus, we define an additional, non-catalytic function of OTULIN in the regulation of SNX27-retromer assembly and recycling to the cell surface.

OTULIN is a linear ubiquitin hydrolase that regulates ubiquitin homeostasis. Here the authors identify the adaptor of the endosomal retromer complex sorting nexin 27 (SNX27) as a binding partner of OTULIN and determine the structure of the OTULIN-SNX27 complex, which reveals a secondary interface through which OTULIN non-catalytically antagonizes SNX27 retromer assembly and cargo loading.

Haploinsufficiency in the ANKS1B gene encoding AIDA-1 leads to a neurodevelopmental syndrome

Neurodevelopmental disorders, including autism spectrum disorder, have complex polygenic etiologies. Single-gene mutations in patients can help define genetic factors and molecular mechanisms underlying neurodevelopmental disorders. Here we describe individuals with monogenic heterozygous microdeletions in ANKS1B, a predicted risk gene for autism and neuropsychiatric diseases. Affected individuals present with a spectrum of neurodevelopmental phenotypes, including autism, attention-deficit hyperactivity disorder, and speech and motor deficits. Neurons generated from patient-derived induced pluripotent stem cells demonstrate loss of the ANKS1B-encoded protein AIDA-1, a brain-specific protein highly enriched at neuronal synapses. A transgenic mouse model of Anks1b haploinsufficiency recapitulates a range of patient phenotypes, including social deficits, hyperactivity, and sensorimotor dysfunction. Identification of the AIDA-1 interactome using quantitative proteomics reveals protein networks involved in synaptic function and the etiology of neurodevelopmental disorders. Our findings formalize a link between the synaptic protein AIDA-1 and a rare, previously undefined genetic disease we term ANKS1B haploinsufficiency syndrome.

Crystal Structure of the PX Domain of SNX27

SNX27 is a component of the retromer complex essential for the recycling of transmembrane receptors. SNX27 contains the N-terminal Phox (PX) domain that binds inositol 1,3-diphosphate (Ins(1,3)P₂) and is important for the SNX27 localization. Here, we determined the crystal structure of human SNX27 PX domain by X-ray crystallography. We found that the sulfate ion is located in the positively charged lipid-binding pocket of the PX domain, which mimics the phospholipid recognition. In addition, we modelled the SNX27-PX-Ins(1,3)P₂ complex to better understand the mechanism of Ins(1,3)P₂ recognition by the PX domain of SNX27.

Diacylglycerol kinase control of protein kinase C

The diacylglycerol kinases (DGK) are lipid kinases that transform diacylglycerol (DAG) into phosphatidic acid (PA) in a reaction that terminates DAG-based signals. DGK provide negative regulation to conventional and novel protein kinase C (PKC) enzymes, limiting local DAG availability in a tissue- and subcellular-restricted manner. Defects in the expression/activity of certain DGK isoforms contribute substantially to cognitive impairment and mental disorders. Abnormal DGK overexpression in tumors facilitates invasion and resistance to chemotherapy preventing tumor immune destruction by tumor-infiltrating lymphocytes. Effective translation of these findings into therapeutic approaches demands a better knowledge of the physical and functional interactions between the DGK and PKC families. DGKζ is abundantly expressed in the nervous and immune system, where physically and functionally interacts with PKCα. The latest discoveries suggest that PDZ-mediated interaction facilitates spatial restriction of PKCα by DGKζ at the cell–cell contact sites in a mechanism where the two enzymes regulate each other. In T lymphocytes, DGKζ interaction with Sorting Nexin 27 (SNX27) guarantees the basal control of PKCα activation. SNX27 is a trafficking component required for normal brain function whose deficit has been linked to Alzheimer's disease (AD) pathogenesis. The enhanced PKCα activation as the result of SNX27 silencing in T lymphocytes aligns with the recent correlation found between gain-of-function PKCα mutations and AD and suggests that disruption of the mechanisms that provides a correct spatial organization of DGKζ and PKCα may lie at the basis of immune and neuronal synapse impairment.

