Recognition and remodeling of endosomal zones by sorting nexins

The proteolipid code determines how cytosolic proteins find and remodel membrane surfaces. Here, we investigate how this process works with sorting nexins Snx1 and Snx3. Both proteins form sorting machines by recognizing membrane zones enriched in phosphatidylinositol 3-phosphate (PI3P), phosphatidylserine (PS) and cholesterol. This co-localized combination forms a unique "lipid codon" or lipidon that we propose is responsible for endosomal targeting, as revealed by structures and interactions of their PX domain-based readers. We outline a membrane recognition and remodeling mechanism for Snx1 and Snx3 involving this code element alongside transmembrane pH gradients, dipole moment-guided docking and specific protein-protein interactions. This generates an initial membrane-protein assembly (memtein) that then recruits retromer and additional PX proteins to recruit cell surface receptors for sorting to the trans-Golgi network (TGN), lysosome and plasma membranes. Post-translational modification (PTM) networks appear to regulate how the sorting machines form and operate at each level. The commonalities and differences between these sorting nexins show how the proteolipid code orchestrates parallel flows of molecular information from ribosome emergence to organelle genesis, and illuminates a universally applicable model of the membrane.

The impact of VPS35 D620N mutation on alternative autophagy and its reversal by estrogen in Parkinson's disease

VPS35 plays a key role in neurodegenerative processes in Alzheimer's disease and Parkinson's disease (PD). Many genetic studies have shown a close relationship between autophagy and PD pathophysiology, and specifically, the PD-causing D620N mutation in VPS35 has been shown to impair autophagy. However, the molecular mechanisms underlying neuronal cell death and impaired autophagy in PD are debated. Notably, increasing evidence suggests that Rab9-dependent "alternative" autophagy, which is driven by a different molecular mechanism that driving ATG5-dependent "conventional" autophagy, also contributes to neurodegenerative process. In this study, we investigated the relationship between alternative autophagy and VPS35 D620N mutant-related PD pathogenesis. We isolated iPSCs from the blood mononuclear cell population of two PD patients carrying the VPS35 D620N mutant. In addition, we used CRISPR-Cas9 to generate SH-SY5Y cells carrying the D620N variant of VPS35. We first revealed that the number of autophagic vacuoles was significantly decreased in ATG5-knockout Mouse Embryonic Fibroblast or ATG5-knockdown patient-derived dopaminergic neurons carrying the VPS35 D620N mutant compared with that of the wild type VPS35 control cells. Furthermore, estrogen, which activates alternative autophagy pathways, increased the number of autophagic vacuoles in ATG5-knockdown VPS35 D620N mutant dopaminergic neurons. Estrogen induces Rab9 phosphorylation, mediated through Ulk1 phosphorylation, ultimately regulating alternative autophagy. Moreover, estrogen reduced the apoptosis rate of VPS35 D620N neurons, and this effect of estrogen was diminished under alternative autophagy knockdown conditions. In conclusion, alternative autophagy might be important for maintaining neuronal homeostasis and may be associated with the neuroprotective effect of estrogen in PD with VPS35 D620N.

VPS35 and α-Synuclein fail to interact to modulate neurodegeneration in rodent models of Parkinson's disease

BACKGROUND

Mutations in the vacuolar protein sorting 35 ortholog (VPS35) gene cause late-onset, autosomal dominant Parkinson's disease (PD), with a single missense mutation (Asp620Asn, D620N) known to segregate with disease in families with PD. The VPS35 gene encodes a core component of the retromer complex, involved in the endosomal sorting and recycling of transmembrane cargo proteins. VPS35-linked PD is clinically indistinguishable from sporadic PD, although it is not yet known whether VPS35-PD brains exhibit α-synuclein-positive brainstem Lewy pathology that is characteristic of sporadic cases. Prior studies have suggested a functional interaction between VPS35 and the PD-linked gene product α-synuclein in lower organisms, where VPS35 deletion enhances α-synuclein-induced toxicity. In mice, VPS35 overexpression is reported to rescue hippocampal neuronal loss in human α-synuclein transgenic mice, potentially suggesting a retromer deficiency in these mice.

METHODS

Here, we employ multiple well-established genetic rodent models to explore a functional or pathological interaction between VPS35 and α-synuclein in vivo.

RESULTS

We find that endogenous α-synuclein is dispensable for nigrostriatal pathway dopaminergic neurodegeneration induced by the viral-mediated delivery of human D620N VPS35 in mice, suggesting that α-synuclein does not operate downstream of VPS35. We next evaluated retromer levels in affected brain regions from human A53T-α-synuclein transgenic mice, but find normal levels of the core subunits VPS35, VPS26 or VPS29. We further find that heterozygous VPS35 deletion fails to alter the lethal neurodegenerative phenotype of these A53T-α-synuclein transgenic mice, suggesting the absence of retromer deficiency in this PD model. Finally, we explored the neuroprotective capacity of increasing VPS35 expression in a viral-based human wild-type α-synuclein rat model of PD. However, we find that the overexpression of wild-type VPS35 is not sufficient for protection against α-synuclein-induced nigral dopaminergic neurodegeneration, α-synuclein pathology and reactive gliosis.

CONCLUSION

Collectively, our data suggest a limited interaction of VPS35 and α-synuclein in neurodegenerative models of PD, and do not provide support for their interaction within a common pathophysiological pathway.

Structural Organization of the Retriever-CCC Endosomal Recycling Complex

The recycling of membrane proteins from endosomes to the cell surface is vital for cell signaling and survival. Retriever, a trimeric complex of VPS35L, VPS26C and VPS29, together with the CCC complex comprising CCDC22, CCDC93, and COMMD proteins, plays a crucial role in this process. The precise mechanisms underlying Retriever assembly and its interaction with CCC have remained elusive. Here, we present the first high-resolution structure of Retriever determined using cryogenic electron microscopy. The structure reveals a unique assembly mechanism, distinguishing it from its remotely related paralog, Retromer. By combining AlphaFold predictions and biochemical, cellular, and proteomic analyses, we further elucidate the structural organization of the entire Retriever-CCC complex and uncover how cancer-associated mutations disrupt complex formation and impair membrane protein homeostasis. These findings provide a fundamental framework for understanding the biological and pathological implications associated with Retriever-CCC-mediated endosomal recycling.

Structural Organization of the Retriever-CCC Endosomal Recycling Complex

The recycling of membrane proteins from endosomes to the cell surface is vital for cell signaling and survival. Retriever, a trimeric complex of VPS35L, VPS26C and VPS29, together with the CCC complex comprising CCDC22, CCDC93, and COMMD proteins, plays a crucial role in this process. The precise mechanisms underlying Retriever assembly and its interaction with CCC have remained elusive. Here, we present the first high-resolution structure of Retriever determined using cryogenic electron microscopy. The structure reveals a unique assembly mechanism, distinguishing it from its remotely related paralog, Retromer. By combining AlphaFold predictions and biochemical, cellular, and proteomic analyses, we further elucidate the structural organization of the entire Retriever-CCC complex and uncover how cancer-associated mutations disrupt complex formation and impair membrane protein homeostasis. These findings provide a fundamental framework for understanding the biological and pathological implications associated with Retriever-CCC-mediated endosomal recycling.

