Regulated membrane recruitment of dynamin-2 mediated by sorting nexin 9

The Chemical Inhibitors of Endocytosis: From Mechanisms to Potential Clinical Applications

Endocytosis is one of the major ways cells communicate with their environment. This process is frequently hijacked by pathogens. Endocytosis also participates in the oncogenic transformation. Here, we review the approaches to inhibit endocytosis, discuss chemical inhibitors of this process, and discuss potential clinical applications of the endocytosis inhibitors.

Role of Clathrin and Dynamin in Clathrin Mediated Endocytosis/Synaptic Vesicle Recycling and Implications in Neurological Diseases

Endocytosis is a process essential to the health and well-being of cell. It is required for the internalisation and sorting of “cargo”—the macromolecules, proteins, receptors and lipids of cell signalling. Clathrin mediated endocytosis (CME) is one of the key processes required for cellular well-being and signalling pathway activation. CME is key role to the recycling of synaptic vesicles [synaptic vesicle recycling (SVR)] in the brain, it is pivotal to signalling across synapses enabling intracellular communication in the sensory and nervous systems. In this review we provide an overview of the general process of CME with a particular focus on two key proteins: clathrin and dynamin that have a central role to play in ensuing successful completion of CME. We examine these two proteins as they are the two endocytotic proteins for which small molecule inhibitors, often of known mechanism of action, have been identified. Inhibition of CME offers the potential to develop therapeutic interventions into conditions involving defects in CME. This review will discuss the roles and the current scope of inhibitors of clathrin and dynamin, providing an insight into how further developments could affect neurological disease treatments.

Common Pathogenic Mechanisms in Centronuclear and Myotubular Myopathies and Latest Treatment Advances

Centronuclear myopathies (CNM) are rare congenital disorders characterized by muscle weakness and structural defects including fiber hypotrophy and organelle mispositioning. The main CNM forms are caused by mutations in: the MTM1 gene encoding the phosphoinositide phosphatase myotubularin (myotubular myopathy), the DNM2 gene encoding the mechanoenzyme dynamin 2, the BIN1 gene encoding the membrane curvature sensing amphiphysin 2, and the RYR1 gene encoding the skeletal muscle calcium release channel/ryanodine receptor. MTM1, BIN1, and DNM2 proteins are involved in membrane remodeling and trafficking, while RyR1 directly regulates excitation-contraction coupling (ECC). Several CNM animal models have been generated or identified, which confirm shared pathological anomalies in T-tubule remodeling, ECC, organelle mispositioning, protein homeostasis, neuromuscular junction, and muscle regeneration. Dynamin 2 plays a crucial role in CNM physiopathology and has been validated as a common therapeutic target for three CNM forms. Indeed, the promising results in preclinical models set up the basis for ongoing clinical trials. Another two clinical trials to treat myotubular myopathy by MTM1 gene therapy or tamoxifen repurposing are also ongoing. Here, we review the contribution of the different CNM models to understanding physiopathology and therapy development with a focus on the commonly dysregulated pathways and current therapeutic targets.

PI(3,4)P2-mediated membrane tubulation promotes integrin trafficking and invasive cell migration

Invadopodia are integrin-mediated adhesions with abundant PI(3,4)P₂ However, the functional role of PI(3,4)P₂ in adhesion signaling remains unclear. Here, we find that the PI(3,4)P₂ biogenesis regulates the integrin endocytosis at invadopodia. PI(3,4)P₂ is locally produced by PIK3CA and SHIP2 and is concentrated at the trailing edge of the invadopodium arc. The PI(3,4)P₂-rich compartment locally forms small puncta (membrane buds) in a SNX9-dependent manner, recruits dynein activator Hook1 through AKTIP, and rearranges into micrometer-long tubular invaginations (membrane tubes). The uncurving membrane tube extends rapidly, follows the retrograde movement of dynein along microtubule tracks, and disconnects from the plasma membrane. Activated integrin-beta3 is locally internalized through the pathway of PI(3,4)P₂-mediated membrane invagination and is then actively recycled. Blockages of PI3K, SHIP2, and SNX9 suppress integrin-beta3 endocytosis, delay adhesion turnover, and impede transwell invasion of MEF-Src and MDA-MB-231 cells. Thus, the production of PI(3,4)P₂ promotes invasive cell migration by stimulating the trafficking of integrin receptor at the invadopodium.

Actin filament- and Wiskott-Aldrich syndrome protein-binding sites on fructose-1,6-bisphosphate aldolase are functionally distinct from the active site

The glycolytic enzyme fructose 1,6-(bis)phosphate aldolase (aldolase) is not only required for efficient utilization of glucose and fructose, but also for cytoskeletal functions like cytokinesis and cell motility. These differing roles are mediated by distinct and discrete binding interactions with aldolase's many binding partners, including actin filaments, Wiskott-Aldrich Syndrome protein (WASP), and Sorting Nexin 9 (SNX9). How these interactions are coordinated on the aldolase homotetramer of 160 kDa is unclear. In this study, the catalytic activity of wild-type aldolase is measured in the presence of actin filaments, and a WASP-derived peptide that binds to aldolase, or both. No appreciable changes in k_cat or K_m values are seen. Then, aldolase variants with substitutions targeting the tryptophan-binding pocket for WASP and SNX9 are created and perturbation of actin filament-, WASP peptide-, and SNX9 peptide-binding are assessed. Those that negatively impacted binding did not show an impact on aldolase catalysis. These results suggest that aldolase can engage in catalysis while simultaneously interacting with cytoskeletal machinery.

The physical basis and practical consequences of biological promiscuity

Proteins interact with metabolites, nucleic acids, and other proteins to orchestrate the myriad catalytic, structural and regulatory functions that support life from the simplest microbes to the most complex multicellular organisms. These molecular interactions are often exquisitely specific, but never perfectly so. Adventitious "promiscuous" interactions are ubiquitous due to the thousands of macromolecules and small molecules crowded together in cells. Such interactions may perturb protein function at the molecular level, but as long as they do not compromise organismal fitness, they will not be removed by natural selection. Although promiscuous interactions are physiologically irrelevant, they are important because they can provide a vast reservoir of potential functions that can provide the starting point for evolution of new functions, both in nature and in the laboratory.