Sorting nexin 27 rescues neuroligin 2 from lysosomal degradation to control inhibitory synapse number

Retromer is an evolutionarily conserved endosomal trafficking complex that mediates the retrieval of cargo proteins from a degradative pathway for sorting back to the cell surface. To promote cargo recycling, the core retromer trimer of VPS (vacuolar protein sorting)26, VPS29 and VPS35 recognises cargo either directly, or through an adaptor protein, the most well characterised of which is the PDZ [postsynaptic density 95 (PSD95), disk large, zona occludens] domain-containing sorting nexin SNX27. Neuroligins (NLGs) are postsynaptic trans-synaptic scaffold proteins that function in the clustering of postsynaptic proteins to maintain synaptic stability. Here, we show that each of the NLGs (NLG1-3) bind to SNX27 in a direct PDZ ligand-dependent manner. Depletion of SNX27 from neurons leads to a decrease in levels of each NLG protein and, for NLG2, this occurs as a result of enhanced lysosomal degradation. Notably, while depletion of the core retromer component VPS35 leads to a decrease in NLG1 and NLG3 levels, NLG2 is unaffected, suggesting that, for this cargo, SNX27 acts independently of retromer. Consistent with loss of SNX27 leading to enhanced lysosomal degradation of NLG2, knockdown of SNX27 results in fewer NLG2 clusters in cultured neurons, and loss of SNX27 or VPS35 reduces the size and number of gephyrin clusters. Together, these data indicate that NLGs are SNX27-retromer cargoes and suggest that SNX27-retromer controls inhibitory synapse number, at least in part through trafficking of NLG2.

Snx27 Deletion Promotes Recovery From Spinal Cord Injury by Neuroprotection and Reduces Macrophage/Microglia Proliferation

Sorting nexin 27 (SNX27) is an endosome-associated cargo adaptor that is involved in various pathologies and development of neurological diseases. However, the role of SNX27 in spinal cord injury (SCI) remains unclear. In this study, we found that SNX27 was up-regulated in injured mice spinal cords by western blot and immunofluorescence. A comparative analysis of Basso mouse scale (BMS), footprint test and corticospinal tract (CST) tracing in Snx27 ^+/+ and Snx27 ^+/- mice revealed that haploinsufficiency of SNX27 ameliorated the clinical symptoms of SCI. Based on the results of western blot and immunofluorescence, mechanistically, we found that SNX27 deficiency suppresses apoptotic caspase-3 induced neuronal death. In addition, SNX27 haploinsufficiency lowers the infiltration and activation of macrophage/microglia by suppressing their proliferation at the SCI lesion site. Together, these results suggest that down-regulation of SNX27 is a potential therapy targeting both acute neuronal death and chronic neuroinflammation, and promoting nerve repair after SCI.

Unique Biology and Single-Channel Properties of GluN2A- and GluN2C-Containing Triheteromeric N-Methyl-D-Aspartate Receptors

Triheteromeric N-methyl-D-aspartate receptors (NMDARs) are assemblies of two different types of GluN2 subunits that endow receptors with properties distinct from their diheteromeric counterparts. Previous studies show an abundance of triheteromeric NMDARs across the central nervous system (CNS), making them an important receptor population to investigate and potential drug target. A recent study by Bhattacharya et al. (1) demonstrated the prevalence of GluN1/GluN2A/GluN2C triheteromeric NMDARs in cerebellar granule cells (CGCs), (2) suggested that GluN2C subunits seldom express as diheteromers, (3) suggested that GluN2A subunits are the preferred partners for GluN2C to functionally express at the cell surface, and (4) revealed unique single-channel properties of these triheteromeric assemblies, which may enable these cells to perform unique tasks. Taken together, this work demonstrates the physiological existence of GluN1/GluN2A/GluN2C receptors in the CGCs.