Structure of the endosomal Commander complex linked to Ritscher-Schinzel syndrome

The Commander complex is required for endosomal recycling of diverse transmembrane cargos and is mutated in Ritscher-Schinzel syndrome. It comprises two sub-assemblies: Retriever composed of VPS35L, VPS26C, and VPS29; and the CCC complex which contains twelve subunits: COMMD1-COMMD10 and the coiled-coil domain-containing (CCDC) proteins CCDC22 and CCDC93. Combining X-ray crystallography, electron cryomicroscopy, and in silico predictions, we have assembled a complete structural model of Commander. Retriever is distantly related to the endosomal Retromer complex but has unique features preventing the shared VPS29 subunit from interacting with Retromer-associated factors. The COMMD proteins form a distinctive hetero-decameric ring stabilized by extensive interactions with CCDC22 and CCDC93. These adopt a coiled-coil structure that connects the CCC and Retriever assemblies and recruits a 16th subunit, DENND10, to form the complete Commander complex. The structure allows mapping of disease-causing mutations and reveals the molecular features required for the function of this evolutionarily conserved trafficking machinery.

Beclin 1 regulates astrocyte phagocytosis and phagosomal recruitment of retromer

Phagocytosis plays an important role in maintaining brain homeostasis and when impaired can result in the accumulation of unwanted cellular material. While microglia are traditionally considered the phagocytes of the brain, astrocytes are also capable of phagocytosis and are the most numerous cells in the brain. In Alzheimer's disease (AD), astrocytes can be found surrounding β-amyloid (Aβ) plaques yet they seem unable to eliminate these deposits, suggesting phagocytosis may be impaired in AD. Mechanisms that might diminish astrocyte phagocytosis in AD are currently unclear. Here, we demonstrate that the autophagy protein beclin 1, which is reduced in AD, plays a role in regulating astrocyte phagocytosis. Specifically, we show that reducing beclin 1 in C6 astrocytes impairs the phagocytosis of latex beads, reduces retromer levels, and impairs retromer recruitment to the phagosomal membrane. Furthermore, we show that these beclin 1-mediated changes are accompanied by reduced expression of the phagocytic receptor Scavenger Receptor Class B type I (SR-BI). Collectively, these findings suggest a critical role for the protein beclin 1 in both receptor trafficking and receptor-mediated phagocytosis in astrocytes. Moreover, these findings provide insight into mechanisms by which astrocytes may become impaired in AD.

VPS35 promotes cell proliferation via EGFR recycling and enhances EGFR inhibitors response in gastric cancer

BACKGROUND

Vacuolar protein sorting-associated protein 35 (VPS35) is a core component of the retromer complex which mediates intracellular protein transport. It is well known that dysfunctional VPS35 functions in the accumulation of pathogenic proteins. In our previous study, VPS35 was found to be a potential gene related to poor prognosis in gastric cancer. However, the biological functions of VPS35 in gastric cancer remain unclear.

METHODS

Cell viability assays were performed to examine whether VPS35 affected cell proliferation. Immunoprecipitation and biotin assays showed that VPS35 bound to epidermal growth factor receptor (EGFR) in the cytoplasm and recycled it to the cell surface. Patient-derived xenografts and organoids were used to evaluate the effect of VPS35 on the response of gastric cancer to EGFR inhibitors.

FINDINGS

VPS35 expression levels were upregulated in tumour tissues and correlated with local tumour invasion and poor survival in patients with gastric cancer. VPS35 promoted cell proliferation and increased tumour growth. Mechanistically, VPS35 selectively bound to endocytosed EGFR in early endosomes and recycled it back to the cell surface, leading to the downstream activation of the ERK1/2 pathway. We also found that high VPS35 expression levels increased the sensitivity of the xenograft and organoid models to EGFR inhibitors.

INTERPRETATION

VPS35 promotes cell proliferation by recycling EGFR to the cell surface, amplifying the network of receptor trafficking. VPS35 expression levels are positively correlated with gastric cancer sensitivity to EGFR inhibitors, which offers a potential method to stratify patients for EGFR inhibitor utilisation.

FUNDING

National Natural Science Foundation of China.

GTPase-activating protein TBC1D5 coordinates with retromer to constrain synaptic growth by inhibiting BMP signaling

Formation and plasticity of neural circuits rely on precise regulation of synaptic growth. At Drosophila neuromuscular junction (NMJ), Bone Morphogenetic Protein (BMP) signaling is critical for many aspects of synapse formation and function. The evolutionarily conserved retromer complex and its associated GTPase-activating protein TBC1D5 are critical regulators of membrane trafficking and cellular signaling. However, their functions in regulating the formation of NMJ are less understood. Here, we report that TBC1D5 is required for inhibition of synaptic growth, and loss of TBC1D5 leads to abnormal presynaptic terminal development, including excessive satellite boutons and branch formation. Ultrastructure analysis reveals that the size of synaptic vesicles and the density of subsynaptic reticulum are increased in TBC1D5 mutant boutons. Disruption of interactions of TBC1D5 with Rab7 and retromer phenocopies the loss of TBC1D5. Unexpectedly, we find that TBC1D5 is functionally linked to Rab6, in addition to Rab7, to regulate synaptic growth. Mechanistically, we show that loss of TBC1D5 leads to upregulated BMP signaling by increasing the protein level of BMP type II receptor Wishful Thinking (Wit) at NMJ. Overall, our data establish that TBC1D5 in coordination with retromer constrains synaptic growth by regulating Rab7 activity, which negatively regulates BMP signaling through inhibiting Wit level.

elegans development and mammalian ciliogenesis

The mammalian retromer consists of subunits VPS26 (either VPS26A or VPS26B), VPS29 and VPS35, and a loosely associated sorting nexin (SNX) heterodimer or a variety of other SNX proteins. Despite involvement in yeast and mammalian cell trafficking, the role of retromer in development is poorly understood, and its impact on primary ciliogenesis remains unknown. Using CRISPR/Cas9 editing, we demonstrate that vps-26-knockout worms have reduced brood sizes, impaired vulval development and decreased body length, all of which have been linked to ciliogenesis defects. Although preliminary studies did not identify worm ciliary defects, and impaired development limited additional ciliogenesis studies, we turned to mammalian cells to investigate the role of retromer in ciliogenesis. VPS35 localized to the primary cilium of mammalian cells, and depletion of VPS26, VPS35, VPS29, SNX1, SNX2, SNX5 or SNX27 led to decreased ciliogenesis. Retromer also coimmunoprecipitated with the centriolar protein, CP110 (also known as CCP110), and was required for its removal from the mother centriole. Herein, we characterize new roles for retromer in C. elegans development and in the regulation of ciliogenesis in mammalian cells, suggesting a novel role for retromer in CP110 removal from the mother centriole.

High glucose-mediated VPS26a downregulation dysregulates neuronal amyloid precursor protein processing and tau phosphorylation

BACKGROUND AND PURPOSE

The relationship between hyperglycaemia-induced retromer dysfunction impairing intracellular trafficking and AD remains unclear, although Diabetes mellitus (DM) is considered a risk factor for Alzheimer's disease (AD). Here, we investigated the effects of high glucose on the retromer, and defined the dysregulation of mechanisms of amyloid precursor protein (APP) processing and tau phosphorylation.

EXPERIMENTAL APPROACH

We used human induced-pluripotent stem cell-derived neuronal differentiated cells and SH-SY5Ys exposed to high glucose to identify the underlying mechanisms. Streptozotocin-induced diabetic mice were used to elucidate whether the retromer contributes to the AD-like pathology.

KEY RESULTS

We found that vacuolar protein sorting-associated protein 26a (VPS26a) was decreased in the hippocampus of diabetic mice and high glucose-treated human neuronal cells. High glucose downregulated VPS26a through ROS/NF-κB/DNA methyltransferase1-mediated promoter hypermethylation. VPS26a recovery blocked retention of APP and cation-independent mannose-6-phosphate receptor in endosomes and promoted transport to the trans-Golgi, which decreased Aβ levels, and improved Cathepsin D activity, reducing p-tau levels, respectively. Retromer enhancement ameliorated synaptic deficits, astrocyte over-activation, and cognitive impairment in diabetic mice.