Fructose acutely stimulates NKCC2 activity in rat thick ascending limbs by increasing surface NKCC2 expression

The thick ascending limb (TAL) reabsorbs 25% of the filtered NaCl through the Na⁺-K⁺-2Cl^- cotransporter (NKCC2). NKCC2 activity is directly related to surface NKCC2 expression and phosphorylation. Higher NaCl reabsorption by TALs is linked to salt-sensitive hypertension, which is linked to consumption of fructose in the diet. However, little is known about the effects of fructose on renal NaCl reabsorption. We hypothesized that fructose, but not glucose, acutely enhances TAL-dependent NaCl reabsorption by increasing NKCC2 activity via stimulation of surface NKCC2 levels and phosphorylation at Thr^96/101. We found that fructose (5 mM) increased transport-related O₂ consumption in TALs by 11.1 ± 3.2% ( P < 0.05). The effect of fructose on O₂ consumption was blocked by furosemide. To study the effect of fructose on NKCC2 activity, we measured the initial rate of NKCC2-dependent thallium influx. We found that 20 min of treatment with fructose (5 mM) increased NKCC2 activity by 58.5 ± 16.9% ( P < 0.05). We then used surface biotinylation to measure surface NKCC2 levels in rat TALs. Fructose increased surface NKCC2 expression in a concentration-dependent manner (22 ± 5,  49 ± 10, and 101 ± 59% of baseline with 1, 5, and 10 mM fructose, respectively, P < 0.05), whereas glucose or a glucose metabolite did not. Fructose did not change NKCC2 phosphorylation at Thre^96/101 or total NKCC2 expression. We concluded that acute fructose treatment increases NKCC2 activity by enhancing surface NKCC2 expression, rather than NKCC2 phosphorylation. Our data suggest that fructose consumption could contribute to salt-sensitive hypertension by stimulating NKCC2-dependent NaCl reabsorption in TALs.

Chlamydia exploits filopodial capture and a macropinocytosis-like pathway for host cell entry

Pathogens hijack host endocytic pathways to force their own entry into eukaryotic target cells. Many bacteria either exploit receptor-mediated zippering or inject virulence proteins directly to trigger membrane reorganisation and cytoskeletal rearrangements. By contrast, extracellular C. trachomatis elementary bodies (EBs) apparently employ facets of both the zipper and trigger mechanisms and are only ~400 nm in diameter. Our cryo-electron tomography of C. trachomatis entry revealed an unexpectedly diverse array of host structures in association with invading EBs, suggesting internalisation may progress by multiple, potentially redundant routes or several sequential events within a single pathway. Here we performed quantitative analysis of actin organisation at chlamydial entry foci, highlighting filopodial capture and phagocytic cups as dominant and conserved morphological structures early during internalisation. We applied inhibitor-based screening and employed reporters to systematically assay and visualise the spatio-temporal contribution of diverse endocytic signalling mediators to C. trachomatis entry. In addition to the recognised roles of the Rac1 GTPase and its associated nucleation-promoting factor (NPF) WAVE, our data revealed an additional unrecognised pathway sharing key hallmarks of macropinocytosis: i) amiloride sensitivity, ii) fluid-phase uptake, iii) recruitment and activity of the NPF N-WASP, and iv) the localised generation of phosphoinositide-3-phosphate (PI3P) species. Given their central role in macropinocytosis and affinity for PI3P, we assessed the role of SNX-PX-BAR family proteins. Strikingly, SNX9 was specifically and transiently enriched at C. trachomatis entry foci. SNX9^-/- cells exhibited a 20% defect in EB entry, which was enhanced to 60% when the cells were infected without sedimentation-induced EB adhesion, consistent with a defect in initial EB-host interaction. Correspondingly, filopodial capture of C. trachomatis EBs was specifically attenuated in SNX9^-/- cells, implicating SNX9 as a central host mediator of filopodial capture early during chlamydial entry. Our findings identify an unanticipated complexity of signalling underpinning cell entry by this major human pathogen, and suggest intriguing parallels with viral entry mechanisms.

Chlamydia trachomatis remains the leading bacterial agent of sexually transmitted disease worldwide and causes a form of blindness called trachoma in Developing nations, which is recognised by the World Health Organisation as a neglected tropical disease. Despite this burden, we know comparatively little about how it causes disease at a molecular level. Chlamydia must live inside human cells to survive, and here we study the mechanism of how it enters cells, which is critical to the lifecycle. We study how the bacterium exploits signalling pathways inside the cell to its own advantage to deform the cell membrane by reorganising the underlying cell skeleton, and identify new factors involved in this process. Our findings suggest intriguing similarities with how some viruses enter cells. A better understanding of these processes may help to develop future vaccines and new treatments.

Sorting nexin 9 (SNX9) is not essential for development and auditory function in mice

Sorting nexins are a large family of evolutionarily conserved proteins that play fundamental roles in endocytosis, endosomal sorting and signaling. As an important member of sorting nexin family, sorting nexin 9 (SNX9) has been shown to participate in coordinating actin polymerization with membrane tubulation and vesicle formation. We previously showed that SNX9 is expressed in mouse auditory hair cells and might regulate actin polymerization in those cells. To further examine the physiological role of SNX9, we generated Snx9 knockout mice using homologous recombination method. Unexpectedly, Snx9 knockout mice have normal viability and fertility, and are morphologically and behaviorally indistinguishable from control mice. Further investigation revealed that the morphology and function of auditory hair cells are not affected by Snx9 inactivation, and Snx9 knockout mice have normal hearing threshold. In conclusion, our data revealed that Snx9-deficient mice do not show defects in development as well as auditory function, suggesting that SNX9 is not essential for mice development and hearing.

