Identification of Alzheimer disease-associated variants in genes that regulate retromer function

A defect in the retromer accessory protein, SNX27, manifests by infantile myoclonic epilepsy and neurodegeneration

The composition of the neuronal cell surface dictates synaptic plasticity and thereby cognitive development. This remodeling of the synapses is governed by the endocytic network which internalize transmembrane proteins, then sort them back to the cell surface or carry them to the lysosome for degradation. The multi-protein retromer complex is central to this selection, capturing specific transmembrane proteins and remodeling the cell membrane to form isolated cargo-enriched transport carriers. We investigated a consanguineous family with four patients who presented in infancy with intractable myoclonic epilepsy and lack of psychomotor development. Using exome analysis, we identified a homozygous deleterious mutation in SNX27, which encodes sorting nexin 27, a retromer cargo adaptor. In western analysis of patient fibroblasts, the encoded mutant protein was expressed at an undetectable level when compared with a control sample. The patients' presentation and clinical course recapitulate that reported for the SNX27 knock-out mouse. Since the cargo proteins for SNX27-mediated sorting include subunits of ionotropic glutamate receptors and endosome-to-cell surface synaptic insertion of AMPA receptors is severely perturbed in SNX27(-/-) neurons, it is proposed that at least part of the neurological aberrations observed in the patients is attributed to defective sorting of ionotropic glutamate receptors. SNX27 deficiency is now added to the growing list of neurodegenerative disorders associated with retromer dysfunction.

Candidate genes for Alzheimer's disease are associated with individual differences in plasma levels of beta amyloid peptides in adults with Down syndrome

We examined the contribution of candidates genes for Alzheimer's disease (AD) to individual differences in levels of beta amyloid peptides in adults with Down syndrom, a population at high risk for AD. Participants were 254 non-demented adults with Down syndrome, 30-78 years of age. Genomic deoxyribonucleic acid was genotyped using an Illumina GoldenGate custom array. We used linear regression to examine differences in levels of Aβ peptides associated with the number of risk alleles, adjusting for age, sex, level of intellectual disability, race and/or ethnicity, and the presence of the APOE ε4 allele. For Aβ42 levels, the strongest gene-wise association was found for a single nucleotide polymorphism (SNP) on CAHLM1; for Aβ40 levels, the strongest gene-wise associations were found for SNPs in IDE and SOD1, while the strongest gene-wise associations with levels of the Aβ42/Aβ40 ratio were found for SNPs in SORCS1. Broadly classified, variants in these genes may influence amyloid precursor protein processing (CALHM1, IDE), vesicular trafficking (SORCS1), and response to oxidative stress (SOD1).

Biogenesis of endosome-derived transport carriers

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

Current and future implications of basic and translational research on amyloid-β peptide production and removal pathways

Inherited variants in multiple different genes are associated with increased risk for Alzheimer's disease (AD). In many of these genes, the inherited variants alter some aspect of the production or clearance of the neurotoxic amyloid β-peptide (Aβ). Thus missense, splice site or duplication mutants in the presenilin 1 (PS1), presenilin 2 (PS2) or the amyloid precursor protein (APP) genes, which alter the levels or shift the balance of Aβ produced, are associated with rare, highly penetrant autosomal dominant forms of Familial Alzheimer's Disease (FAD). Similarly, the more prevalent late-onset forms of AD are associated with both coding and non-coding variants in genes such as SORL1, PICALM and ABCA7 that affect the production and clearance of Aβ. This review summarises some of the recent molecular and structural work on the role of these genes and the proteins coded by them in the biology of Aβ. We also briefly outline how the emerging knowledge about the pathways involved in Aβ generation and clearance can be potentially targeted therapeutically. This article is part of Special Issue entitled "Neuronal Protein".