CONCLUSION AND IMPLICATIONS

In conclusion, VPS26a is a promising candidate for the inhibition of DM-associated AD pathogenesis by modulating APP processing and tau phosphorylation.

α-synuclein and phosphoinositide-binding proteins: α-synuclein inhibits the association of PX- but not FYVE-containing proteins with vesicles in vivo

By an unknown mechanism, alpha-synuclein (α-syn) inhibits autophagy in yeast and human cells. Herein, using the yeast Saccharomyces cerevisiae, we tested the hypothesis that α-syn disrupts autophagy by inhibiting the required association of sorting nexin 4 (Snx4) with phagophores. Snx4 contains a phox (PX) homology domain that selectively binds membranes enriched in phosphatidylinositol 3-phosphate (PI3P). Using fluorescence microscopy, we show that upon nitrogen starvation, 70% of the cells exhibited green puncta (phagophores); whereas identically treated cells expressing α-syn exhibited a significantly lower percentage of cells (30%) with such puncta. Our interpretation is that α-syn outcompetes Snx4 for binding to membranes enriched in PI3P, resulting in fewer phagophores and consequently inefficient induction of autophagy. As a control, we tested whether α-syn disrupts the binding of Vps27-GFP to late endosomes/multivesicular bodies (MVBs). Vps27 contains a PI3P-binding domain called FYVE. α-Syn did not disrupt the binding of Vps27-GFP to late endosomes. α-Syn likely inhibits the binding of PX- but not FYVE-containing proteins to PI3P because FYVE domains bind more than two-orders of magnitude tighter than PX domains. We propose that in all cells, whether yeast or human, α-syn has the potential to inhibit protein trafficking pathways that are dependent on PX-domain proteins such as sorting nexins.

Microglial VPS35 deficiency impairs Aβ phagocytosis and Aβ-induced disease-associated microglia, and enhances Aβ associated pathology

Background

Vacuolar sorting protein 35 (VPS35), a key component of the retromer, plays an essential role in selectively retrieval of transmembrane proteins from endosomes to trans-Golgi networks. Dysfunctional retromer is a risk factor for neurodegenerative disorders, including Alzheimer’s disease (AD). Microglial VPS35 deficiency is found in AD patients’ brain; however, it remains unclear if and how microglial VPS35-loss contributes to AD development.

Methods

We used mice with VPS35 cKO (conditional knockout) in microglial cells in 5XFAD, an AD mouse model. The AD related brain pathology (Aβ and glial activation), behavior, and phagocytosis of Aβ were accessed by a combination of immunofluorescence staining analyses and neurological behavior tests.

Results

A decrease in learning and memory function, but increases in insoluble, fibrillar, and plaques of β-amyloids (Aβ), dystrophic neurites, and reactive astrocytes are observed in microglial VPS35 deficient 5XFAD mice. Further examining microglial phenotype demonstrates necessity of microglial VPS35 in disease-associated microglia (DAM) development and microglial uptake of Aβ, revealing a tight association of microglial Aβ uptake with DAM development.

Conclusions

Together, these results uncovered a mechanism by which microglial VPS35-deficiency precipitates AD pathology in 5XFAD mice likely by impairing DAM development and DAM mediated Aβ uptake and clearance, and thus accelerating the cognition decline.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02422-0.

Monitoring Endosomal Cargo Retrieval to the Trans-Golgi Network by Microscopic and Biochemical Approaches

The endosomal recycling pathway plays a crucial role in diverse physiologically important biological processes such as cell-to-cell signaling, nutrient uptake, immune response, and autophagy. A selective subset of these recycling cargoes, mostly transmembrane proteins, is retrieved from endosomes to the trans-Golgi network (TGN) by a retrograde transport process. Endosome-to-TGN retrograde trafficking is crucial for maintaining cellular homeostasis and signaling by preventing proteins and lipids from degradation in the lysosome. Many of the membrane sorting machinery, such as the retromer complex and sorting nexins (SNXs) are involved in endosomal retrieval and recycling of various transmembrane proteins. Recent technological advances in the resolution of light microscopy and unbiased analytical approaches in quantitative image analysis enable us to explore and understand the regulation of membrane trafficking pathways in greater detail. In this chapter, we describe quantitative imaging-based methods for analyzing the roles of proteins involved in the retrograde trafficking in retromer dependent or independent fashion, using cation-independent mannose-6-phosphate receptor (CIM6PR) as an example.

Biochemical basis for an interaction between SNX27 and the flexible SNX1 N-terminus

Metazoans require the sorting nexin (SNX) protein, SNX27, to recycle hundreds of important transmembrane protein receptors from endosomes to the plasma membrane. Cargo recycling by SNX27 requires its interaction with retromer, a heterotrimer known to assemble on membranes with multiple sorting nexins, including SNX-BAR proteins and SNX3. SNX27 has also been functionally linked to SNX-BARs, but the molecular basis of this interaction has been unknown. We identify a direct biochemical interaction between the conserved and flexible SNX1/SNX2 N-terminus and full-length SNX27 using purified proteins in pulldown experiments. Sequence alignments indicate both SNX1 and SNX2 contain two short and conserved stretches of acidic residues bearing a DxF motif in their flexible N-terminal regions. Biochemical pulldown and mapping experiments reveal forty residues in the N-terminus of either SNX1 or SNX2 can mediate binding to SNX27. SNX27 truncation analysis demonstrates the SNX27 FERM domain binds the SNX1 N-terminus. Calorimetry experiments quantified binding between the SNX1 N-terminus and SNX27 in the low micromolar affinity range (K_D ∼10 μM) and suggest the second DxF motif may play a more prominent role in binding. Mutation of either DxF sequence in SNX1 abrogates measurable binding to SNX27 in the calorimeter. Modelling from both predicted and experimentally determined structures suggests the SNX27 FERM domain could accommodate both DxF motifs simultaneously. Together, these data suggest SNX27 is directly linked to specific SNX-BAR proteins through binding acidic motifs in the SNX1 or SNX2 N-terminus.

Endosomal traffic and glutamate synapse activity are increased in VPS35 D620N mutant knock-in mouse neurons, and resistant to LRRK2 kinase inhibition

Vacuolar protein sorting 35 (VPS35) regulates neurotransmitter receptor recycling from endosomes. A missense mutation (D620N) in VPS35 leads to autosomal-dominant, late-onset Parkinson's disease. Here, we study the basic neurobiology of VPS35 and Parkinson's disease mutation effects in the D620N knock-in mouse and the effect of leucine-rich repeat kinase 2 (LRRK2) inhibition on synaptic phenotypes. The study was conducted using a VPS35 D620N knock-in mouse that expresses VPS35 at endogenous levels. Protein levels, phosphorylation states, and binding ratios in brain lysates from knock-in mice and wild-type littermates were assayed by co-immunoprecipitation and western blot. Dendritic protein co-localization, AMPA receptor surface expression, synapse density, and glutamatergic synapse activity in primary cortical cultures from knock-in and wild-type littermates were assayed using immunocytochemistry and whole-cell patch clamp electrophysiology. In brain tissue, we confirm VPS35 forms complexes with LRRK2 and AMPA-type glutamate receptor GluA1 subunits, in addition to NMDA-type glutamate receptor GluN1 subunits and D2-type dopamine receptors. Receptor and LRRK2 binding was unaltered in D620N knock-in mice, but we confirm the mutation results in reduced binding of VPS35 with WASH complex member FAM21, and increases phosphorylation of the LRRK2 kinase substrate Rab10, which is reversed by LRRK2 kinase inhibition in vivo. In cultured cortical neurons from knock-in mice, pRab10 is also increased, and reversed by LRRK2 inhibition. The mutation also results in increased endosomal recycling protein cluster density (VPS35-FAM21 co-clusters and Rab11 clusters), glutamate transmission, and GluA1 surface expression. LRRK2 kinase inhibition, which reversed Rab10 hyper-phosphorylation, did not rescue elevated glutamate release or surface GluA1 expression in knock-in neurons, but did alter AMPAR traffic in wild-type cells. The results improve our understanding of the cell biology of VPS35, and the consequences of the D620N mutation in developing neuronal networks. Together the data support a chronic synaptopathy model for latent neurodegeneration, providing phenotypes and candidate pathophysiological stresses that may drive eventual transition to late-stage parkinsonism in VPS35 PD. The study demonstrates the VPS35 mutation has effects that are independent of ongoing LRRK2 kinase activity, and that LRRK2 kinase inhibition alters basal physiology of glutamate synapses in vitro.

