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Rac mediates the formation of plasma membrane ruffles from which activated TrkA is internalized. TrkA-PC12 cells were cotransfected with Rac-T7 and Pincher-HA (A). PC12 cells were transfected with TrkA (B Left) or cotransfected with TrkA and Rac-T7 (B Right) or with TrkA and RacV12-GFP (C). Cells were or were not treated with NGF for the indicated times and fixed and stained as described in Materials and Methods. P-Trk (red, A Upper, B, and C), phalloidin (Actin, green, A), anti-T7 (Rac-T7, cyan in A, green in B), RacV12-GFP (green in C), anti-Pincher (red in A Lower). (Scale bar: 2 m.)
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Why neurotrophins and their Trk receptors promote neuronal differentiation and survival whereas receptor tyrosine kinases for other growth factors, such as EGF, do not, has been a long-standing question in neurobiology. We provide evidence that one difference lies in the selective ability of Trk to generate long-lived signaling endosomes. We show t...
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... This observation also demonstrates that Nav1 is involved with extracellular cues beyond netrin during neuron development. Furthermore, it has been suggested that Trk-containing macropinosomes are marked by the protein Pincher to avoid degradation, and thereby perpetuate long-range signals in response to neurotrophins to promote neural growth (Valdez et al., 2005(Valdez et al., , 2007. Transduction of a variety of extracellular guidance cues may be a general characteristic of the Navigator molecules, possibly acting via fluid-phase uptake mechanisms like macropinocytosis (Figure 7). ...
Neuron navigators (Navigators) are cytoskeletal-associated proteins important for neuron migration, neurite growth, and axon guidance, but they also function more widely in other tissues. Recent studies have revealed novel cellular functions of Navigators such as macropinocytosis, and have implicated Navigators in human disorders of axon growth. Navigators are present in most or all bilaterian animals: vertebrates have three Navigators (NAV1-3), Drosophila has one (Sickie), and Caenorhabditis elegans has one (Unc-53). Structurally, Navigators have conserved N- and C-terminal regions each containing specific domains. The N-terminal region contains a calponin homology (CH) domain and one or more SxIP motifs, thought to interact with the actin cytoskeleton and mediate localization to microtubule plus-end binding proteins, respectively. The C-terminal region contains two coiled-coil domains, followed by a AAA+ family nucleoside triphosphatase domain of unknown activity. The Navigators appear to have evolved by fusion of N- and C-terminal region homologs present in simpler organisms. Overall, Navigators participate in the cytoskeletal response to extracellular cues via microtubules and actin filaments, in conjunction with membrane trafficking. We propose that uptake of fluid-phase cues and nutrients and/or downregulation of cell surface receptors could represent general mechanisms that explain Navigator functions. Future studies developing new models, such as conditional knockout mice or human cerebral organoids may reveal new insights into Navigator function. Importantly, further biochemical studies are needed to define the activities of the Navigator AAA+ domain, and to study potential interactions among different Navigators and their binding partners.
... Enlargement of Rab5-positive endosomes was associated with neurofibrillary tangles and amyloid deposition. Neurotrophic factors such as nerve growth factor (NGF) bind to their Trk receptors and internalize into Rab5-positive endosomes to initiate downstream signaling [38]. It is interesting to note that, the signaling endosome is retained as Rab5 early endosomes, and does not progress to Rab7 late endosomes, during their transit within the long axons [39]. ...
Increased interest in the aging and Alzhei-mer's disease (AD)-related impairments in autophagy in the brain raise important questions about regulation and treatment. Since many steps in endocytosis and autophagy depend on GTPases, new measures of cellular GTP levels are needed to evaluate energy regulation in aging and AD. The recent development of ratiometric GTP sensors (GEVALS) and findings that GTP levels are not homogenous inside cells raise new issues of regulation of GTPases by the local availability of GTP. In this review, we highlight the metabolism of GTP in relation to the Rab GTPases involved in formation of early endosomes, late endosomes, and lysosomal transport to execute the autophagic degradation of damaged cargo. Specific GTPases control macroautophagy (mitophagy), microautophagy, and chaperone-mediated autophagy (CMA). By inference, local GTP levels would control autophagy, if not in excess. Additional levels of control are imposed by the redox state of the cell, including thioredoxin involvement. Throughout this review, we emphasize the age-related changes that could contribute to deficits in GTP and AD. We conclude with prospects for boosting GTP levels and reversing age-related oxida-tive redox shift to restore autophagy. Therefore, GTP levels could regulate the numerous GTPases involved in endocytosis, autophagy, and vesicular trafficking. In aging, metabolic adaptation to a sedentary lifestyle could impair mitochondrial function generating less GTP and redox energy for healthy management of amyloid and tau proteostasis, synaptic function, and inflammation. GTP, the other energy currency, in aging and AD Synthesis of the high-energy molecules ATP and GTP both rely on the redox state of NAD + /NADH and their sufficient concentrations. ATP synthesis from glycolysis is primarily powered by the
... The duration of ERK activation can be either transient and short-lived or sustained, lasting several hours (Muta et al., 2019). Sustained ERK signaling can occur because of activated receptors that are translocated upon ligand stimulation to long-lived signaling endosomes, where they avoid degradation (Bergeron et al., 2016;Valdez et al., 2007;Villaseñor et al., 2015). Notably, ligand-induced IL-4 receptor dimers become enriched in cortical endosomes after IL-4 treatment to stimulate downstream signaling (Kurgonaite et al., 2015), which may explain our observation that M2-type polarization of macrophages is associated with sustained ERK activation between 2 and 8 h after IL-4 treatment. ...