Retromer in Alzheimer disease, Parkinson disease and other neurological disorders

Retromer is a protein assembly that has a central role in endosomal trafficking, and retromer dysfunction has been linked to a growing number of neurological disorders. First linked to Alzheimer disease, retromer dysfunction causes a range of pathophysiological consequences that have been shown to contribute to the core pathological features of the disease. Genetic studies have established that retromer dysfunction is also pathogenically linked to Parkinson disease, although the biological mechanisms that mediate this link are only now being elucidated. Most recently, studies have shown that retromer is a tractable target in drug discovery for these and other disorders of the nervous system.

Nuclear FAM21 participates in NF-κB-dependent gene regulation in pancreatic cancer cells

The pentameric WASH complex is best known for its role in regulating receptor trafficking from retromer-rich endosomal subdomains. FAM21 functions to stabilize the WASH complex through its N-terminal head domain and localizes it to endosomes by directly binding the retromer through its extended C-terminal tail. Herein, we used affinity purification combined with mass spectrometry to identify additional FAM21-interacting proteins. Surprisingly, multiple components of the nuclear factor κB (NF-κB) pathway were identified, including the p50 and p65 (RelA) NF-κB subunits. We show that FAM21 interacts with these components and regulates NF-κB-dependent gene transcription at the level of p65 chromatin binding. We further demonstrate that FAM21 contains a functional monopartite nuclear localization signal sequence (NLS) as well as a CRM1/exportin1-dependent nuclear export signal (NES), both of which work jointly with the N-terminal head domain and C-terminal retromer recruitment domain to regulate FAM21 cytosolic and nuclear subcellular localization. Finally, our findings indicate that FAM21 depletion sensitizes pancreatic cancer cells to gemcitabine and 5-fluorouracil. Thus, FAM21 not only functions as an integral component of the cytoplasmic WASH complex, but also modulates NF-κB gene transcription in the nucleus.

Autophagy-related protein 7 deficiency in amyloid β (Aβ) precursor protein transgenic mice decreases Aβ in the multivesicular bodies and induces Aβ accumulation in the Golgi

Alzheimer disease (AD) is biochemically characterized by increased levels of amyloid β (Aβ) peptide, which aggregates into extracellular Aβ plaques in AD brains. Before plaque formation, Aβ accumulates intracellularly in both AD brains and in the brains of AD model mice, which may contribute to disease progression. Autophagy, which is impaired in AD, clears cellular protein aggregates and participates in Aβ metabolism. In addition to a degradative role of autophagy in Aβ metabolism we recently showed that Aβ secretion is inhibited in mice lacking autophagy-related gene 7 (Atg7) in excitatory neurons in the mouse forebrain. This inhibition of Aβ secretion leads to intracellular accumulation of Aβ. Here, we used fluorescence and immunoelectron microscopy to elucidate the subcellular localization of the intracellular Aβ accumulation which accumulates in Aβ precursor protein mice lacking Atg7. Autophagy deficiency causes accumulation of p62(+) aggregates, but these aggregates do not contain Aβ. However, knockdown of Atg7 induced Aβ accumulation in the Golgi and a concomitant reduction of Aβ in the multivesicular bodies. This indicates that Atg7 influences the transport of Aβ possibly derived from Golgi to multivesicular bodies.

Genome-wide RNAi screen reveals a role for multipass membrane proteins in endosome-to-golgi retrieval

Endosome-to-Golgi retrieval is an essential membrane trafficking pathway required for many important physiological processes and linked to neurodegenerative disease and infection by bacterial and viral pathogens. The prototypical cargo protein for this pathway is the cation-independent mannose 6-phosphate receptor (CIMPR), which delivers lysosomal hydrolases to endosomes. Efficient retrieval of CIMPR to the Golgi requires the retromer complex, but other aspects of the endosome-to-Golgi retrieval pathway are poorly understood. Employing an image-based antibody-uptake assay, we conducted a genome-wide RNAi loss-of-function screen for novel regulators of this trafficking pathway and report ∼90 genes that are required for endosome-to-Golgi retrieval of a CD8-CIMPR reporter protein. Among these regulators of endosome-to-Golgi retrieval are a number of multipass membrane-spanning proteins, a class of proteins often overlooked with respect to a role in membrane trafficking. We further demonstrate a role for three multipass membrane proteins, SFT2D2, ZDHHC5, and GRINA, in endosome-to-Golgi retrieval.