Large G protein α-subunit XLαs limits clathrin-mediated endocytosis and regulates tissue iron levels in vivo

Alterations in the activity/levels of the extralarge G protein α-subunit (XLαs) are implicated in various human disorders, such as perinatal growth retardation. Encoded by GNAS, XLαs is partly identical to the α-subunit of the stimulatory G protein (Gsα), but the cellular actions of XLαs remain poorly defined. Following an initial proteomic screen, we identified sorting nexin-9 (SNX9) and dynamins, key components of clathrin-mediated endocytosis, as binding partners of XLαs. Overexpression of XLαs in HEK293 cells inhibited internalization of transferrin, a process that depends on clathrin-mediated endocytosis, while its ablation by CRISPR/Cas9 in an osteocyte-like cell line (Ocy454) enhanced it. Similarly, primary cardiomyocytes derived from XLαs knockout (XLKO) pups showed enhanced transferrin internalization. Early postnatal XLKO mice showed a significantly higher degree of cardiac iron uptake than wild-type littermates following iron dextran injection. In XLKO neonates, iron and ferritin levels were elevated in heart and skeletal muscle, where XLαs is normally expressed abundantly. XLKO heart and skeletal muscle, as well as XLKO Ocy454 cells, showed elevated SNX9 protein levels, and siRNA-mediated knockdown of SNX9 in XLKO Ocy454 cells prevented enhanced transferrin internalization. In transfected cells, XLαs also inhibited internalization of the parathyroid hormone and type 2 vasopressin receptors. Internalization of transferrin and these G protein-coupled receptors was also inhibited in cells expressing an XLαs mutant missing the Gα portion, but not Gsα or an N-terminally truncated XLαs mutant unable to interact with SNX9 or dynamin. Thus, XLαs restricts clathrin-mediated endocytosis and plays a critical role in iron/transferrin uptake in vivo.

Sorting Nexin 9 facilitates podocin endocytosis in the injured podocyte

The irreversibility of glomerulosclerotic changes depends on the degree of podocyte injury. We have previously demonstrated the endocytic translocation of podocin to the subcellular area in severely injured podocytes and found that this process is the primary disease trigger. Here we identified the protein sorting nexin 9 (SNX9) as a novel facilitator of podocin endocytosis in a yeast two-hybrid analysis. SNX9 is involved in clathrin-mediated endocytosis, actin rearrangement and vesicle transport regulation. Our results revealed and confirmed that SNX9 interacts with podocin exclusively through the Bin–Amphiphysin–Rvs (BAR) domain of SNX9. Immunofluorescence staining revealed the expression of SNX9 in response to podocyte adriamycin-induced injury both in vitro and in vivo. Finally, an analysis of human glomerular disease biopsy samples demonstrated strong SNX9 expression and co-localization with podocin in samples representative of severe podocyte injury, such as IgA nephropathy with poor prognosis, membranous nephropathy and focal segmental glomerulosclerosis. In conclusion, we identified SNX9 as a facilitator of podocin endocytosis in severe podocyte injury and demonstrated the expression of SNX9 in the podocytes of both nephropathy model mice and human patients with irreversible glomerular disease.

Endocytosis, Metastasis and Beyond: Multiple Facets of SNX9

Sorting nexin (SNX)9 was first discovered as an endocytic accessory protein involved in clathrin-mediated endocytosis. However, recent data suggest that SNX9 is a multifunctional scaffold that coordinates membrane trafficking and remodeling with changes in actin dynamics to affect diverse cellular processes. Here, we review the accumulated knowledge on SNX9 with an emphasis on its recently identified roles in clathrin-independent endocytic pathways, cell invasion, and cell division, which have implications for SNX9 function in human disease, including cancer.

Developmentally programmed germ cell remodelling by endodermal cell cannibalism

Primordial germ cells (PGCs) in many species associate intimately with endodermal cells, but the significance of such interactions is largely unexplored. Here, we show that Caenorhabditis elegans PGCs form lobes that are removed and digested by endodermal cells, dramatically altering PGC size and mitochondrial content. We demonstrate that endodermal cells do not scavenge lobes PGCs shed, but rather, actively remove lobes from the cell body. CED-10 (Rac)-induced actin, DYN-1 (dynamin) and LST-4 (SNX9) transiently surround lobe necks and are required within endodermal cells for lobe scission, suggesting that scission occurs through a mechanism resembling vesicle endocytosis. These findings reveal an unexpected role for endoderm in altering the contents of embryonic PGCs, and define a form of developmentally programmed cell remodelling involving intercellular cannibalism. Active roles for engulfing cells have been proposed in several neuronal remodelling events, suggesting that intercellular cannibalism may be a more widespread method used to shape cells than previously thought.

Sorting nexin 9 negatively regulates invadopodia formation and function in cancer cells

The ability of cancer cells to degrade the extracellular matrix and invade interstitial tissues contributes to their metastatic potential. We recently showed that overexpression of sorting nexin 9 (SNX9) leads to increased cell invasion and metastasis in animal models, which correlates with increased SNX9 protein expression in metastases from human mammary cancers. Here, we report that SNX9 expression is reduced relative to neighboring normal tissues in primary breast tumors, and progressively reduced in more aggressive stages of non-small-cell lung cancers. We show that SNX9 is localized at invadopodia where it directly binds the invadopodia marker TKS5 and negatively regulates invadopodia formation and function. SNX9 depletion increases invadopodia number and the local recruitment of MT1-MMP by decreasing its internalization. Together, these effects result in increased localized matrix degradation. We further identify SNX9 as a Src kinase substrate and show that this phosphorylation is important for SNX9 activity in regulating cell invasion, but is dispensable for its function in regulating invadopodia. The diversified changes associated with SNX9 expression in cancer highlight its importance as a central regulator of cancer cell behavior.

PtdIns(4,5)P₂ and PtdIns3P coordinate to regulate phagosomal sealing for apoptotic cell clearance

A coincidence detection mechanism regulates phagosomal sealing and couples it with phosphoinositide conversion from PtdIns(4,5)P₂ enrichment on unsealed phagosomes to PtdIns3P enrichment on fully sealed phagosomes.

Phagocytosis requires phosphoinositides (PIs) as both signaling molecules and localization cues. How PIs coordinate to control phagosomal sealing and the accompanying switch of organelle identity is unclear. In this study, we followed dynamic changes in PIs during apoptotic cell clearance in Caenorhabditis elegans. We found that phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P₂) and phosphatidylinositol-3-phosphate (PtdIns3P), which accumulate transiently on unsealed and fully sealed phagosomes, respectively, are both involved in phagosome closure. We identified PtdIns3P phosphatase MTM-1 as an effector of PtdIns(4,5)P₂ to promote phagosomal sealing. MTM-1 coordinates with the class II PI3 kinase PIKI-1 to control PtdIns3P levels on unsealed phagosomes. The SNX9 family protein LST-4 is required for sealing, and its association with unsealed phagosomes is regulated by PtdIns(4,5)P₂, PIKI-1, and MTM-1. Loss of LST-4 or its retention on phagosomes disrupts sealing and suppresses PtdIns3P accumulation, indicating close coupling of the two events. Our findings support a coincidence detection mechanism by which phagosomal sealing is regulated and coupled with conversion from PtdIns(4,5)P₂ enrichment on unsealed phagosomes to PtdIns3P enrichment on fully sealed phagosomes.