To degrade or not to degrade: mechanisms and significance of endocytic recycling

Newly endocytosed integral cell surface proteins are typically either directed for degradation or subjected to recycling back to the plasma membrane. The sorting of integral cell surface proteins, including signalling receptors, nutrient transporters, ion channels, adhesion molecules and polarity markers, within the endolysosomal network for recycling is increasingly recognized as an essential feature in regulating the complexities of physiology at the cell, tissue and organism levels. Historically, endocytic recycling has been regarded as a relatively passive process, where the majority of internalized integral proteins are recycled via a nonspecific sequence-independent 'bulk membrane flow' pathway. Recent work has increasingly challenged this view. The discovery of sequence-specific sorting motifs and the identification of cargo adaptors and associated coat complexes have begun to uncover the highly orchestrated nature of endosomal cargo recycling, thereby providing new insight into the function and (patho)physiology of this process.

GIRK currents in VTA dopamine neurons control the sensitivity of mice to cocaine-induced locomotor sensitization

GABA_BR-dependent activation of G protein-gated inwardly rectifying potassium channels (GIRK or K_IR3) provides a well-known source of inhibition in the brain, but the details on how this important inhibitory pathway affects neural circuits are lacking. We used sorting nexin 27 (SNX27), an endosomal adaptor protein that associates with GIRK2c and GIRK3 subunits, to probe the role of GIRK channels in reward circuits. A conditional knockout of SNX27 in both substantia nigra pars compacta and ventral tegmental area (VTA) dopamine neurons leads to markedly smaller GABA_BR- and dopamine D₂R-activated GIRK currents, as well as to suprasensitivity to cocaine-induced locomotor sensitization. Expression of the SNX27-insensitive GIRK2a subunit in SNX27-deficient VTA dopamine neurons restored GIRK currents and GABA_BR-dependent inhibition of spike firing, while also resetting the mouse's sensitivity to cocaine-dependent sensitization. These results establish a link between slow inhibition mediated by GIRK channels in VTA dopamine neurons and cocaine addiction, revealing a therapeutic target for treating addiction.

Triheteromeric GluN1/GluN2A/GluN2C NMDARs with Unique Single-Channel Properties Are the Dominant Receptor Population in Cerebellar Granule Cells

NMDA-type glutamate receptors (NMDARs) are ligand-gated ion channels that mediate excitatory neurotransmission in the CNS. Here we describe functional and single-channel properties of triheteromeric GluN1/GluN2A/GluN2C receptors, which contain two GluN1, one GluN2A, and one GluN2C subunits. This NMDAR has three conductance levels and opens in bursts similar to GluN1/GluN2A receptors but with a single-channel open time and open probability reminiscent of GluN1/GluN2C receptors. The deactivation time course of GluN1/GluN2A/GluN2C receptors is intermediate to GluN1/GluN2A and GluN1/GluN2C receptors and is not dominated by GluN2A or GluN2C. We show that triheteromeric GluN1/GluN2A/GluN2C receptors are the predominant NMDARs in cerebellar granule cells and propose that co-expression of GluN2A and GluN2C in cerebellar granule cells occludes cell surface expression of diheteromeric GluN1/GluN2C receptors. This new insight into neuronal GluN1/GluN2A/GluN2C receptors highlights the complexity of NMDAR signaling in the CNS.

SNX27 links DGKζ to the control of transcriptional and metabolic programs in T lymphocytes

Sorting nexin 27 (SNX27) recycles PSD-95, Dlg1, ZO-1 (PDZ) domain-interacting membrane proteins and is essential to sustain adequate brain functions. Here we define a fundamental SNX27 function in T lymphocytes controlling antigen-induced transcriptional activation and metabolic reprogramming. SNX27 limits the activation of diacylglycerol (DAG)-based signals through its high affinity PDZ-interacting cargo DAG kinase ζ (DGKζ). SNX27 silencing in human T cells enhanced T cell receptor (TCR)-stimulated activator protein 1 (AP-1)- and nuclear factor κB (NF-κB)-mediated transcription. Transcription did not increase upon DGKζ silencing, suggesting that DGKζ function is dependent on SNX27. The enhanced transcriptional activation in SNX27-silenced cells contrasted with defective activation of the mammalian target of rapamycin (mTOR) pathway. The analysis of Snx27^−/− mice supported a role for SNX27 in the control of T cell growth. This study broadens our understanding of SNX27 as an integrator of lipid-based signals with the control of transcription and metabolic pathways.