Mistargeting of secretory cargo in retromer-deficient cells

Intracellular trafficking is a basic and essential cellular function required for delivery of proteins to the appropriate subcellular destination; this process is especially demanding in professional secretory cells, which synthesize and secrete massive quantities of cargo proteins via regulated exocytosis. The Drosophila larval salivary glands are composed of professional secretory cells that synthesize and secrete mucin proteins at the onset of metamorphosis. Using the larval salivary glands as a model system, we have identified a role for the highly conserved retromer complex in trafficking of secretory granule membrane proteins. We demonstrate that retromer-dependent trafficking via endosomal tubules is induced at the onset of secretory granule biogenesis, and that recycling via endosomal tubules is required for delivery of essential secretory granule membrane proteins to nascent granules. Without retromer function, nascent granules do not contain the proper membrane proteins; as a result, cargo from these defective granules is mistargeted to Rab7-positive endosomes, where it progressively accumulates to generate dramatically enlarged endosomes. Retromer complex dysfunction is strongly associated with neurodegenerative diseases, including Alzheimer's disease, characterized by accumulation of amyloid β (Aβ). We show that ectopically expressed amyloid precursor protein (APP) undergoes regulated exocytosis in salivary glands and accumulates within enlarged endosomes in retromer-deficient cells. These results highlight recycling of secretory granule membrane proteins as a critical step during secretory granule maturation and provide new insights into our understanding of retromer complex function in secretory cells. These findings also suggest that missorting of secretory cargo, including APP, may contribute to the progressive nature of neurodegenerative disease.

The ESCRT machinery regulates retromer-dependent transcytosis of septate junction components in Drosophila

Loss of ESCRT function in Drosophila imaginal discs is known to cause neoplastic overgrowth fueled by mis-regulation of signaling pathways. Its impact on junctional integrity, however, remains obscure. To dissect the events leading to neoplasia, we used transmission electron microscopy (TEM) on wing imaginal discs temporally depleted of the ESCRT-III core component Shrub. We find a specific requirement for Shrub in maintaining septate junction (SJ) integrity by transporting the claudin Megatrachea (Mega) to the SJ. In absence of Shrub function, Mega is lost from the SJ and becomes trapped on endosomes coated with the endosomal retrieval machinery retromer. We show that ESCRT function is required for apical localization and mobility of retromer positive carrier vesicles, which mediate the biosynthetic delivery of Mega to the SJ. Accordingly, loss of retromer function impairs the anterograde transport of several SJ core components, revealing a novel physiological role for this ancient endosomal agent.

Proteins are large molecules responsible for a variety of activities that cells needs to perform to survive; from respiration to copying DNA before cells divide. To perform these roles proteins need to be transported to the correct cell compartment, or to the cell membrane. This protein trafficking depends on the endosomal system, a set of membrane compartments that can travel within the cell and act as a protein sorting hub. This system needs its own proteins to work properly. In particular, there are two sets of proteins that are crucial for the endosomal systems activity: a group of proteins known as the ESCRT (endosomal sorting complex required for transport) machinery and a complex called retromer. The retromer complex regulates recycling of receptor proteins so they can be reused, while the ESCRT machinery mediates degradation of proteins that the cell does not require anymore. In the epithelia of fruit fly larvae – the tissues that form layers of cells, usually covering an organ but also making structures like wings – defects in ESCRT activity lead to a loss of tissue integrity.

This loss of tissue integrity suggests that the endosomal system might be involved in transporting proteins that form cellular junctions, the multiprotein complexes that establish contacts between cells or between a cell and the extracellular space. In arthropods such as the fruit fly, the adherens junction and the septate junction are two types of cellular junctions important for the integrity of epithelia integrity. Adherens junctions allow cells to adhere to each other, while septate junctions stop nutrient molecules, ions and water from leaking into the tissue. The role of the endosomal system in trafficking the proteins that form septate junctions remains a mystery.

To better understand the role of the endosomal system in regulating cell junctions and tissue integrity, Pannen et al. blocked the activity of either the ESCRT or retromer in wing imaginal discs – the future wings – of fruit fly larvae. Pannen et al. then analyzed the effects of these endosomal defects on cellular junctions using an imaging technique called transmission electron microscopy. The results showed that both ESCRT and retromer activities are necessary for the correct delivery of septate junction components to the cell membrane. However, neither retromer nor ESCRT were required for the delivery of adherens junction proteins.

These findings shed light on how retromer and the ESCRT machinery are involved in the epithelial tissue integrity of fruit fly larvae through their effects on cell junctions. Humans have their own versions of the ESCRT, retromer, and cell junction proteins, all of which are very similar to their fly counterparts. Since defects in the human versions of these proteins have been associated with a variety of diseases, from infections to cancer, these results may have implications for research into treating those diseases.

Retromer retrieves the Wilson disease protein ATP7B from endolysosomes in a copper-dependent manner

The Wilson disease protein, ATP7B maintains copper (herein referring to the Cu+ ion) homeostasis in the liver. ATP7B traffics from trans-Golgi network to endolysosomes to export excess copper. Regulation of ATP7B trafficking to and from endolysosomes is not well understood. We investigated the fate of ATP7B after copper export. At high copper levels, ATP7B traffics primarily to acidic, active hydrolase (cathepsin-B)-positive endolysosomes and, upon subsequent copper chelation, returns to the trans-Golgi network (TGN). At high copper, ATP7B colocalizes with endolysosomal markers and with a core member of retromer complex, VPS35. Knocking down VPS35 did not abrogate the copper export function of ATP7B or its copper-responsive anterograde trafficking to vesicles; rather upon subsequent copper chelation, ATP7B failed to relocalize to the TGN, which was rescued by overexpressing wild-type VPS35. Overexpressing mutants of the retromer complex-associated proteins Rab7A and COMMD1 yielded a similar non-recycling phenotype of ATP7B. At high copper, VPS35 and ATP7B are juxtaposed on the same endolysosome and form a large complex that is stabilized by in vivo photoamino acid labeling and UV-crosslinking. We demonstrate that retromer regulates endolysosome to TGN trafficking of copper transporter ATP7B in a manner that is dependent upon intracellular copper.

EMAC, Retromer, and VSRs: do they connect?