Macrophages undergoing M1- versus M2-type polarization differ significantly in their cell metabolism and cellular functions. Here, global quantitative time-course proteomics and phosphoproteomics paired with transcriptomics provide a comprehensive characterization of temporal changes in cell metabolism, cellular functions, and signaling pathways that occur during the induction phase of M1- versus M2-type polarization. Significant differences in, especially, metabolic pathways are observed, including changes in glucose metabolism, glycosaminoglycan metabolism, and retinoic acid signaling. Kinase-enrichment analysis shows activation patterns of specific kinases that are distinct in M1- versus M2-type polarization. M2-type polarization inhibitor drug screens identify drugs that selectively block M2- but not M1-type polarization, including mitogen-activated protein kinase kinase (MEK) and histone deacetylase (HDAC) inhibitors. These datasets provide a comprehensive resource to identify specific signaling and metabolic pathways that are critical for macrophage polarization. In a proof-of-principle approach, we use these datasets to show that MEK signaling is required for M2-type polarization by promoting peroxisome proliferator-activated receptor-γ (PPARγ)-induced retinoic acid signaling.
... For instance, CME is crucial for the insertion and internalization of neurotransmitter receptors, such as GLUA1, GLUA2, and GLUA3 subunits of AMPA (a-amino-3hydroxy-5-methyl-4-isoxazolepropionic acid) receptors (AMPARs) [276][277][278][279][280], and GABAA receptors [281][282][283], while caveolin-dependent CIE endocytosis has been suggested to regulate the turnover of dopamine D1 receptor [284] and the localization of N-methyl-Daspartate (NMDA) receptors subunits GLUN1, GLUN2, and GLUN2B to postsynaptic lipid rafts [285]. CME is also believed to act as a predominant endocytic pathway for the internalization of BDNFbound TRKB receptors in neurons [8, [286][287][288], although the involvement of CIE and macropinocytosis has also been described [136,289,290]. Following the endocytosis at distal axons, activated TRK complexes are internalized into a so-called 'signaling endosome', an organelle of endosome/amphisome-like identity, which continues to signal moving in a microtubule-mediated manner to the cell body [288,[291][292][293][294][295]. ...
Endocytosis is an essential cellular process required for multiple physiological functions, including communication with the extracellular environment, nutrient uptake, and signaling by the cell surface receptors. In a broad sense, endocytosis is accomplished through either constitutive or ligand‐induced invagination of the plasma membrane, which results in the formation of the plasma membrane‐retrieved endocytic vesicles, which can either be sent for degradation to the lysosomes or recycled back to the PM. This additional function of endocytosis in membrane retrieval has been adopted by excitable cells, such as neurons, for membrane equilibrium maintenance at synapses. The last two decades were especially productive with respect to the identification of brain‐specific functions of the endocytic machinery, which additionally include but not limited to regulation of neuronal differentiation and migration, maintenance of neuron morphology and synaptic plasticity, and prevention of neurotoxic aggregates spreading. In this review, we highlight the current knowledge of brain‐specific functions of endocytic machinery with a specific focus on three brain cell types, neuronal progenitor cells, neurons, and glial cells.
... Neurotrophins (nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), NT-3, NT4) are a family of neurotrophic factors that, together with their cognate receptors the tropomyosin receptor kinases (TRKA, TRKB, TRKC) and the p75 neurotrophin receptor, regulate neuronal death and survival, axonal growth and dendritic branching. It is now well established that CME is the predominant endocytic mode involved in the internalization of ligand-bound TRK receptors in growth cones (Beattie et al., 2000;Cosker & Segal, 2014;Howe et al., 2001;Zheng et al., 2008), although the involvement of CIE has also been described Shao et al., 2002;Valdez et al., 2007). Following the endocytosis at distal axons, activated TRK complexes are internalized into a so-called "signaling endosome", an organelle which continues to signal moving in a microtubule-mediated manner to the cell body (Howe et al., 2001;Suo et al., 2014). ...