Dynamic control of RSK complexes by phosphoswitch-based regulation

Assembly and disassembly of protein-protein complexes needs to be dynamically controlled and phosphoswitches based on linear motifs are crucial in this process. Extracellular signal-regulated kinase 2 (ERK2) recognizes a linear-binding motif at the C-terminal tail (CTT) of ribosomal S6 kinase 1 (RSK1), leading to phosphorylation and subsequent activation of RSK1. The CTT also contains a classical PDZ domain-binding motif which binds RSK substrates (e.g. MAGI-1). We show that autophosphorylation of the disordered CTT promotes the formation of an intramolecular charge clamp, which efficiently masks critical residues and indirectly hinders ERK binding. Thus, RSK1 CTT operates as an autoregulated phosphoswitch: its phosphorylation at specific sites affects its protein-binding capacity and its conformational dynamics. These biochemical feedbacks, which form the structural basis for the rapid dissociation of ERK2-RSK1 and RSK1-PDZ substrate complexes under sustained epidermal growth factor (EGF) stimulation, were structurally characterized and validated in living cells. Overall, conformational changes induced by phosphorylation in disordered regions of protein kinases, coupled to allosteric events occurring in the kinase domain cores, may provide mechanisms that contribute to the emergence of complex signaling activities. In addition, we show that phosphoswitches based on linear motifs can be functionally classified as ON and OFF protein-protein interaction switches or dimmers, depending on the specific positioning of phosphorylation target sites in relation to functional linear-binding motifs. Moreover, interaction of phosphorylated residues with positively charged residues in disordered regions is likely to be a common mechanism of phosphoregulation.

DATABASE

Structural data are available in the PDB database under the accession numbers 5N7D, 5N7F and 5N7G. NMR spectral assignation data are available in the BMRB database under the accession numbers 27213 and 27214.

Regulation of Transporters and Channels by Membrane-Trafficking Complexes in Epithelial Cells

The vectorial secretion and absorption of fluid and solutes by epithelial cells is dependent on the polarized expression of membrane solute transporters and channels at the apical and basolateral membranes. The establishment and maintenance of this polarized expression of transporters and channels are affected by divers protein-trafficking complexes. Moreover, regulation of the magnitude of transport is often under control of physiological stimuli, again through the interaction of transporters and channels with protein-trafficking complexes. This review highlights the value in utilizing transporters and channels as cargo to characterize core trafficking machinery by which epithelial cells establish and maintain their polarized expression, and how this machinery regulates fluid and solute transport in response to physiological stimuli.

The emerging role of retromer in neuroprotection

Efficient sorting and transportation of integral membrane proteins, such as ion channels, nutrient transporters, signalling receptors, cell-cell and cell-matrix adhesion molecules is essential for the function of cellular organelles and hence organism development and physiology. Retromer is a master controller of integral membrane protein sorting and transport through one of the major sorting station within eukaryotic cells, the endosomal network. Subtle de-regulation of retromer is an emerging theme in the pathoetiology of Parkinson's disease. Here we summarise recent advances in defining the neuroprotective role of retromer and how its de-regulation may contribute to Parkinson's disease by interfering with: lysosomal health and protein degradation, association with accessory proteins including the WASH complex and mitochondrial health.