Eukaryotic organisms share many common features in terms of endomembrane trafficking. This fact has helped plant scientists to propose testable hypotheses on how plant intracellular membrane trafficking is achieved and regulated based on knowledge from yeast and mammals. However, when a new compartment has been identified in a plant cell that has a vesicle tethering complex located at a position which is completely different to its counterpart in yeast and mammalian cells, caution is demanded when interpreting possible interactions with other trafficking elements. This is exemplified by the recently discovered EMAC (ER and microtubule-associated compartment). It has been postulated that this compartment is the recipient of vacuolar sorting receptors (VSRs) transported retrogradely via "retromer vesicles" from a post-Golgi location. Unfortunately, this suggestion was based entirely on our knowledge of retromer from yeast and mammalian cells, and did not take into account the available literature on the composition, localization, and function of the plant retromer. It also lacked reference to recent contradictory findings on VSR trafficking. In this short article, we have tried to rectify this situation, pointing out that plant retromer may not function as a pentameric complex of two subunits: the retromer core and the sorting nexins.

Cancer-driving mutations and variants of components of the membrane trafficking core machinery

The core machinery for vesicular membrane trafficking broadly comprises of coat proteins, RABs, tethering complexes and SNAREs. As cellular membrane traffic modulates key processes of mitogenic signaling, cell migration, cell death and autophagy, its dysregulation could potentially results in increased cell proliferation and survival, or enhanced migration and invasion. Changes in the levels of some components of the core machinery of vesicular membrane trafficking, likely due to gene amplifications and/or alterations in epigenetic factors (such as DNA methylation and micro RNA) have been extensively associated with human cancers. Here, we provide an overview of association of membrane trafficking with cancer, with a focus on mutations and variants of coat proteins, RABs, tethering complex components and SNAREs that have been uncovered in human cancer cells/tissues. The major cellular and molecular cancer-driving or suppression mechanisms associated with these components of the core membrane trafficking machinery shall be discussed.

Endosomal dysfunction in iPSC-derived neural cells from Parkinson's disease patients with VPS35 D620N

Mutations in the Vacuolar protein sorting 35 (VPS35) gene have been linked to familial Parkinson’s disease (PD), PARK17. VPS35 is a key component of the retromer complex, which plays a central role in endosomal trafficking. However, whether and how VPS35 deficiency or mutation contributes to PD pathogenesis remain unclear. Here, we analyzed human induced pluripotent stem cell (iPSC)-derived neurons from PD patients with the VPS35 D620N mutation and addressed relevant disease mechanisms. In the disease group, dopaminergic (DA) neurons underwent extensive apoptotic cell death. The movement of Rab5a- or Rab7a-positive endosomes was slower, and the endosome fission and fusion frequencies were lower in the PD group than in the healthy control group. Interestingly, vesicles positive for cation-independent mannose 6-phosphate receptor transported by retromers were abnormally localized in glial cells derived from patient iPSCs. Furthermore, we found α-synuclein accumulation in TH positive DA neurons. Our results demonstrate the induction of cell death, endosomal dysfunction and α -synuclein accumulation in neural cells of the PD group. PARK17 patient-derived iPSCs provide an excellent experimental tool for understanding the pathophysiology underlying PD.

The cellular machinery of post-endocytic APP trafficking in Alzheimer's disease: A future target for therapeutic intervention?

Recent data establish multiple defects in endocytic functions as early events initiating various neurodegenerative disorders, including Alzheimer's disease (AD). The genetic landscape resulting from genome-wide association studies (GWAS) reveals changes in post-endocytic trafficking of amyloid precursor protein (APP) in neurons leading to an increase in amyloidogenic processing, deficits in amyloid beta (Aβ) clearance, increases in intracellular Aβ, and other endosomal pathogenic phenotypes. Multiple genetic factors regulate each segment of endosomal and post-endosomal trafficking. Intriguingly, several studies indicate endosomal dysfunctions preceding Aβ pathology and tau phosphorylation. In this chapter we highlight the role of various GWAS-identified endosomal and post-endosomal gene products in initiating AD pathologies. We also summarize the functions of various genetic modifiers of post-endocytic trafficking of APP that may work as targets for therapeutic intervention in AD.

Mammalian copper homeostasis requires retromer-dependent recycling of the high-affinity copper transporter 1

The concentration of essential micronutrients, such as copper (used here to describe both Cu⁺ and Cu²⁺), within the cell is tightly regulated to avoid their adverse deficiency and toxicity effects. Retromer-mediated sorting and recycling of nutrient transporters within the endo-lysosomal network is an essential process in regulating nutrient balance. Cellular copper homeostasis is regulated primarily by two transporters: the copper influx transporter copper transporter 1 (CTR1; also known as SLC31A1), which controls the uptake of copper, and the copper-extruding ATPase ATP7A, a recognised retromer cargo. Here, we show that in response to fluctuating extracellular copper, retromer controls the delivery of CTR1 to the cell surface. Following copper exposure, CTR1 is endocytosed to prevent excessive copper uptake. We reveal that internalised CTR1 localises on retromer-positive endosomes and, in response to decreased extracellular copper, retromer controls the recycling of CTR1 back to the cell surface to maintain copper homeostasis. In addition to copper, CTR1 plays a central role in the trafficking of platinum. The efficacy of platinum-based cancer drugs has been correlated with CTR1 expression. Consistent with this, we demonstrate that retromer-deficient cells show reduced sensitivity to the platinum-based drug cisplatin.

Summary: CTR1 (SLC31A1) is the only known mammalian importer of copper. We show that CTR1 is a retromer complex cargo protein, and that retromer is required for cellular sensitivity to extracellular copper.

Acute inactivation of retromer and ESCPE-1 leads to time-resolved defects in endosomal cargo sorting

Human retromer, a heterotrimer of VPS26 (VPS26A or VPS26B), VPS35 and VPS29, orchestrates the endosomal retrieval of internalised cargo and promotes their cell surface recycling, a prototypical cargo being the glucose transporter GLUT1 (also known as SLC2A1). The role of retromer in the retrograde sorting of the cation-independent mannose 6-phosphate receptor (CI-MPR, also known as IGF2R) from endosomes back to the trans-Golgi network remains controversial. Here, by applying knocksideways technology, we develop a method for acute retromer inactivation. While retromer knocksideways in HeLa and H4 human neuroglioma cells resulted in time-resolved defects in cell surface sorting of GLUT1, we failed to observe a quantifiable defect in CI-MPR sorting. In contrast, knocksideways of the ESCPE-1 complex – a key regulator of retrograde CI-MPR sorting – revealed time-resolved defects in CI-MPR sorting. Together, these data are consistent with a comparatively limited role for retromer in ESCPE-1-mediated CI-MPR retrograde sorting, and establish a methodology for acute retromer and ESCPE-1 inactivation that will aid the time-resolved dissection of their functional roles in endosomal cargo sorting.

Summary: Retromer, a master controller of endosomal cargo sorting, is deregulated in neurodegenerative disease. Here, we develop and apply a retromer knocksideways methodology to quantify endosomal cargo sorting upon acute perturbation.

Endosomal microdomains: Formation and function

It is widely recognized that after endocytosis, internalized cargo is delivered to endosomes that act as sorting stations. The limiting membrane of endosomes contain specialized subregions, or microdomains, that represent distinct functions of the endosome, including regions competing for cargo capture leading to degradation or recycling. Great progress has been made in defining the endosomal protein coats that sort cargo in these domains, including Retromer that recycles transmembrane cargo, and ESCRT (endosomal sorting complex required for transport) that degrades transmembrane cargo. In this review, we discuss recent work that is beginning to unravel how such coat complexes contribute to the creation and maintenance of endosomal microdomains. We highlight data that indicates that adjacent microdomains do not act independently but rather interact to cross-regulate. We posit that these interactions provide an agile means for the cell to adjust sorting in response to extracellular signals and intracellular metabolic cues.