Pervanadate induces Mammalian Ste20 Kinase 3 (MST3) tyrosine phosphorylation but not activation

The yeast Ste20 (sterile) protein kinase, which is a serine/threonine kinase, responds to the stimulation of the G proteincoupled receptor (GPCR) pheromone receptor. Ste20 protein kinase serves as the critical component that links signaling from the GPCR/G proteins to the mitogen-activated protein kinase (MAPK) cascade in yeast. The yeast Ste20p functions as a MAP kinase kinase kinase kinase (MAP4K) in the pheromone response. Ste20-like kinases are structurally conserved from yeast to mammals. The mechanism by which MAP4K links GPCR to the MAPK pathway is less clearly defined in vertebrates. In addition to MAP4K, the tyrosine kinase cascade bridges G proteins and the MAPK pathway in vertebrate cells. Mammalian Ste20 Kinase 3 (MST3) has been categorized into the Ste20 family and has been reported to function in the regulation of cell polarity and migration. However, whether MST3 tyrosine phosphorylation regulates diverse signaling pathways is unknown. In this study, the tyrosine phosphatase inhibitor pervanadate was found to induce MST3 tyrosine phosphorylation in intact cells, and the activity of tyrosine-phosphorylated MST3 was measured. This tyrosine-directed phosphorylation was independent of MST3 activity. Parameters including protein conformation, Triton concentration and ionic concentration influenced the sensitivity of MST3 activity. Taken together, our data suggests that the serine/threonine kinase MST3 undergoes tyrosinedirected phosphorylation. The tyrosine-phosphorylated MST3 may create a docking site for the structurally conserved SH2/SH3 (Src Homology 2 and 3) domains within the Src oncoprotein. The unusual tyrosinephosphorylated MST3 may recruit MST3 to various signaling components.

SNX9 promotes metastasis by enhancing cancer cell invasion via differential regulation of RhoGTPases

Despite current advances in cancer research, metastasis remains the leading factor in cancer-related deaths. Here, we identify sorting nexin 9 (SNX9) as a new regulator of breast cancer metastasis. We detected an increase in SNX9 expression in human breast cancer metastases compared with primary tumors and demonstrated that SNX9 expression in MDA-MB-231 breast cancer cells is necessary to maintain their ability to metastasize in a chick embryo model. Reciprocally, SNX9 knockdown impairs the process. In vitro studies using several cancer cell lines derived from a variety of human tumors revealed a role for SNX9 in cell invasion and identified mechanisms responsible for this novel function. We showed that SNX9 controls the activation of RhoA and Cdc42 GTPases and also regulates cell motility via the modulation of well-known molecules involved in metastasis, namely RhoA-ROCK and N-WASP. In addition, we have discovered that SNX9 is required for RhoGTPase-dependent, clathrin-independent endocytosis, and in this capacity, can functionally substitute to the bona fide Rho GAP, GRAF1 (GTPase Regulator Associated with Focal Adhesion Kinase). Together, our data establish novel roles for SNX9 as a multifunctional protein scaffold that regulates, and potentially coordinates, several cellular processes that together can enhance cancer cell metastasis.

Evolutionary Changes on the Way to Clathrin-Mediated Endocytosis in Animals

Endocytic pathways constitute an evolutionarily ancient system that significantly contributed to the eukaryotic cell architecture and to the diversity of cell type-specific functions and signaling cascades, in particular of metazoans. Here we used comparative proteomic studies to analyze the universal internalization route in eukaryotes, clathrin-mediated endocytosis (CME), to address the issues of how this system evolved and what are its specific features. Among 35 proteins crucially required for animal CME, we identified a subset of 22 proteins common to major eukaryotic branches and 13 gradually acquired during evolution. Based on exploration of structure-function relationship between conserved homologs in sister, distantly related and early diverged branches, we identified novel features acquired during evolution of endocytic proteins on the way to animals: Elaborated way of cargo recruitment by multiple sorting proteins, structural changes in the core endocytic complex AP2, the emergence of the Fer/Cip4 homology domain-only protein/epidermal growth factor receptor substrate 15/intersectin functional complex as an additional interaction hub and activator of AP2, as well as changes in late endocytic stages due to recruitment of dynamin/sorting nexin 9 complex and involvement of the actin polymerization machinery. The evolutionary reconstruction showed the basis of the CME process and its subsequent step-by-step development. Documented changes imply more precise regulation of the pathway, as well as CME specialization for the uptake of specific cargoes and cell type-specific functions.

Suppression of fructose-bisphosphate aldolase C expression as a predictor of advanced oral squamous cell carcinoma

BACKGROUND

Glycolysis machinery regulates cancer cell behavior. However, the roles of these glycolysis enzymes in oral squamous cell carcinoma (OSCC) progression remain unknown.

METHODS

Fructose-bisphosphate aldolase C (ALDOC) expression in OSCC patients and cell lines was detected using quantitative real-time polymerase chain reaction (PCR). The functions of ALDOC in migration and invasion were determined using gain and loss of function approaches. An orthotopic OSCC animal model was performed to investigate the effects of ALDOC on metastasis and tumorigenesis in vivo.

RESULTS

ALDOC expression is negatively significantly correlated with clinical outcome and cell migration in vitro and in vivo. ALDOC blocks adenosine triphosphate generation and lactate production, and mutation constructs of Arg42 and Lys146 functionally restore ALDOC-inhibited cell migration and invasion.

CONCLUSION

ALDOC functions as an OSCC prognosis marker clinically, and suppresses migration and invasion by its catalytic domain of Arg42 and Lys146. © 2015 Wiley Periodicals, Inc. Head Neck 38: E1075-E1085, 2016.