SNX27 Deletion Causes Hydrocephalus by Impairing Ependymal Cell Differentiation and Ciliogenesis

Hydrocephalus is a brain disorder derived from CSF accumulation due to defects in CSF clearance. Although dysfunctional apical cilia in the ependymal cell layer are causal to the onset of hydrocephalus, mechanisms underlying proper ependymal cell differentiation are largely unclear. SNX27 is a trafficking component required for normal brain function and was shown previously to suppress γ-secretase-dependent amyloid precursor protein and Notch cleavage. However, it was unclear how SNX27-dependent γ-secretase inhibition could contribute to brain development and pathophysiology. Here, we describe and characterize an Snx27-deleted mouse model for the ependymal layer defects of deciliation and hydrocephalus. SNX27 deficiency results in reductions in ependymal cells and cilia density, as well as severe postnatal hydrocephalus. Inhibition of Notch intracellular domain signaling with γ-secretase inhibitors reversed ependymal cells/cilia loss and dilation of lateral ventricles in Snx27-deficient mice, giving strong indication that Snx27 deletion triggers defects in ependymal layer formation and ciliogenesis through Notch hyperactivation. Together, these results suggest that SNX27 is essential for ependymal cell differentiation and ciliogenesis, and its deletion can promote hydrocephalus pathogenesis.

SIGNIFICANCE STATEMENT

Down's syndrome (DS) in humans and mouse models has been shown previously to confer a high risk for the development of pathological hydrocephalus. Because we have previously described SNX27 as a component that is consistently downregulated in DS, we present here a robust Snx27-deleted mouse model that produces hydrocephalus and associated ciliary defects with complete penetrance. In addition, we find that γ-secretase/Notch modulation may be a candidate drug target in SNX27-associated hydrocephalus such as that observed in DS. Based on these findings, we anticipate that future study will determine whether modulation of a SNX27/Notch/γ-secretase pathway can also be of therapeutic interest to congenital hydrocephalus.

Sorting nexin 27 interactome in T-lymphocytes identifies zona occludens-2 dynamic redistribution at the immune synapse

T Lymphocyte recognition of antigens leads to the formation of a highly organized structure termed immune synapse (IS) by analogy with the neuronals synapse. Sorting nexin 27 (SNX27) controls the endosomal traffic of PSD95, Dlg1, ZO-1 (PDZ) domain-interacting proteins, and its alteration is associated with impaired synaptic function and neurological diseases. In T-lymphocytes, SNX27-positive vesicles polarize to the IS, the identity of SNX27 interactors in these conditions nonetheless remains unknown. Here we used proteomics to analyze the SNX27 interactome purified from IS-forming T cells, and confirmed the conserved nature of the SNX27/WASH/retromer association in hematopoietic cells. Furthermore, our comparative interactome analysis of SNX27 wild-type and a mutant-deficient for PDZ cargo recognition identified the epithelial cell-cell junction protein zona occludens-2 (ZO-2) as an IS component. Biochemistry and microscopy approaches in T cells confirmed SNX27/ZO-2 PDZ-dependent interaction, and demonstrated its role controlling the dynamic localization of ZO-2 at the IS. This study broadens our knowledge of SNX27 function in T lymphocytes, and suggests that pathways that delimit polarized structures in nervous and epithelial systems also participate in IS regulation.

Atypical parkinsonism-associated retromer mutant alters endosomal sorting of specific cargo proteins

Mutations in the retromer complex, which is involved in sorting integral membrane proteins from endosomes to cellular compartments, are associated with atypical parkinsonism, but how these mutations affect retromer function remains unclear. Through a quantitative proteomic analysis of the retromer interactome, McMillan et al. reveal a new mechanism for perturbed endosomal sorting in parkinsonism.

The retromer complex acts as a scaffold for endosomal protein complexes that sort integral membrane proteins to various cellular destinations. The retromer complex is a heterotrimer of VPS29, VPS35, and VPS26. Two of these paralogues, VPS26A and VPS26B, are expressed in humans. Retromer dysfunction is associated with neurodegenerative disease, and recently, three VPS26A mutations (p.K93E, p.M112V, and p.K297X) were discovered to be associated with atypical parkinsonism. Here, we apply quantitative proteomics to provide a detailed description of the retromer interactome. By establishing a comparative proteomic methodology, we identify how this interactome is perturbed in atypical parkinsonism-associated VPS26A mutants. In particular, we describe a selective defect in the association of VPS26A (p.K297X) with the SNX27 cargo adaptor. By showing how a retromer mutant leads to altered endosomal sorting of specific PDZ ligand–containing cargo proteins, we reveal a new mechanism for perturbed endosomal cargo sorting in atypical parkinsonism.