CLN3 regulates endosomal function by modulating Rab7A-effector interactions

Mutations in CLN3 are a cause of juvenile neuronal ceroid lipofuscinosis (JNCL), also known as Batten disease. Clinical manifestations include cognitive regression, progressive loss of vision and motor function, epileptic seizures and a significantly reduced lifespan. CLN3 localizes to endosomes and lysosomes, and has been implicated in intracellular trafficking and autophagy. However, the precise molecular function of CLN3 remains to be elucidated. Previous studies showed an interaction between CLN3 and Rab7A, a small GTPase that regulates several functions at late endosomes. We confirmed this interaction in live cells and found that CLN3 is required for the efficient endosome-to-TGN trafficking of the lysosomal sorting receptors because it regulates the Rab7A interaction with retromer. In cells lacking CLN3 or expressing CLN3 harbouring a disease-causing mutation, the lysosomal sorting receptors were degraded. We also demonstrated that CLN3 is required for the Rab7A-PLEKHM1 interaction, which is required for fusion of autophagosomes to lysosomes. Overall, our data provide a molecular explanation behind phenotypes observed in JNCL and give an indication of the pathogenic mechanism behind Batten disease.This article has an associated First Person interview with the first author of the paper.

Recognising the signals for endosomal trafficking

The endosomal compartment is a major sorting station controlling the balance between endocytic recycling and lysosomal degradation, and its homeostasis is emerging as a central factor in various neurodegenerative diseases such as Alzheimer's and Parkinson's. Membrane trafficking is generally coordinated by the recognition of specific signals in transmembrane protein cargos by different transport machineries. A number of different protein trafficking complexes are essential for sequence-specific recognition and retrieval of endosomal cargos, recycling them to other compartments and acting to counter-balance the default endosomal sorting complex required for transport-mediated degradation pathway. In this review, we provide a summary of the key endosomal transport machineries, and the molecular mechanisms by which different cargo sequences are specifically recognised.

DNAJC13 p.Asn855Ser, implicated in familial parkinsonism, alters membrane dynamics of sorting nexin 1

DNAJC13 (RME-8) is a core co-chaperone that facilitates membrane recycling and cargo sorting of endocytosed proteins. DNAJ/Hsp40 (heat shock protein 40) proteins are highly conserved throughout evolution and mediate the folding of nascent proteins, and the unfolding, refolding or degradation of misfolded proteins while assisting in associated-membrane translocation. DNAJC13 is one of five DNAJ 'C' class chaperone variants implicated in monogenic parkinsonism. Here we examine the effect of the DNAJC13 disease-linked mutation (p.Asn855Ser) on its interacting partners, focusing on sorting nexin 1 (SNX1) membrane dynamics in primary cortical neurons derived from a novel Dnajc13 p.Asn855Ser knock-in (DKI) mouse model. Dnajc13 p.Asn855Ser mutant and wild type protein expression were equivalent in mature heterozygous cultures (DIV21). While SNX1-positive puncta density, area, and WASH-retromer assembly were comparable between cultures derived from DKI and wild type littermates, the formation of SNX1-enriched tubules in DKI neuronal cultures was significantly increased. Thus, Dnajc13 p.Asn855Ser disrupts SNX1 membrane-tubulation and trafficking, analogous to results from RME-8 depletion studies. The data suggest the mutation confers a dominant-negative gain-of-function in RME-8. Implications for the pathogenesis of Parkinson's disease are discussed.

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.

elegans epidermis integrity

Rab proteins are conserved small GTPases that coordinate intracellular trafficking essential to cellular function and homeostasis. RAB-6.2 is a highly conserved C. elegans ortholog of human RAB6 proteins. RAB-6.2 is expressed in most tissues in C. elegans and is known to function in neurons and in the intestine to mediate retrograde trafficking. Here, we show that RAB-6.2 is necessary for cuticle integrity and impermeability in C. elegans RAB-6.2 functions in the epidermis to instruct skin integrity. Significantly, we show that expression of a mouse RAB6A cDNA can rescue defects in C. elegans epidermis caused by lack of RAB-6.2, suggesting functional conservation across phyla. We also show that the novel function of RAB-6.2 in C. elegans cuticle development is distinct from its previously described function in neurons. Exocyst mutants partially phenocopy rab-6.2-null animals, and rab-6.2-null animals phenocopy mutants that have defective surface glycosylation. These results suggest that RAB-6.2 may mediate the trafficking of one or many secreted glycosylated cuticle proteins directly, or might act indirectly by trafficking glycosylation enzymes to their correct intracellular localization.

Analysis of RNA Expression Profiles Identifies Dysregulated Vesicle Trafficking Pathways in Creutzfeldt-Jakob Disease

Functional genomics applied to the study of RNA expression profiles identified several abnormal molecular processes in experimental prion disease. However, only a few similar studies have been carried out to date in a naturally occurring human prion disease. To better characterize the transcriptional cascades associated with sporadic Creutzfeldt-Jakob disease (sCJD), the most common human prion disease, we investigated the global gene expression profile in samples from the frontal cortex of 10 patients with sCJD and 10 non-neurological controls by microarray analysis. The comparison identified 333 highly differentially expressed genes (hDEGs) in sCJD. Functional enrichment Gene Ontology analysis revealed that hDEGs were mainly associated with synaptic transmission, including GABA (q value = 0.049) and glutamate (q value = 0.005) signaling, and the immune/inflammatory response. Furthermore, the analysis of cellular components performed on hDEGs showed a compromised regulation of vesicle-mediated transport with mainly up-regulated genes related to the endosome (q value = 0.01), lysosome (q value = 0.04), and extracellular exosome (q value < 0.01). A targeted analysis of the retromer core component VPS35 (vacuolar protein sorting-associated protein 35) showed a down-regulation of gene expression (p value= 0.006) and reduced brain protein levels (p value= 0.002). Taken together, these results confirm and expand previous microarray expression profile data in sCJD. Most significantly, they also demonstrate the involvement of the endosomal-lysosomal system. Since the latter is a common pathogenic pathway linking together diseases, such as Alzheimer's and Parkinson's, it might be the focus of future studies aimed to identify new therapeutic targets in neurodegenerative diseases.

Sorting nexin-21 is a scaffold for the endosomal recruitment of huntingtin

The endo-lysosomal network serves an essential role in determining the fate of endocytosed transmembrane proteins and their associated proteins and lipids. Sorting nexins (SNXs) play a central role in the functional organisation of this network. Comprising over 30 proteins in humans, SNXs are classified into sub-groups based on the presence of additional functional domains. Sorting nexin-20 (SNX20) and sorting nexin-21 (SNX21) comprise the SNX-PXB proteins. The presence of a predicted protein-protein interaction domain, termed the PX-associated B (PXB) domain, has led to the proposal that they function as endosome-associated scaffolds. Here, we used unbiased quantitative proteomics to define the SNX21 interactome. We reveal that the N-terminal extension of SNX21 interacts with huntingtin (Htt) whereas the PXB domain appears to associate with septins, a family of cytoskeletal- and membrane-associated proteins. In establishing that these interactions are sufficient for SNX21 to recruit Htt and septins on to an endosomal population, we reveal a scaffolding function for this sorting nexin. Our work paves the way for a more-detailed mechanistic analysis of the role(s) of the SNX-PXB proteins in endosomal biology.

Summary: A potential scaffolding function for SNX21 paves the way for a more-detailed mechanistic analysis of the role(s) of this protein in endosomal biology.