Metabolic clues to salubrious longevity in the brain of the longest-lived rodent: the naked mole-rat

Naked mole-rats (NMRs) are the oldest-living rodent species. Living underground in a thermally stable ecological niche, NMRs have evolved certain exceptional traits, resulting in sustained health spans, negligible cognitive decline, and a pronounced resistance to age-related disease. Uncovering insights into mechanisms underlying these extraordinary traits involved in successful aging may conceivably provide crucial clues to extend the human life span and health span. One of the most fundamental processes inside the cell is the production of ATP, which is an essential fuel in driving all other energy-requiring cellular activities. Not surprisingly, a prominent hallmark in age-related diseases, such as neurodegeneration and cancer, is the impairment and dysregulation of metabolic pathways. Using a two-dimensional polyacrylamide gel electrophoresis proteomics approach, alterations in expression and phosphorylation levels of metabolic proteins in the brains of NMRs, aged 2-24 years, were evaluated in an age-dependent manner. We identified 13 proteins with altered levels and/or phosphorylation states that play key roles in various metabolic pathways including glycolysis, β-oxidation, the malate-aspartate shuttle, the Tricarboxylic Acid Cycle (TCA) cycle, the electron transport chain, NADPH production, as well as the production of glutamate. New insights into potential pathways involved in metabolic aspects of successful aging have been obtained by the identification of key proteins through which the NMR brain responds and adapts to the aging process and how the NMR brain adapted to resist age-related degeneration. This study examines the changes in the proteome and phosphoproteome in the brain of the naked mole-rat aged 2-24 years. We identified 13 proteins (labeled in red) with altered expression and/or phosphorylation levels that are conceivably associated with sustained metabolic functions in the oldest NMRs that may promote a sustained health span and life span.

Aldolase sequesters WASP and affects WASP/Arp2/3-stimulated actin dynamics

In addition to its roles in sugar metabolism, fructose-1,6-bisphosphate aldolase (aldolase) has been implicated in cellular functions independent from these roles, termed "moonlighting functions." These moonlighting functions likely involve the known aldolase-actin interaction, as many proteins with which aldolase interacts are involved in actin-dependent processes. Specifically, aldolase interacts both in vitro and in cells with Wiskott-Aldrich Syndrome Protein (WASP), a protein involved in controlling actin dynamics, yet the function of this interaction remains unknown. Here, the effect of aldolase on WASP-dependent processes in vitro and in cells is investigated. Aldolase inhibits WASP/Arp2/3-dependent actin polymerization in vitro. In cells, knockdown of aldolase results in a decreased rate of cell motility and cell spreading, two WASP-dependent processes. Expression of exogenous aldolase rescues these defects. Whether these effects of aldolase on WASP-dependent processes were due to aldolase catalysis or moonlighting functions is tested using aldolase variants defective in either catalytic or actin-binding activity. While the actin-binding deficient aldolase variant is unable to inhibit actin polymerization in vitro and is unable to rescue cell motility defects in cells, the catalytically inactive aldolase is able to perform these functions, providing evidence that aldolase moonlighting plays a role in WASP-mediated processes.

Sorting nexin 9 recruits clathrin heavy chain to the mitotic spindle for chromosome alignment and segregation

Sorting nexin 9 (SNX9) and clathrin heavy chain (CHC) each have roles in mitosis during metaphase. Since the two proteins directly interact for their other cellular function in endocytosis we investigated whether they also interact for metaphase and operate on the same pathway. We report that SNX9 and CHC functionally interact during metaphase in a specific molecular pathway that contributes to stabilization of mitotic spindle kinetochore (K)-fibres for chromosome alignment and segregation. This function is independent of their endocytic role. SNX9 residues in the clathrin-binding low complexity domain are required for CHC association and for targeting both CHC and transforming acidic coiled-coil protein 3 (TACC3) to the mitotic spindle. Mutation of these sites to serine increases the metaphase plate width, indicating inefficient chromosome congression. Therefore SNX9 and CHC function in the same molecular pathway for chromosome alignment and segregation, which is dependent on their direct association.

Targeting of several glycolytic enzymes using RNA interference reveals aldolase affects cancer cell proliferation through a non-glycolytic mechanism

In cancer, glucose uptake and glycolysis are increased regardless of the oxygen concentration in the cell, a phenomenon known as the Warburg effect. Several (but not all) glycolytic enzymes have been investigated as potential therapeutic targets for cancer treatment using RNAi. Here, four previously untargeted glycolytic enzymes, aldolase A, glyceraldehyde 3-phosphate dehydrogenase, triose phosphate isomerase, and enolase 1, are targeted using RNAi in Ras-transformed NIH-3T3 cells. Of these enzymes, knockdown of aldolase causes the greatest effect, inhibiting cell proliferation by 90%. This defect is rescued by expression of exogenous aldolase. However, aldolase knockdown does not affect glycolytic flux or intracellular ATP concentration, indicating a non-metabolic cause for the cell proliferation defect. Furthermore, this defect could be rescued with an enzymatically dead aldolase variant that retains the known F-actin binding ability of aldolase. One possible model for how aldolase knockdown may inhibit transformed cell proliferation is through its disruption of actin-cytoskeleton dynamics in cell division. Consistent with this hypothesis, aldolase knockdown cells show increased multinucleation. These results are compared with other studies targeting glycolytic enzymes with RNAi in the context of cancer cell proliferation and suggest that aldolase may be a useful target in the treatment of cancer.

Dynamin2, clathrin, and lipid rafts mediate endocytosis of the apical Na/K/2Cl cotransporter NKCC2 in thick ascending limbs

Steady-state surface levels of the apical Na/K/2Cl cotransporter NKCC2 regulate NaCl reabsorption by epithelial cells of the renal thick ascending limb (THAL). We reported that constitutive endocytosis of NKCC2 controls NaCl absorption in native THALs; however, the pathways involved in NKCC2 endocytosis are unknown. We hypothesized that NKCC2 endocytosis at the apical surface depends on dynamin-2 and clathrin. Measurements of steady-state surface NKCC2 and the rate of NKCC2 endocytosis in freshly isolated rat THALs showed that inhibition of endogenous dynamin-2 with dynasore blunted NKCC2 endocytosis by 56 ± 11% and increased steady-state surface NKCC2 by 67 ± 27% (p < 0.05). Expression of the dominant negative Dyn2K44A in THALs slowed the rate of NKCC2 endocytosis by 38 ± 8% and increased steady-state surface NKCC2 by 37 ± 8%, without changing total NKCC2 expression. Inhibition of clathrin-mediated endocytosis with chlorpromazine blunted NKCC2 endocytosis by 54 ± 6%, while preventing clathrin from interacting with synaptojanin also blunted NKCC2 endocytosis by 52 ± 5%. Disruption of lipid rafts blunted NKCC2 endocytosis by 39 ± 4% and silencing caveolin-1 by 29 ± 4%. Simultaneous inhibition of clathrin- and lipid raft-mediated endocytosis completely blocked NKCC2 internalization. We concluded that dynamin-2, clathrin, and lipid rafts mediate NKCC2 endocytosis and maintain steady-state apical surface NKCC2 in native THALs. These are the first data identifying the endocytic pathway for apical NKCC2 endocytosis.