GIRK2 splice variants and neuronal G protein-gated K+ channels: implications for channel function and behavior

Many neurotransmitters directly inhibit neurons by activating G protein-gated inwardly rectifying K+ (GIRK) channels, thereby moderating the influence of excitatory input on neuronal excitability. While most neuronal GIRK channels are formed by GIRK1 and GIRK2 subunits, distinct GIRK2 isoforms generated by alternative splicing have been identified. Here, we compared the trafficking and function of two isoforms (GIRK2a and GIRK2c) expressed individually in hippocampal pyramidal neurons lacking GIRK2. GIRK2a and GIRK2c supported comparable somato-dendritic GIRK currents in Girk2 -/- pyramidal neurons, although GIRK2c achieved a more uniform subcellular distribution in pyramidal neurons and supported inhibitory postsynaptic currents in distal dendrites better than GIRK2a. While over-expression of either isoform in dorsal CA1 pyramidal neurons restored contextual fear learning in a conditional Girk2 -/- mouse line, GIRK2a also enhanced cue fear learning. Collectively, these data indicate that GIRK2 isoform balance within a neuron can impact the processing of afferent inhibitory input and associated behavior.

GPCR Signaling and Trafficking: The Long and Short of It

Emerging findings disclose unexpected components of G protein-coupled receptor (GPCR) signaling and cell biology. Select GPCRs exhibit classical signaling, that is restricted to cell membranes, as well as newly described persistent signaling that depends on internalization of the GPCR bound to β-arrestins. Termination of non-canonical endosomal signaling requires intraluminal acidification and sophisticated protein trafficking machineries. Recent studies reveal the structural determinants of the trafficking chaperones. This review summarizes advances in GPCR signaling and trafficking with a focus on the parathyroid hormone receptor (PTHR) as a prototype, and on the actin-sorting nexin 27 (SNX27)-retromer tubule (ASRT) complex, an endosomal sorting hub responsible for recycling and preservation of cell surface receptors. The findings are integrated into a model of PTHR trafficking with implications for signal transduction, bone growth, and mineral ion metabolism.

G Protein-Gated Potassium Channels: A Link to Drug Addiction

G protein-gated inwardly rectifying potassium (GIRK) channels are regulators of neuronal excitability in the brain. Knockout mice lacking GIRK channels display altered behavioral responses to multiple addictive drugs, implicating GIRK channels in addictive behaviors. Here, we review the effects of GIRK subunit deletions on the behavioral response to psychostimulants, such as cocaine and methamphetamine. Additionally, exposure of mice to psychostimulants produces alterations in the surface expression of GIRK channels in multiple types of neurons within the reward system of the brain. Thus, we compare the subcellular mechanisms by which drug exposure appears to alter GIRK expression in multiple cell types and provide an outlook on future studies examining the role of GIRK channels in addiction. A greater understanding of how GIRK channels are regulated by addictive drugs may enable the development of therapies to prevent or treat drug abuse.

Ablation of SNX6 leads to defects in synaptic function of CA1 pyramidal neurons and spatial memory

SNX6 is a ubiquitously expressed PX-BAR protein that plays important roles in retromer-mediated retrograde vesicular transport from endosomes. Here we report that CNS-specific Snx6 knockout mice exhibit deficits in spatial learning and memory, accompanied with loss of spines from distal dendrites of hippocampal CA1 pyramidal cells. SNX6 interacts with Homer1b/c, a postsynaptic scaffold protein crucial for the synaptic distribution of other postsynaptic density (PSD) proteins and structural integrity of dendritic spines. We show that SNX6 functions independently of retromer to regulate distribution of Homer1b/c in the dendritic shaft. We also find that Homer1b/c translocates from shaft to spines by protein diffusion, which does not require SNX6. Ablation of SNX6 causes reduced distribution of Homer1b/c in distal dendrites, decrease in surface levels of AMPAR and impaired AMPAR-mediated synaptic transmission. These findings reveal a physiological role of SNX6 in CNS excitatory neurons.