Cholera toxin inhibits SNX27-retromer-mediated delivery of cargo proteins to the plasma membrane

Cholera toxin (CT) causes severe diarrhea by increasing intracellular cAMP leading to a PKA-dependent increase in Cl^- secretion through CFTR and decreased Na⁺ absorption through inhibition of Na⁺/H⁺ exchanger 3 (NHE3; also known as SLC9A3). The mechanism(s) by which CT inhibits NHE3 is partially understood, although no drug therapy has been successful at reversing this inhibition. We now describe that CT phosphorylates an amino acid in the PDZ domain of SNX27, which inhibits SNX27-mediated trafficking of NHE3 from the early endosomes to the plasma membrane (PM), and contributes to reduced basal NHE3 activity through a mechanism that involves reduced PM expression and reduced endocytic recycling. Importantly, mutagenesis studies (Ser to Asp) showed that the effect of this phosphorylation of SNX27 phenocopies the effects seen upon loss of SNX27 function, affecting PM trafficking of cargo proteins that bind SNX27-retromer. Additionally, CT destabilizes retromer function by decreasing the amount of core retromer proteins. These effects of CT can be partially rescued by enhancing retromer stability by using 'pharmacological chaperones'. Moreover, pharmacological chaperones can be used to increase basal and cholera toxin-inhibited NHE3 activity and fluid absorption by intestinal epithelial cells.This article has an associated First Person interview with the first author of the paper.

Inhibition of TBC1D5 activates Rab7a and can enhance the function of the retromer cargo-selective complex

The retromer complex is a vital component of the endosomal protein sorting machinery necessary for sorting into both the endosome-to-Golgi retrieval pathway and also the endosome-to-cell-surface recycling pathway. Retromer mediates cargo selection through a trimeric complex comprising VPS35, VPS29 and VPS26, which is recruited to endosomes by binding to Rab7a and Snx3. Retromer function is linked to two distinct neurodegenerative diseases, Parkinson's disease and Alzheimer's disease and modulating retromer function has been proposed as an avenue to explore for a putative therapy in these conditions. We hypothesised that activating Rab7a to promote the recruitment of retromer to endosomes could positively modulate its activity. Here, we show that inhibition of the GTPase activating protein TBC1D5 can enhance Rab7a activation and lead to a gain of function for retromer.

Highlighted Article: Enhancement of retromer complex function through inhibition of TBC1D5, a Rab GTPase-activating protein for Rab7a, leads to a gain of function for retromer-mediated endosomal sorting.

Retromer associates with the cytoplasmic amino-terminus of polycystin-2

Autosomal dominant polycystic kidney disease (ADPKD) is the most common monogenic human disease, with around 12.5 million people affected worldwide. ADPKD results from mutations in either PKD1 or PKD2, which encode the atypical G-protein coupled receptor polycystin-1 (PC1) and the transient receptor potential channel polycystin-2 (PC2), respectively. Although altered intracellular trafficking of PC1 and PC2 is an underlying feature of ADPKD, the mechanisms which govern vesicular transport of the polycystins through the biosynthetic and endosomal membrane networks remain to be fully elucidated. Here, we describe an interaction between PC2 and retromer, a master controller for the sorting of integral membrane proteins through the endo-lysosomal network. We show that association of PC2 with retromer occurs via a region in the PC2 cytoplasmic amino-terminal domain, independently of the retromer-binding Wiskott-Aldrich syndrome and scar homologue (WASH) complex. Based on observations that retromer preferentially interacts with a trafficking population of PC2, and that ciliary levels of PC1 are reduced upon mutation of key residues required for retromer association in PC2, our data are consistent with the identification of PC2 as a retromer cargo protein.This article has an associated First Person interview with the first author of the paper.

Retromer Dysfunction and Neurodegenerative Disease

In recent years, genomic, animal and cell biology studies have implicated deficiencies in retromer-mediated trafficking of proteins in an increasing number of neurodegenerative diseases including Alzheimer's Disease (AD), Parkinson's Disease (PD) and Frontotemporal Lobar Degener-ation (FTLD). The retromer complex, which is highly conserved across all eukaryotes, regulates the sorting of transmembrane proteins out of endo-somes to the cell surface or to the trans-Golgi network. Within retromer, cargo selection and binding are performed by a trimer of the Vps26, Vps29 and Vps35 proteins, named the "Cargo-Selective Complex (CSC)". Sorting of cargo into tubules for distribution to the trans-Golgi network or the cell sur-face is achieved through the dimeric sorting nexin (SNX) component of retromer and accessory proteins such as the WASH complex which medi-ates the formation of discrete endosomal tubules enabling the sorting of cargo into distinct pathways through production of filamentous actin patch-es. In the present article, we review the molecular structure and function of the retromer and summarize the evidence linking retromer dysfunction to neurodegenerative disease.

The retromer, sorting nexins and the plant endomembrane protein trafficking

Protein sorting in the endomembrane system is responsible for the coordination of cellular functions. Plant intracellular trafficking has its own unique features, which include specific regulatory aspects of endosomal sorting and recycling of cargo proteins, mediated by the retromer complex. Recent work has led to significant progress in understanding the role of Arabidopsis retromer subunits in recycling vacuolar sorting receptors and plasma membrane proteins. As a consequence, members of the sorting nexin (SNX) protein family and their interaction partners have emerged as critical protein trafficking regulators, in particular with regard to adaptation to environmental change, such as temperature fluctuations and nutrient deficiency. In this Review, we discuss the known and proposed functions of the comparatively small Arabidopsis SNX protein family. We review the available information on the role of the three Bin-Amphiphysin-Rvs (BAR)-domain-containing Arabidopsis thaliana (At)SNX proteins and discuss their function in the context of their potential participation in the plant retromer complex. We also summarize the role of AtSNX1-interacting proteins in different aspects of SNX-dependent protein trafficking and comment on the potential function of three novel, as yet unexplored, Arabidopsis SNX proteins.

The Phox Homology (PX) Domain

The phox-homology (PX) domain is a phosphoinositide-binding domain conserved in all eukaryotes and present in 49 human proteins. Proteins containing PX domains, many of which are also known as sorting nexins (SNXs), have a large variety of functions in membrane trafficking, cell signaling, and lipid metabolism in association with membranes of the secretory and endocytic system. In this review we discuss the structural basis for both canonical lipid interactions with the endosome-enriched lipid phosphatidylinositol-3-phosphate (PtdIns3P) as well as non-canonical lipids that promote membrane association. We also describe recent advances in defining the diverse mechanisms by which PX domains interact with other proteins including the retromer trafficking complex and proteins secreted by bacterial pathogens. Like other membrane interacting domains, the attachment of PX domain proteins to specific membranes is often facilitated by additional interactions that contribute to binding avidity, and we discuss this coincidence detection for several known examples.

Retromer and the cation-independent mannose 6-phosphate receptor-Time for a trial separation?

The retromer cargo-selective complex (CSC) comprising Vps35, Vps29 and Vps26 mediates the endosome-to-Golgi retrieval of the cation-independent mannose 6-phosphate receptor (CIMPR). Or does it? Recently published data have questioned the validity of this long-established theory. Here, the evidence for and against a role for the retromer CSC in CIMPR endosome-to-Golgi retrieval is examined in the light of the new data that the SNX-BAR dimer is actually responsible for CIMPR retrieval.

Cellular interactome analysis of vaccinia virus K7 protein identifies three transport machineries as binding partners for K7

Identification of viral-host interacting proteins will contribute to understanding of how poxvirus exploits the host cellular machinery. The vaccinia virus gene K7R encodes a conserved protein K7 in most orthopoxviruses. To gain insight into the biology of K7, we investigated the cellular interactome of K7 by GST pulldown coupled with mass spectrometry. The top categories of identified proteins contained components of trafficking machineries. We selected key components of three transport machineries including coatomer, retromer, and CHEVI to further confirm their binding abilities to K7. Di-lysine motif of K7 is required for its interaction with coatomer, while C terminal leucines in K7 are critical for association of retromer. Our study uncovers the viral-host interactome of vaccinia K7 and reveals three host transport machineries as binding partners of K7, which might have important roles in poxvirus' life cycles.