SNX9, SNX18 and SNX33 are required for progression through and completion of mitosis

Mitosis involves considerable membrane remodelling and vesicular trafficking to generate two independent cells. Consequently, endocytosis and endocytic proteins are required for efficient mitotic progression and completion. Several endocytic proteins also participate in mitosis in an endocytosis-independent manner. Here, we report that the sorting nexin 9 (SNX9) subfamily members - SNX9, SNX18 and SNX33 - are required for progression and completion of mitosis. Depletion of any one of these proteins using siRNA induces multinucleation, an indicator of cytokinesis failure, as well as an accumulation of cytokinetic cells. Time-lapse microscopy on siRNA-treated cells revealed a role for SNX9 subfamily members in progression through the ingression and abscission stages of cytokinesis. Depletion of these three proteins disrupted MRLC(S19) localization during ingression and recruitment of Rab11-positive recycling endosomes to the intracellular bridge between nascent daughter cells. SNX9 depletion also disrupted the localization of Golgi during cytokinesis. Endocytosis of transferrin was blocked during cytokinesis by depletion of the SNX9 subfamily members, suggesting that these proteins participate in cytokinesis in an endocytosis-dependent manner. In contrast, depletion of SNX9 did not block transferrin uptake during metaphase but did delay chromosome alignment and segregation, suggesting that SNX9 plays an additional non-endocytic role at early mitotic stages. We conclude that SNX9 subfamily members are required for mitosis through both endocytosis-dependent and -independent processes.

Charcot-Marie-Tooth disease and intracellular traffic

Mutations of genes whose primary function is the regulation of membrane traffic are increasingly being identified as the underlying causes of various important human disorders. Intriguingly, mutations in ubiquitously expressed membrane traffic genes often lead to cell type- or organ-specific disorders. This is particularly true for neuronal diseases, identifying the nervous system as the most sensitive tissue to alterations of membrane traffic. Charcot-Marie-Tooth (CMT) disease is one of the most common inherited peripheral neuropathies. It is also known as hereditary motor and sensory neuropathy (HMSN), which comprises a group of disorders specifically affecting peripheral nerves. This peripheral neuropathy, highly heterogeneous both clinically and genetically, is characterized by a slowly progressive degeneration of the muscle of the foot, lower leg, hand and forearm, accompanied by sensory loss in the toes, fingers and limbs. More than 30 genes have been identified as targets of mutations that cause CMT neuropathy. A number of these genes encode proteins directly or indirectly involved in the regulation of intracellular traffic. Indeed, the list of genes linked to CMT disease includes genes important for vesicle formation, phosphoinositide metabolism, lysosomal degradation, mitochondrial fission and fusion, and also genes encoding endosomal and cytoskeletal proteins. This review focuses on the link between intracellular transport and CMT disease, highlighting the molecular mechanisms that underlie the different forms of this peripheral neuropathy and discussing the pathophysiological impact of membrane transport genetic defects as well as possible future ways to counteract these defects.

Dynamin, a membrane-remodelling GTPase

Dynamin, the founding member of a family of dynamin-like proteins (DLPs) implicated in membrane remodelling, has a critical role in endocytic membrane fission events. The use of complementary approaches, including live-cell imaging, cell-free studies, X-ray crystallography and genetic studies in mice, has greatly advanced our understanding of the mechanisms by which dynamin acts, its essential roles in cell physiology and the specific function of different dynamin isoforms. In addition, several connections between dynamin and human disease have also emerged, highlighting specific contributions of this GTPase to the physiology of different tissues.

Phosphoinositides in the mammalian endo-lysosomal network

The endo-lysosomal system is an interconnected tubulo-vesicular network that acts as a sorting station to process and distribute internalised cargo. This network accepts cargoes from both the plasma membrane and the biosynthetic pathway, and directs these cargos either towards the lysosome for degradation, the peri-nuclear recycling endosome for return to the cell surface, or to the trans-Golgi network. These intracellular membranes are variously enriched in different phosphoinositides that help to shape compartmental identity. These lipids act to localise a number of phosphoinositide-binding proteins that function as sorting machineries to regulate endosomal cargo sorting. Herein we discuss regulation of these machineries by phosphoinositides and explore how phosphoinositide-switching contributes toward sorting decisions made at this platform.

Membrane trafficking and phagosome maturation during the clearance of apoptotic cells

Apoptosis is a cellular suicide process that quietly and efficiently eliminates unwanted or damaged cells. In metazoans, cells that undergo apoptosis are swiftly internalized by phagocytes and subsequently degraded inside phagosomes through phagosome maturation, a process that involves the fusion between phagosomes and multiple kinds of intracellular organelles and the gradual acidification of phagosomal lumen. In recent years, rapid progress has been made, in particular, through studies conducted in the model organism, the nematode Caenorhabditis elegans, in understanding the membrane trafficking events and molecular mechanisms that govern the degradation of apoptotic cells through phagosome maturation. These studies revealed the novel and essential functions of a large number of proteins, including the large GTPase dynamin, multiple Rab small GTPases and their regulatory proteins, the lipid second messenger PtdIns(3)P and its effectors, and unexpectedly, the phagosomal receptors for apoptotic cells, in promoting phagosome maturation. Further, novel signaling pathways essential for phagosome maturation have been delineated. Here, we discuss these exciting new findings, which have significantly deepened and broadened our understanding of the mechanisms that regulate the interaction between intracellular organelles and phagosomes.

Analysis of yeast endocytic site formation and maturation through a regulatory transition point

ETOC: During yeast endocytic site formation, Ede1p (yeast Eps15), but not clathrin light chain, is important for the recruitment of most other early-arriving proteins to endocytic sites. Cargo and clathrin light chain may play roles in regulating the transition of endocytic sites out of the “intermediate coat” stage of endocytosis.