DOI:http://dx.doi.org/10.7554/eLife.20991.001

Neurons are the building blocks of the nervous system. These cells generally consist of a round portion called the cell body and a long cable-like axon. The cell body bears numerous branches called dendrites, which are in turn covered in spines. Neurons communicate with one another at junctions – or synapses – that typically form between the end of the axon of one cell and a dendritic spine on another.

Specialized proteins stabilize the dendritic spines and enable the cells to exchange messages across the synapse. However, it is the cell body – rather than the dendrites – that produces most of these proteins. Structures called molecular motors transport proteins to their destinations within the cell along fixed tracks, similar to how a freight train carries cargo over the rail network. One of the key molecular motors within neurons is called dynein‒dynactin. This in turn interacts with other proteins called adaptors, enabling it to transport specific types of cargo.

Niu, Dai, Liu et al. have now examined the role of SNX6, an adaptor protein for the dynein‒dynactin motor. Mice that have been genetically modified to lack SNX6 in their brains have fewer spines on their dendrites compared with normal mice. This was particularly true for dendrites that contain AMPAR, a protein that receives signals sent across synapses. Niu, Dai, Liu et al. showed that SNX6 interacts with another protein called Homer1b/c and is responsible for distributing this protein in dendrites far from the cell body. The Homer1b/c protein helps to stabilize dendritic spines and to regulate the number of AMPAR proteins within them. Mice that lack SNX6 therefore have less Homer1b/c in the dendrites furthest from the cell body, and fewer spines on these dendrites too. These mice also have fewer AMPAR proteins at their synapses than control mice.

Mice that lack SNX6 show impaired learning and memory compared to control mice. This is consistent with the fact that changes in the strength of synapses that possess AMPAR proteins are thought to underlie learning and memory. Additional experiments are required to explore these relationships further, and to determine whether SNX6 helps to localize any other proteins that also contribute to changes in the strength of synapses.

DOI:http://dx.doi.org/10.7554/eLife.20991.002

Retromer- and WASH-dependent sorting of nutrient transporters requires a multivalent interaction network with ANKRD50

Retromer and the associated actin-polymerizing WASH complex are essential for the endocytic recycling of a wide range of integral membrane proteins. A hereditary Parkinson's-disease-causing point mutation (D620N) in the retromer subunit VPS35 perturbs retromer's association with the WASH complex and also with the uncharacterized protein ankyrin-repeat-domain-containing protein 50 (ANKRD50). Here, we firmly establish ANKRD50 as a new and essential component of the SNX27-retromer-WASH super complex. Depletion of ANKRD50 in HeLa or U2OS cells phenocopied the loss of endosome-to-cell-surface recycling of multiple transmembrane proteins seen upon suppression of SNX27, retromer or WASH-complex components. Mass-spectrometry-based quantification of the cell surface proteome of ANKRD50-depleted cells identified amino acid transporters of the SLC1A family, among them SLC1A4, as additional cargo molecules that depend on ANKRD50 and retromer for their endocytic recycling. Mechanistically, we show that ANKRD50 simultaneously engages multiple parts of the SNX27-retromer-WASH complex machinery in a direct and co-operative interaction network that is needed to efficiently recycle the nutrient transporters GLUT1 (also known as SLC2A1) and SLC1A4, and potentially many other surface proteins.