Rab proteins and the compartmentalization of the endosomal system

Of the approximately 70 human Rab GTPases, nearly three-quarters are involved in endocytic trafficking. Significant plasticity in endosomal membrane transport pathways is closely coupled to receptor signaling and Rab GTPase-regulated scaffolds. Here we review current literature pertaining to endocytic Rab GTPase localizations, functions, and coordination with regulatory proteins and effectors. The roles of Rab GTPases in (1) compartmentalization of the endocytic pathway into early, recycling, late, and lysosomal routes; (2) coordination of individual transport steps from vesicle budding to fusion; (3) effector interactomes; and (4) integration of GTPase and signaling cascades are discussed.

Rab proteins and the compartmentalization of the endosomal system

Of the approximately 70 human Rab GTPases, nearly three-quarters are involved in endocytic trafficking. Significant plasticity in endosomal membrane transport pathways is closely coupled to receptor signaling and Rab GTPase-regulated scaffolds. Here we review current literature pertaining to endocytic Rab GTPase localizations, functions, and coordination with regulatory proteins and effectors. The roles of Rab GTPases in (1) compartmentalization of the endocytic pathway into early, recycling, late, and lysosomal routes; (2) coordination of individual transport steps from vesicle budding to fusion; (3) effector interactomes; and (4) integration of GTPase and signaling cascades are discussed.

The role of the retromer complex in aging-related neurodegeneration: a molecular and genomic review

The retromer coat complex is a vital component of the intracellular trafficking mechanism sorting cargo from the endosomes to the trans-Golgi network or to the cell surface. In recent years, genes encoding components of the retromer coat complex and members of the vacuolar protein sorting 10 (Vps10) family of receptors, which play pleiotropic functions in protein trafficking and intracellular/intercellular signaling in neuronal and non-neuronal cells and are primary cargos of the retromer complex, have been implicated as genetic risk factors for sporadic and autosomal dominant forms of several neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease and frontotemporal lobar degeneration. In addition to their functions in protein trafficking, the members of the Vps10 receptor family (sortilin, SorL1, SorCS1, SorCS2, and SorCS3) modulate neurotrophic signaling pathways. Both sortilin and SorCS2 act as cell surface receptors to mediate acute responses to proneurotrophins. In addition, sortilin can modulate the intracellular response to brain-derived neurotrophic factor (BDNF) by direct control of BDNF levels and regulating anterograde trafficking of Trk receptors to the synapse. This review article summarizes the emerging data from this rapidly growing field of intracellular trafficking signaling in the pathogenesis of neurodegeneration.

Isolating pathogenic mechanisms embedded within the hippocampal circuit through regional vulnerability

Some of the most common and devastating disorders of the brain target the hippocampal formation. The hippocampal formation is a complex circuit of interconnected regions, and it is assumed that clues into the causes of these disorders are embedded within the circuit. Neuroimaging tools have been optimized to interrogate the malfunctioning hippocampal circuit, and by applying these tools to patients in the earliest stages of disease and to animal models, patterns of regional vulnerability have been established for Alzheimer's disease, schizophrenia, and cognitive aging. More recently, studies have begun deciphering the cellular and molecular reasons underlying regional dysfunction. Collectively, this information clarifies the pathophysiology of these disorders and informs on therapeutic strategies.