Rab7 palmitoylation is required for efficient endosome-to-TGN trafficking

Retromer is a multimeric protein complex that mediates endosome-to-trans-Golgi network (TGN) and endosome-to-plasma membrane trafficking of integral membrane proteins. Dysfunction of this complex has been linked to Alzheimer's disease and Parkinson's disease. The recruitment of retromer to endosomes is regulated by Rab7 (also known as RAB7A) to coordinate endosome-to-TGN trafficking of cargo receptor complexes. Rab7 is also required for the degradation of internalized integral membrane proteins, such as the epidermal growth factor receptor (EGFR). We found that Rab7 is palmitoylated and that this modification is not required for membrane anchoring. Palmitoylated Rab7 colocalizes efficiently with and has a higher propensity to interact with retromer than nonpalmitoylatable Rab7. Rescue of Rab7 knockout cells by expressing wild-type Rab7 restores efficient endosome-to-TGN trafficking, while rescue with nonpalmitoylatable Rab7 does not. Interestingly, Rab7 palmitoylation does not appear to be required for the degradation of EGFR or for its interaction with its effector, Rab-interacting lysosomal protein (RILP). Overall, our results indicate that Rab7 palmitoylation is required for the spatiotemporal recruitment of retromer and efficient endosome-to-TGN trafficking of the lysosomal sorting receptors.

Control of mitochondrial homeostasis by endocytic regulatory proteins

Mitochondria play essential roles in cellular energy processes, including ATP production, control of reactive oxygen species (ROS) and apoptosis. While mitochondrial function is regulated by the dynamics of fusion and fission, mitochondrial homeostasis remains incompletely understood. Recent studies implicate dynamin-2 and dynamin-related protein-1 (Drp1, also known as DNM1L), as GTPases involved in mitochondrial fission. Here, we identify the ATPase and endocytic protein EHD1 as a novel regulator of mitochondrial fission. EHD1 depletion induces a static and elongated network of mitochondria in the cell. However, unlike dynamin-2 and Drp1, whose depletion protects cells from staurosporine-induced mitochondrial fragmentation, EHD1-depleted cells remain sensitive to staurosporine, suggesting a different mechanism for EHD1 function. Recent studies have demonstrated that VPS35 and the retromer complex influence mitochondrial homeostasis either by Mul1-mediated ubiquitylation and degradation of the fusion protein Mfn2, or by removal of inactive Drp1 from the mitochondrial membrane. We demonstrate that EHD1 and its interaction partner rabankyrin-5 interact with the retromer complex to influence mitochondrial dynamics, likely by inducing VPS35-mediated removal of inactive Drp1 from mitochondrial membranes. Our study sheds light on mitochondrial dynamics, expanding a new paradigm of endocytic protein regulation of mitochondrial homeostasis.

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.

Cargo selectivity of yeast sorting nexins

Sorting nexins are PX domain-containing proteins that bind phospholipids and often act in membrane trafficking where they help to select cargo. However, the functions and cargo specificities of many sorting nexins are unknown. Here, a high-throughput imaging screen was used to identify new sorting nexin cargo in the yeast Saccharomyces cerevisiae. Deletions of 9 different sorting nexins were screened for mislocalization of a set of green fluorescent protein (GFP)-tagged membrane proteins found at the plasma membrane, Golgi or endosomes. This identified 27 proteins that require 1 or more sorting nexins for their correct localization, 23 of which represent novel sorting nexin cargo. Nine hits whose sorting was dependent on Snx4, the sorting nexin-containing retromer complex, or both retromer and Snx3, were examined in detail to search for potential sorting motifs. We identified cytosolic domains of Ear1, Ymd8 and Ymr010w that conferred retromer-dependent sorting on a chimeric reporter and identified conserved residues required for this sorting in a functional assay. This work defined a consensus sequence for retromer and Snx3-dependent sorting.

Sorting receptor SORLA: cellular mechanisms and implications for disease

Sorting-related receptor with A-type repeats (SORLA) is an intracellular sorting receptor that directs cargo proteins, such as kinases, phosphatases, and signaling receptors, to their correct location within the cell. The activity of SORLA assures proper function of cells and tissues, and receptor dysfunction is the underlying cause of common human malignancies, including Alzheimer’s disease, atherosclerosis, and obesity. Here, we discuss the molecular mechanisms that govern sorting of SORLA and its cargo in multiple cell types, and why genetic defects in this receptor results in devastating diseases.

Increased copper toxicity in Saccharomyces cerevisiae lacking VPS35, a component of the retromer and monogenic Parkinson disease gene in humans

The Saccharomyces cerevisiae gene VPS35 encodes a component of the retromer complex which is involved in vesicle transport from endosomes to the trans-Golgi network. Yeast and human VPS35 orthologs are highly conserved and mutations in human VPS35 cause an autosomal dominant form of late-onset Parkinson disease (PD). We now show that deletion of VPS35 in yeast (vps35Δ) leads to a dose-dependent growth defect towards copper. This increased sensitivity could be rescued by transformation with yeast wild-type VPS35 but not by the expression of a construct harboring the yeast equivalent (i.e. D686N) of the most commonly identified VPS35-associated PD mutation, p.D620N. In addition, we show that expression of one copy of α-synuclein, which is known to directly interact with copper, leads to a pronounced aggravation of copper toxicity in vps35Δ cells, thereby linking the regulation of copper homeostasis by Vps35p in yeast to one of the key molecules in PD pathophysiology.

Retromer in Osteoblasts Interacts With Protein Phosphatase 1 Regulator Subunit 14C, Terminates Parathyroid Hormone's Signaling, and Promotes Its Catabolic Response

Parathyroid hormone (PTH) plays critical, but distinct, roles in bone remodeling, including bone formation (anabolic response) and resorption (catabolic response). Although its signaling and function have been extensively investigated, it just began to be understood how distinct functions are induced by PTH activating a common receptor, the PTH type 1 receptor (PTH1R), and how PTH1R signaling is terminated. Here, we provide evidence for vacuolar protein sorting 35 (VPS35), a major component of retromer, in regulating PTH1R trafficking, turning off PTH signaling, and promoting its catabolic function. VPS35 is expressed in osteoblast (OB)-lineage cells. VPS35-deficiency in OBs impaired PTH_(1–34)-promoted PTH1R translocation to the trans-Golgi network, enhanced PTH_(1–34)-driven signaling, and reduced PTH_(1–34)'s catabolic response in culture and in mice. Further mechanical studies revealed that VPS35 interacts with not only PTH1R, but also protein phosphatase 1 regulatory subunit 14C (PPP1R14C), an inhibitory subunit of PP1 phosphatase. PPP1R14C also interacts with PTH1R, which is necessary for the increased endosomal PTH1R signaling and decreased PTH_(1–34)'s catabolic response in VPS35-deficient OB-lineage cells. Taken together, these results suggest that VPS35 deregulates PTH1R-signaling likely by its interaction with PTH1R and PPP1R14C. This event is critical for the control of PTH_(1–34)-signaling dynamics, which may underlie PTH-induced catabolic response and adequate bone remodeling.

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VPS35 terminates PTH_(1-34)-induced cell surface and endosomal signalings

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Osteoblastic VPS35 promotes PTH_(1-34)-driven catabolic response

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VPS35 interacts with PPP1R14C

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PPP1R14C also interacts with PTH1R and promotes PTH_(1-34)-induced endosomal signaling

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PPP1R14C is necessary for the increased endosomal PTH1R signaling and decreased PTH_(1-34)’s catabolic response in VPS35-deficient OB-lineage cells