Interaction of the Human Papillomavirus E6 Oncoprotein with Sorting Nexin 27 Modulates Endocytic Cargo Transport Pathways

A subset of high-risk Human Papillomaviruses (HPVs) are the causative agents of a large number of human cancers, of which cervical is the most common. Two viral oncoproteins, E6 and E7, contribute directly towards the development and maintenance of malignancy. A characteristic feature of the E6 oncoproteins from cancer-causing HPV types is the presence of a PDZ binding motif (PBM) at its C-terminus, which confers interaction with cellular proteins harbouring PDZ domains. Here we show that this motif allows E6 interaction with Sorting Nexin 27 (SNX27), an essential component of endosomal recycling pathways. This interaction is highly conserved across E6 proteins from multiple high-risk HPV types and is mediated by a classical PBM-PDZ interaction but unlike many E6 targets, SNX27 is not targeted for degradation by E6. Rather, in HPV-18 positive cell lines the association of SNX27 with components of the retromer complex and the endocytic transport machinery is altered in an E6 PBM-dependent manner. Analysis of a SNX27 cargo, the glucose transporter GLUT1, reveals an E6-dependent maintenance of GLUT1 expression and alteration in its association with components of the endocytic transport machinery. Furthermore, knockdown of E6 in HPV-18 positive cervical cancer cells phenocopies the loss of SNX27, both in terms of GLUT1 expression levels and its vesicular localization, with a concomitant marked reduction in glucose uptake, whilst loss of SNX27 results in slower cell proliferation in low nutrient conditions. These results demonstrate that E6 interaction with SNX27 can alter the recycling of cargo molecules, one consequence of which is modulation of nutrient availability in HPV transformed tumour cells.

Phosphoinositide binding by the SNX27 FERM domain regulates its localization at the immune synapse of activated T-cells

Sorting nexin 27 (SNX27) controls the endosomal-to-cell-surface recycling of diverse transmembrane protein cargos. Crucial to this function is the recruitment of SNX27 to endosomes which is mediated by the binding of phosphatidylinositol-3-phosphate (PtdIns3P) by its phox homology (PX) domain. In T-cells, SNX27 localizes to the immunological synapse in an activation-dependent manner, but the molecular mechanisms underlying SNX27 translocation remain to be clarified. Here, we examined the phosphoinositide-lipid-binding capabilities of full-length SNX27, and discovered a new PtdInsP-binding site within the C-terminal 4.1, ezrin, radixin, moesin (FERM) domain. This binding site showed a clear preference for bi- and tri-phosphorylated phophoinositides, and the interaction was confirmed through biophysical, mutagenesis and modeling approaches. At the immunological synapse of activated T-cells, cell signaling regulates phosphoinositide dynamics, and we find that perturbing phosphoinositide binding by the SNX27 FERM domain alters the SNX27 distribution in both endosomal recycling compartments and PtdIns(3,4,5)P3-enriched domains of the plasma membrane during synapse formation. Our results suggest that SNX27 undergoes dynamic partitioning between different membrane domains during immunological synapse assembly, and underscore the contribution of unique lipid interactions for SNX27 orchestration of cargo trafficking.

A role for sorting nexin 27 in AMPA receptor trafficking

Sorting nexin 27 (SNX27), a PDZ domain-containing endosomal protein, was recently shown to modulate glutamate receptor recycling in Down's syndrome. However, the precise molecular role of SNX27 in GluA1 trafficking is unclear. Here we report that SNX27 is enriched in dendrites and spines, along with recycling endosomes. Significantly, the mobilization of SNX27 along with recycling endosomes into spines was observed. Mechanistically, SNX27 interacts with K-ras GTPase via the RA domain; and following chemical LTP stimuli, K-ras is recruited to SNX27-enriched endosomes through a Ca(2+)/CaM-dependent mechanism, which in turn drives the synaptic delivery of homomeric GluA1 receptors. Impairment of SNX27 prevents LTP and associated trafficking of AMPARs. These results demonstrate a role for SNX27 in neuronal plasticity, provide a molecular explanation for the K-ras signal during LTP and identify SNX27 as the PDZ-containing molecular linker that couples the plasticity stimuli to the delivery of postsynaptic cargo.