A membrane proximal helix in the cytosolic domain of the human APP interacting protein LR11/SorLA deforms liposomes

Over the last decade, compelling evidence has linked the development of Alzheimer's disease (AD) to defective intracellular trafficking of the amyloid precursor protein (APP). Faulty APP trafficking results in an overproduction of Aβ peptides, which is generally agreed to be the primary cause of AD-related pathogenesis. LR11 (SorLA), a type I transmembrane sorting receptor, has emerged as a key regulator of APP trafficking and processing. It directly interacts with APP and diverts it away from amyloidogenic processing. The 54-residue cytosolic domain of LR11 is essential for its proper intracellular localization and trafficking which, in turn, determines the fate of APP. Here, we have found a surprising membrane-proximal amphipathic helix in the cytosolic domain of LR11. Moreover, a peptide corresponding to this region folds into an α-helical structure in the presence of liposomes and transforms liposomes to small vesicles and tubule-like particles. We postulate that this amphipathic helix may contribute to the dynamic remodeling of membrane structure and facilitate LR11 intracellular transport.

Pharmacological chaperones stabilize retromer to limit APP processing

Retromer is a multiprotein complex that trafficks cargo out of endosomes. The neuronal retromer traffics the amyloid-precursor protein (APP) away from endosomes, a site where APP is cleaved into pathogenic fragments in Alzheimer's disease. Here we determined whether pharmacological chaperones can enhance retromer stability and function. First, we relied on the crystal structures of retromer proteins to help identify the 'weak link' of the complex and to complete an in silico screen of small molecules predicted to enhance retromer stability. Among the hits, an in vitro assay identified one molecule that stabilized retromer against thermal denaturation. Second, we turned to cultured hippocampal neurons, showing that this small molecule increases the levels of retromer proteins, shifts APP away from the endosome, and decreases the pathogenic processing of APP. These findings show that pharmacological chaperones can enhance the function of a multiprotein complex and may have potential therapeutic implications for neurodegenerative diseases.

Going forward with retromer

The retromer complex is well known to mediate retrograde transport from endosomes to the Golgi. In a recent issue of Neuron, Choy et al. (2014) identify a function for retromer in supporting fast, local delivery of neurotransmitter receptors from endosomes to the dendritic plasma membrane.

RME-8 coordinates the activity of the WASH complex with the function of the retromer SNX dimer to control endosomal tubulation

Retromer is a vital element of the endosomal protein sorting machinery and comprises two subcomplexes that operate together to sort membrane proteins (cargo) and tubulate membranes. Tubules are formed by a dimer of sorting nexins, a key component of which is SNX1. Cargo selection is mediated by the VPS35-VPS29-VPS26 trimer, which additionally recruits the WASH complex through VPS35 binding to the WASH complex subunit FAM21. Loss of function of the WASH complex leads to dysregulation of endosome tubulation, although it is unclear how this occurs. Here, we show that FAM21 also binds to the SNX1-interacting DNAJ protein RME-8. Loss of RME-8 causes altered kinetics of SNX1 membrane association and a pronounced increase in highly branched endosomal tubules. Building on previous observations from other laboratories, we show that these tubules contain membrane proteins that are dependent upon WASH complex activity for their localization to the plasma membrane. Therefore, we propose that the interaction between RME-8 and the WASH complex provides a means to coordinate the activity of the WASH complex with the membrane-tubulating function of the sorting nexins at sites where retromer-mediated endosomal protein sorting occurs.

Retromer: a master conductor of endosome sorting

The endosomal network comprises an interconnected network of membranous compartments whose primary function is to receive, dissociate, and sort cargo that originates from the plasma membrane and the biosynthetic pathway. A major challenge in cell biology is to achieve a thorough molecular description of how this network operates, and in so doing, how defects contribute to the etiology and pathology of human disease. We discuss the increasing body of evidence that implicates an ancient evolutionary conserved complex, termed "retromer," as a master conductor in the complex orchestration of multiple cargo-sorting events within the endosomal network.