The earliest stages of endocytic site formation and the regulation of endocytic site maturation are not well understood. Here we analyzed the order in which the earliest proteins are detectable at endocytic sites in budding yeast and found that an uncharacterized protein, Pal1p/Ydr348cp, is also present at the initial stages of endocytosis. Because Ede1p (homologue of Eps15) and clathrin are the early-arriving proteins most important for cargo uptake, their roles during the early stages of endocytosis were examined more comprehensively. Ede1p is necessary for efficient recruitment of most early-arriving proteins, but not for the recruitment of the adaptor protein Yap1802p, to endocytic sites. The early-arriving proteins, as well as the later-arriving proteins Sla2p and Ent1/2p (homologues of Hip1R and epsins), were found to have longer lifetimes in CLC1-knockout yeast, which indicates that clathrin light chain facilitates the transition from the intermediate to late coat stages. Cargo also arrives during the early stages of endocytosis, and therefore its effect on endocytic machinery dynamics was investigated. Our results are consistent with a role for cargo in regulating the transition of endocytic sites from the early stages of formation to the late stages during which vesicle formation occurs.

Phosphoinositides and the regulation of tubular-based endosomal sorting

From the pioneering work of Mabel and Lowell Hokin in the 1950s, the biology of this specific isomer of hexahydroxycyclohexane and its phosphorylated derivatives, in the form of inositol phosphates and phosphoinositides, has expanded to fill virtually every corner of cell biology, whole-organism physiology and development. In the present paper, I give a personal view of the role played by phosphoinositides in regulating the function of the endosomal network, and, in so doing, highlight some of the basic properties through which phosphoinositides regulate cell function.

Let's go bananas: revisiting the endocytic BAR code

Against the odds of membrane resistance, members of the BIN/Amphiphysin/Rvs (BAR) domain superfamily shape membranes and their activity is indispensable for a plethora of life functions. While crystal structures of different BAR dimers advanced our understanding of membrane shaping by scaffolding and hydrophobic insertion mechanisms considerably, especially life-imaging techniques and loss-of-function studies of clathrin-mediated endocytosis with its gradually increasing curvature show that the initial idea that solely BAR domain curvatures determine their functions is oversimplified. Diagonal placing, lateral lipid-binding modes, additional lipid-binding modules, tilde shapes and formation of macromolecular lattices with different modes of organisation and arrangement increase versatility. A picture emerges, in which BAR domain proteins create macromolecular platforms, that recruit and connect different binding partners and ensure the connection and coordination of the different events during the endocytic process, such as membrane invagination, coat formation, actin nucleation, vesicle size control, fission, detachment and uncoating, in time and space, and may thereby offer mechanistic explanations for how coordination, directionality and effectiveness of a complex process with several steps and key players can be achieved.

Systems biology and physical biology of clathrin-mediated endocytosis

In this review, we describe the application of experimental data and modeling of intracellular endocytic trafficking mechanisms with a focus on the process of clathrin-mediated endocytosis. A detailed parts-list for the protein-protein interactions in clathrin-mediated endocytosis has been available for some time. However, recent experimental, theoretical, and computational tools have proved to be critical in establishing a sequence of events, cooperative dynamics, and energetics of the intracellular process. On the experimental front, total internal reflection fluorescence microscopy, photo-activated localization microscopy, and spinning-disk confocal microscopy have focused on assembly and patterning of endocytic proteins at the membrane, while on the theory front, minimal theoretical models for clathrin nucleation, biophysical models for membrane curvature and bending elasticity, as well as methods from computational structural and systems biology, have proved insightful in describing membrane topologies, curvature mechanisms, and energetics.

Three sorting nexins drive the degradation of apoptotic cells in response to PtdIns(3)P signaling

LST-4/SNX9, SNX-1, and SNX-6 together drive the degradation of apoptotic cells, as PtdIns(3)P effectors, during Caenorhabditis elegans development. By inducing regional membrane curvature and maintaining RAB-7 GTPase on phagosomes, these three sorting nexins stimulate the fusion of endocytic organelles with phagosomes.

Apoptotic cells are swiftly engulfed by phagocytes and degraded inside phagosomes. Phagosome maturation requires phosphatidylinositol 3-phosphate [PtdIns(3)P], yet how PtdIns(3)P triggers phagosome maturation remains largely unknown. Through a genome-wide PtdIns(3)P effector screen in the nematode Caenorhabditis elegans, we identified LST-4/SNX9, SNX-1, and SNX-6, three BAR domain-containing sorting nexins, that act in two parallel pathways to drive PtdIns(3)P-mediated degradation of apoptotic cells. We found that these proteins were enriched on phagosomal surfaces through association with PtdIns(3)P and through specific protein–protein interaction, and they promoted the fusion of early endosomes and lysosomes to phagosomes, events essential for phagosome maturation. Specifically, LST-4 interacts with DYN-1 (dynamin), an essential phagosome maturation initiator, to strengthen DYN-1’s association to phagosomal surfaces, and facilitates the maintenance of the RAB-7 GTPase on phagosomal surfaces. Furthermore, both LST-4 and SNX-1 promote the extension of phagosomal tubules to facilitate the docking and fusion of intracellular vesicles. Our findings identify the critical and differential functions of two groups of sorting nexins in phagosome maturation and reveal a signaling cascade initiated by phagocytic receptor CED-1, mediated by PtdIns(3)P, and executed through these sorting nexins to degrade apoptotic cells.

Expression of sorting nexin 18 (SNX18) is dynamically regulated in developing spinal motor neurons

The sorting nexin (SNX) family proteins, which contain a Phox homology (PX) domain, play crucial roles in regulating the intracellular membrane trafficking of the endocytic pathway. The proper coordination of this pathway is important for axonal elongation; however, little is known about the expression and intracellular dynamics of the SNX members during the formation of the nervous system. Here the authors found that SNX18, which belongs to the Src-homology-3-PX-Bin/Amphiphysin/Rvs domain-containing SNX subfamily, was specifically expressed in differentiating motor neurons in the chick and mouse embryonic spinal cord. The expression of SNX18 in embryonic spinal motor neurons was transient and was downregulated as the neurons matured. The authors further demonstrated that the localization of EGFP-SNX18 in growth cones was dynamically regulated and accumulated especially at areas in contact with permissive substrates. These findings collectively suggest that SNX18 may play an active role in axonal elongation.