Multiple aspects of neuronal physiology crucially depend on two cellular pathways, autophagy and endocytosis. During endocytosis, extracellular components either unbound or recognized by membrane‐localized receptors (termed “cargo”) become internalized into plasma membrane‐derived vesicles. These can serve to either recycle the material back to the plasma membrane or send it for degradation to lysosomes. Autophagy also uses lysosomes as a terminal degradation point, although instead of degrading the plasma membrane‐derived cargo, autophagy eliminates detrimental cytosolic material and intracellular organelles, which are transported to lysosomes by means of double‐membrane vesicles, referred to as autophagosomes. Neurons, like all non‐neuronal cells, capitalize on autophagy and endocytosis to communicate with the environment and maintain protein and organelle homeostasis. Additionally, the highly polarized, post‐mitotic nature of neurons made them adopt these two pathways for cell‐specific functions. These include the maintenance of the synaptic vesicle pool in the pre‐synaptic terminal and the long‐distance transport of signaling molecules. Originally discovered independently from each other, it is now clear that autophagy and endocytosis are closely interconnected and share several common participating molecules. Considering the crucial role of autophagy and endocytosis in cell type‐specific functions in neurons, it is not surprising that defects in both pathways have been linked to the pathology of numerous neurodegenerative diseases. In this review, we highlight the recent knowledge of the role of endocytosis and autophagy in neurons with a special focus on synaptic physiology and discuss how impairments in genes coding for autophagy and endocytosis proteins can cause neurodegeneration.
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... A group of small ras-related GTPase (rab) proteins control vesicular transport to early late endosomes and other organelles along the endosome-lysosomal pathway [137]. Early endosome effector rab5 and late endosome component rab7 regulate nervous growth factor (NGF) signaling [138,139]. Ginsberg et al. collected a population of neurons only from deceased subjects using LMD and analyzed endosomal markers selected by a customized microarray analysis. As a result, there has been a significant upregulation of the early endosome effector genes, including the late endosome genes rab4 and rab5 [140]. ...
... Le complexe NGF/TrkA est internalisé soit par la voie classique dépendante des clathrines (Beattie et al., 2000;Howe et al., 2001) ou par macropinocytose médiée par la protéine PINCHER (Philippidou et al., 2011;Valdez et al., 2005). Le mécanisme de régulation est inconnu mais les deux processus semblent nécessiter les fonctions de la GTPase dynamine (Valdez et al., 2007;Ye et al., 2003) et pourraient être facilités par l'activation des voies PLC-γ1 (Bodmer et al., 2011) et PI3K (Kuruvilla et al., 2000). Différents endosomes peuvent ainsi être formés et aboutirent à des voies différentes. ...
Le laboratoire INSERM U908 a montré le rôle déterminant du facteur de croissance NGF dans l’agressivité du cancer du sein. De précédentes études ont montré que l’inhibition de l’activité de TrkA, le récepteur du NGF, aboutissait à une diminution de la taille des tumeurs in vitro mais aussi dans des modèles pré-cliniques. Néanmoins, ces inhibiteurs ont donné lieu à des essais cliniques décevants. La résistance à ces inhibiteurs est en partie due à l’association de récepteurs membranaires. Plus précisément, mon travail de thèse a permis de démontrer qu’une forme particulière du récepteur CD44 interagit avec TrkA ce qui conduit à une résistance aux traitements. De plus, j’ai également montré l’existence d’une interaction entre TrkA et deux autres récepteurs. Ainsi, j’ai pu déterminer précisément comment ces récepteurs coopèrent afin de développer des inhibiteurs ciblant leur interaction. Ces inhibiteurs pourraient donc s’avérer efficaces pour le traitement de certains cancers du sein. Ma thèse souligne l’importance des complexes de récepteurs dans la plasticité des cellules cancéreuses et leur capacité à s’adapter pour résister aux thérapies ciblées.
... The difference in ERK activity dynamics can be accounted for in part by the amount and dynamics of receptors for these growth factors present at the cell surface. Upon activation by ligands, EGF receptor (EGFR) is rapidly internalized and degraded, whereas TrkA, a receptor for NGF, is translocated to long-lived signaling endosomes, where it avoids degradation and mediates sustained signaling [8,43]. When EGFR is overexpressed, EGF stimulation drives sustained but not transient ERK activation and induces differentiation of PC12 cells [44], suggesting that the decrease in the cell surface EGFR upon ligand stimulation is a critical limiting factor for the duration of ERK activation. ...