Hyperleucinemia causes hippocampal retromer deficiency linking diabetes to Alzheimer's disease

Type 2 diabetes (T2D) is a major risk factor for late-onset Alzheimer's disease (AD). A variety of metabolic changes related to T2D (e.g. hyperinsulinemia, hyperglycemia, and elevated branched-chain amino acids) have been proposed as mechanistic links, but the basis for this association remains unknown. Retromer-dependent trafficking is implicated in the pathogenesis of AD, and two key retromer proteins, VPS35 and VPS26, are deficient in the hippocampal formation of AD patients. We characterized VPS35 levels in five different mouse models of T2D/obesity to identify specific metabolic factors that could affect retromer in the brain. Mouse models in which hyperleucinemia was present displayed hippocampus-selective retromer deficiency. Wild-type lean mice fed a high leucine diet also developed hippocampal-selective retromer deficiency, and neuronal-like cells grown in high ambient leucine had reduced retromer complex proteins. Our results suggest that hyperleucinemia may account, in part, for the association of insulin resistance/T2D with AD.

A mechanism for retromer endosomal coat complex assembly with cargo

Retromer is an evolutionarily conserved protein complex composed of the VPS26, VPS29, and VPS35 proteins that selects and packages cargo proteins into transport carriers that export cargo from the endosome. The mechanisms by which retromer is recruited to the endosome and captures cargo are unknown. We show that membrane recruitment of retromer is mediated by bivalent recognition of an effector of PI3K, SNX3, and the RAB7A GTPase, by the VPS35 retromer subunit. These bivalent interactions prime retromer to capture integral membrane cargo, which enhances membrane association of retromer and initiates cargo sorting. The role of RAB7A is severely impaired by a mutation, K157N, that causes Charcot-Marie-Tooth neuropathy 2B. The results elucidate minimal requirements for retromer assembly on the endosome membrane and reveal how PI3K and RAB signaling are coupled to initiate retromer-mediated cargo export.

Protein sorting motifs in the cytoplasmic tail of SorCS1 control generation of Alzheimer's amyloid-β peptide

Endosomal sorting of the Alzheimer amyloid precursor protein (APP) plays a key role in the biogenesis of the amyloid-β (Aβ) peptide. Genetic lesions underlying Alzheimer's disease (AD) can act by interfering with this physiological process. Specifically, proteins involved in trafficking between endosomal compartments and the trans-Golgi network (TGN) [including the retromer complex (Vps35, Vps26) and its putative receptors (sortilin, SorL1, SorCS1)] have been implicated in the molecular pathology of late-onset AD. Previously, we demonstrated a role for SorCS1 in APP metabolism and Aβ production and, while we implicated a role for the retromer in this regulation, the underlying mechanism remained poorly understood. Here, we provide evidence for a motif within the SorCS1c cytoplasmic tail that, when manipulated, results in perturbed sorting of APP and/or its fragments to endosomal compartments, decreased retrograde TGN trafficking, and increased Aβ production in H4 neuroglioma cells. These perturbations apparently do not involve turnover of the cell surface APP pool, but rather they involve intracellular APP and/or its fragments, downstream of APP endocytosis.

SorLA is a molecular link for retromer-dependent sorting of the Amyloid precursor protein

Deficiency in the retromer sorting pathway is known to be associated with the onset of Alzheimer disease (AD), and has been suggested to involve regulation of Amyloid precursor protein (APP) trafficking. Absence of the APP sorting receptor sorLA is also associated to AD, as amyloidogenic processing of APP is increased due to missorting. Reduced activity of either retromer or sorLA thus both lead to enhanced amyloidogenic APP processing, and these pathways are therefore important factors for understanding the development of AD. It is therefore key to outline the neuronal APP trafficking in order to determine the mechanisms that influence AD onset.

Retromer-mediated endosomal protein sorting: all WASHed up!

Endosomal protein sorting governs the fate of many physiologically important proteins involved in a panoply of cellular functions. Recent discoveries have revealed a vital role for endosomally localised branched actin patches in facilitating protein sorting. The formation of the actin patches has been shown to require the function of the WASH complex - the major endosomal actin polymerisation-promoting complex - which stimulates the activity of the ubiquitously expressed Arp2/3 complex. Another key component of the endosomal protein-sorting machinery is the retromer complex. Studies now show that retromer mediates the recruitment of the WASH complex and its regulators to endosomes. In this review, recent progress in understanding the role of the WASH complex along with retromer in endosomal protein sorting is discussed.

George-Hyslop P, Mayeux R, Haines JL, Pericak-Vance MA, Yoshida M, Nishida N, Tokunaga K, Yamamoto K, Tsuji S, Kanazawa I, Ihara Y, Schellenberg GD, Farrer LA, Kuwano R. SORL1 is genetically associated with late-onset Alzheimer's disease in Japanese, Koreans and Caucasians

To discover susceptibility genes of late-onset Alzheimer's disease (LOAD), we conducted a 3-stage genome-wide association study (GWAS) using three populations: Japanese from the Japanese Genetic Consortium for Alzheimer Disease (JGSCAD), Koreans, and Caucasians from the Alzheimer Disease Genetic Consortium (ADGC). In Stage 1, we evaluated data for 5,877,918 genotyped and imputed SNPs in Japanese cases (n = 1,008) and controls (n = 1,016). Genome-wide significance was observed with 12 SNPs in the APOE region. Seven SNPs from other distinct regions with p-values <2×10(-5) were genotyped in a second Japanese sample (885 cases, 985 controls), and evidence of association was confirmed for one SORL1 SNP (rs3781834, P = 7.33×10(-7) in the combined sample). Subsequent analysis combining results for several SORL1 SNPs in the Japanese, Korean (339 cases, 1,129 controls) and Caucasians (11,840 AD cases, 10,931 controls) revealed genome wide significance with rs11218343 (P = 1.77×10(-9)) and rs3781834 (P = 1.04×10(-8)). SNPs in previously established AD loci in Caucasians showed strong evidence of association in Japanese including rs3851179 near PICALM (P = 1.71×10(-5)) and rs744373 near BIN1 (P = 1.39×10(-4)). The associated allele for each of these SNPs was the same as in Caucasians. These data demonstrate for the first time genome-wide significance of LOAD with SORL1 and confirm the role of other known loci for LOAD in Japanese. Our study highlights the importance of examining associations in multiple ethnic populations.

Snx3 regulates recycling of the transferrin receptor and iron assimilation

Sorting of endocytic ligands and receptors is critical for diverse cellular processes. The physiological significance of endosomal sorting proteins in vertebrates, however, remains largely unknown. Here we report that sorting nexin 3 (Snx3) facilitates the recycling of transferrin receptor (Tfrc) and thus is required for the proper delivery of iron to erythroid progenitors. Snx3 is highly expressed in vertebrate hematopoietic tissues. Silencing of Snx3 results in anemia and hemoglobin defects in vertebrates due to impaired transferrin (Tf)-mediated iron uptake and its accumulation in early endosomes. This impaired iron assimilation can be complemented with non-Tf iron chelates. We show that Snx3 and Vps35, a component of the retromer, interact with Tfrc to sort it to the recycling endosomes. Our findings uncover a role of Snx3 in regulating Tfrc recycling, iron homeostasis, and erythropoiesis. Thus, the identification of Snx3 provides a genetic tool for exploring erythropoiesis and disorders of iron metabolism.