The extracellular signal-regulated kinase (ERK) signaling pathway regulates a variety of biological processes including cell proliferation, survival, and differentiation. Since ERK activation promotes proliferation of many types of cells, its deregulated/constitutive activation is among general mechanisms for cancer. Recent advances in bioimaging techniques have enabled to visualize ERK activity in real-time at the single-cell level. Emerging evidence from such approaches suggests unexpectedly complex spatiotemporal dynamics of ERK activity in living cells and animals and their crucial roles in determining cellular responses. In this review, we discuss how ERK activity dynamics are regulated and how they affect biological processes including cell fate decisions, cell migration, embryonic development, tissue homeostasis, and tumorigenesis.
... They showed that sorting of TrkA to signalling carriers depends on the Rho-GTPase Rac and the macropinocytic adaptor Pincher. These markers characterise a particular type of postendocytic organelle that, in contrast to signalling endosomes containing EGF receptors, was protected from lysosomal degradation [126][127][128]. Interestingly, the mechanism of internalisation varies depending on the nature of the receptor, type of neuron, and ligand concentration. ...
... An important morphological feature to help us to distinguish whether the retrograde signalling carriers are endosomes, autophagosomes or MVBs is their singleor multiple-membrane nature. Early studies on the generation of signalling endosomes in PC12 cells showed that after Pincher-mediated macroendocytosis, TrkA receptors were rapidly associated to MVBs [128]. Interestingly, when subcellular fractionation was used to isolate NGF-containing postendocytic organelles in PC12 cells, TrkA and p75 NTR were found in different populations of endosomal vesicles, translucent and electron dense, respectively. ...
... Evidence showing that multiple-membrane organelles are a common destination for retrogradely transported neurotrophic factors in the soma and dendrites appears to be consistent between cell lines and neuronal types, including superior cervical ganglia and hippocampal neurons [126][127][128] and embryonic stem cells-derived motor neurons [153]. Whether the signalling complexes are transported in multiple-membrane organelles along the axon, however, is less clear from ultrastructural studies. ...
Neurons are highly complex and polarised cells that must overcome a series of logistic challenges to maintain homeostasis across their morphological domains. A very clear example is the propagation of neurotrophic signalling from distal axons, where target‐released neurotrophins bind to their receptors and initiate signalling, towards the cell body, where nuclear and cytosolic responses are integrated. The mechanisms of propagation of neurotrophic signalling have been extensively studied and, eventually, the model of a ‘signalling endosome’, transporting activated receptors and associated complexes, has emerged. Nevertheless, the exact nature of this organelle remains elusive. In this Review, we examine the evidence for the retrograde transport of neurotrophins and their receptors in endosomes, outline some of their diverse physiological and pathological roles, and discuss the main interactors, morphological features and trafficking destinations of a highly flexible endosomal signalling organelle with multiple molecular signatures. This article is protected by copyright. All rights reserved.
... All of these Rac1 effectors are critical for actin organization at the plasma membrane, which in addition to controlling formation of membrane protrusions such as lamellipodia in migrating cells, also influences endocytosis Soriano-Castell et al. 2017). For instance, Rac1 impacts on CIE of receptors, such as IL-2R (Grassart et al. 2008;Lamaze et al. 2001), fluid phase ingestion, TrkA receptor internalization via macropinocytosis (Valdez et al. 2007), as well as phagocytosis of pathogens (Etienne-Manneville and Hall 2002;Criss et al. 2001). Interestingly, the recently identified novel Rac1 effector amyotrophic lateral sclerosis 2 (ALS2) gene, which is vital for motor neurons, is a Rab5-GEF driving Rac1 activation for macropinocytosis and the subsequent fusion of macropinosomes with EEs (Kunita et al. 2007). ...
The endocytic compartment is not only the functional continuity of the plasma membrane but consists of a diverse collection of intracellular heterogeneous complex structures that transport, amplify, sustain, and/or sort signaling molecules. Over the years, it has become evident that early, late, and recycling endosomes represent an interconnected vesicular-tubular network able to form signaling platforms that dynamically and efficiently translate extracellular signals into biological outcome. Cell activation, differentiation, migration, death, and survival are some of the endpoints of endosomal signaling. Hence, to understand the role of the endosomal system in signal transduction in space and time, it is therefore necessary to dissect and identify the plethora of decoders that are operational in the different steps along the endocytic pathway. In this chapter, we focus on the regulation of spatiotemporal signaling in cells, considering endosomes as central platforms, in which several small GTPases proteins of the Ras superfamily, in particular Ras and Rac1, actively participate to control cellular processes like proliferation and cell mobility.