Vps10 family proteins and the retromer complex in aging-related neurodegeneration and diabetes

Members of the vacuolar protein sorting 10 (Vps10) family of receptors (including sortilin, SorL1, SorCS1, SorCS2, and SorCS3) play pleiotropic functions in protein trafficking and intracellular and intercellular signaling in neuronal and non-neuronal cells. Interactions have been documented between Vps10 family members and the retromer coat complex, a key component of the intracellular trafficking apparatus that sorts cargo from the early endosome to the trans-Golgi network. In recent years, genes encoding several members of the Vps10 family of proteins, as well as components of the retromer coat complex, have been implicated as genetic risk factors for sporadic and autosomal dominant forms of neurodegenerative diseases, including Alzheimer's disease, frontotemporal lobar degeneration, and Parkinson's disease, with risk for type 2 diabetes mellitus and atherosclerosis. In addition to their functions in protein trafficking, the Vps10 family proteins modulate neurotrophic signaling pathways. Sortilin can impact the intracellular response to brain-derived neurotrophic factor (BDNF) by regulating anterograde trafficking of Trk receptors to the synapse and direct control of BDNF levels, while both sortilin and SorCS2 function as cell surface receptors to mediate acute responses to proneurotrophins. This mini-review and symposium will highlight the emerging data from this rapidly growing area of research implicating the Vps10 family of receptors and the retromer in physiological intracellular trafficking signaling by neurotrophins and in the pathogenesis of neurodegeneration.

Rab7 mutants associated with Charcot-Marie-Tooth disease cause delayed growth factor receptor transport and altered endosomal and nuclear signaling

Rab7 belongs to the Ras superfamily of small GTPases and is a master regulator of early to late endocytic membrane transport. Four missense mutations in the late endosomal Rab7 GTPase (L129F, K157N, N161T, and V162M) cause the autosomal dominant peripheral neuropathy Charcot-Marie-Tooth type 2B (CMT2B) disease. As yet, the pathological mechanisms connecting mutant Rab7 protein expression to altered neuronal function are undefined. Here, we analyze the effects of Rab7 CMT2B mutants on epidermal growth factor (EGF)-dependent intracellular signaling and trafficking. Three different cell lines expressing Rab7 CMT2B mutants and stimulated with EGF exhibited delayed trafficking of EGF to LAMP1-positive late endosomes and lysosomes and slowed EGF receptor (EGFR) degradation. Expression of all Rab7 CMT2B mutants altered the Rab7 activation cycle, leading to enhanced and prolonged EGFR signaling as well as variable increases in p38 and ERK1/2 activation. However, due to reduced nuclear translocation of p38 and ERK1/2, the downstream nuclear activation of Elk-1 was decreased along with the expression of immediate early genes like c-fos and Egr-1 by the disease mutants. In conclusion, our results demonstrate that Rab7 CMT2B mutants impair growth factor receptor trafficking and, in turn, alter p38 and ERK1/2 signaling from perinuclear, clustered signaling endosomes. The resulting down-regulation of EGFR-dependent nuclear transcription that is crucial for normal axon outgrowth and peripheral innervation offers a crucial new mechanistic insight into disease pathogenesis that is relevant to other neurodegenerative diseases.

The retromer complex - endosomal protein recycling and beyond

The retromer complex is a vital element of the endosomal protein sorting machinery that is conserved across all eukaryotes. Retromer is most closely associated with the endosome-to-Golgi retrieval pathway and is necessary to maintain an active pool of hydrolase receptors in the trans-Golgi network. Recent progress in studies of retromer have identified new retromer-interacting proteins, including the WASH complex and cargo such as the Wntless/MIG-14 protein, which now extends the role of retromer beyond the endosome-to-Golgi pathway and has revealed that retromer is required for aspects of endosome-to-plasma membrane sorting and regulation of signalling events. The interactions between the retromer complex and other macromolecular protein complexes now show how endosomal protein sorting is coordinated with actin assembly and movement along microtubules, and place retromer squarely at the centre of a complex set of protein machinery that governs endosomal protein sorting. Dysregulation of retromer-mediated endosomal protein sorting leads to various pathologies, including neurodegenerative diseases such as Alzheimer disease and spastic paraplegia and the mechanisms underlying these pathologies are starting to be understood. In this Commentary, I will highlight recent advances in the understanding of retromer-mediated endosomal protein sorting and discuss how retromer contributes to a diverse set of physiological processes.