A novel, retromer-independent role for sorting nexins 1 and 2 in RhoG-dependent membrane remodeling

The sorting nexins SNX1 and SNX2 are members of the retromer complex involved in protein sorting within the endocytic pathway. While retromer-dependent functions of SNX1 and SNX2 have been well documented, potential retromer-independent roles remain unclear. Here, we show that SNX1 and SNX2 interact with the Rac1 and RhoG guanine nucleotide exchange factor Kalirin-7. Simultaneous overexpression of SNX1 or SNX2 and Kalirin-7 in epithelial cells causes partial redistribution of both SNX isoforms to the plasma membrane, and results in RhoG-dependent lamellipodia formation that requires functional Phox homology (PX) and Bin/Amphiphysin/Rvs (BAR) domains of SNX, but is Rac1- and retromer-independent. Conversely, depletion of endogenous SNX1 or SNX2 inhibits Kalirin-7-mediated lamellipodia formation. Finally, we demonstrate that SNX1 and SNX2 interact directly with inactive RhoG, suggesting a novel role for these SNX proteins in recruiting an inactive Rho GTPase to its exchange factor.

Vesicle scission: dynamin

Dynamin is a large GTPase involved in endocytic vesicle formation, but its exact role and mechanism are subjects of long-standing debate. Despite recent advances in the structural analyses of isolated dynamin domains and the faithful reconstitution of dynamin-dependent membrane fission in model membrane systems, the mechanism of its action remains poorly understood at the molecular level. Here, I will review current progress in elucidating dynamin action in vesicle scission and highlight the most visible gaps in knowledge that limit the development of a coherent and complete model for its role in vesicle biogenesis. Coordinated functions of BAR domain-containing binding partners are also discussed.

The E3 ubiquitin ligase Itch regulates sorting nexin 9 through an unconventional substrate recognition domain

The level of intracellular proteins is mainly regulated through modifications by ubiquitin ligases that target them for degradation. Members of the NEDD4 family of E3 ubiquitin ligases, such as Itch (atrophin-1 interacting protein 4), possess up to four WW domains for specific association with PY motif-containing substrates. We have identified sorting nexin 9 (SNX9), a protein involved in endocytic processes, as a new substrate of Itch. Itch ubiquitylates SNX9 and regulates intracellular SNX9 levels. Using truncated proteins, we found that the interaction with SNX9 is mediated by the proline-rich domain (PRD) of Itch, a domain distinct from the conventional WW recognition domain, and the SH3 domain of SNX9. Interaction with the PRD of Itch is essential for SNX9 ubiquitylation and degradation. Furthermore, this effect is specific for Itch, as NEDD4, a related PRD-containing E3 ligase, does not bind SNX9. SNX18, a second member of the SNX family containing an SH3 domain, was also found to bind to Itch. Our results indicate that the pool of substrates of NEDD4 family E3 ubiquitin ligases extends beyond proteins containing PY motifs.

SNX18 shares a redundant role with SNX9 and modulates endocytic trafficking at the plasma membrane

SNX18 and SNX9 are members of a subfamily of SNX (sorting nexin) proteins with the same domain structure. Although a recent report showed that SNX18 and SNX9 localize differently in cells and appear to function in different trafficking pathways, concrete evidence regarding whether they act together or separately in intracellular trafficking is still lacking. Here, we show that SNX18 has a similar role to SNX9 in endocytic trafficking at the plasma membrane, rather than having a distinct role. SNX18 and SNX9 are expressed together in most cell lines, but to a different extent. Like SNX9, SNX18 interacts with dynamin and stimulates the basal GTPase activity of dynamin. It also interacts with neuronal Wiskott-Aldrich syndrome protein (N-WASP) and synaptojanin, as does SNX9. SNX18 and SNX9 can form a heterodimer and colocalize in tubular membrane structures. Depletion of SNX18 by small hairpin RNA inhibited transferrin uptake. SNX18 successfully compensates for SNX9 deficiency during clathrin-mediated endocytosis and vice versa. Total internal reflection fluorescence microscopy in living cells shows that a transient burst of SNX18 recruitment to clathrin-coated pits coincides spatiotemporally with a burst of dynamin and SNX9. Taken together, our results suggest that SNX18 functions with SNX9 in multiple pathways of endocytosis at the plasma membrane and that they are functionally redundant.

Dynamin 2 and human diseases

Dynamin 2 (DNM2) mutations cause autosomal dominant centronuclear myopathy, a rare form of congenital myopathy, and intermediate and axonal forms of Charcot-Marie-Tooth disease, a peripheral neuropathy. DNM2 is a large GTPase mainly involved in membrane trafficking through its function in the formation and release of nascent vesicles from biological membranes. DNM2 participates in clathrin-dependent and clathrin-independent endocytosis and intracellular membrane trafficking (from endosomes and Golgi apparatus). Recent studies have also implicated DNM2 in exocytosis. DNM2 belongs to the machinery responsible for the formation of vesicles and regulates the cytoskeleton providing intracellular vesicle transport. In addition, DNM2 tightly interacts with and is involved in the regulation of actin and microtubule networks, independent from membrane trafficking processes. We summarize here the molecular, biochemical, and functional data on DNM2 and discuss the possible pathophysiological mechanisms via which DNM2 mutations can lead to two distinct neuromuscular disorders.

Mechanism of aldolase control of sorting nexin 9 function in endocytosis

Sorting nexin 9 (SNX9) functions in a complex with the GTPase dynamin-2 at clathrin-coated pits, where it provokes fission of vesicles to complete endocytosis. Here the SNX9.dynamin-2 complex binds to clathrin and adapter protein complex 2 (AP-2) that line these pits, and this occurs through interactions of the low complexity domain (LC4) of SNX9 with AP-2. Intriguingly, localization of the SNX9.dynamin-2 complex to clathrin-coated pits is blocked by interactions with the abundant glycolytic enzyme aldolase, which also binds to the LC4 domain of SNX9. The crystal structure of the LC4 motif of human SNX9 in complex with aldolase explains the biochemistry and biology of this interaction, where SNX9 binds near the active site of aldolase via residues 165-171 that are also required for the interactions of SNX9 with AP-2. Accordingly, SNX9 binding to aldolase is structurally precluded by the binding of substrate to the active site. Interactions of SNX9 with aldolase are far more extensive and differ from those of the actin-nucleating factor WASP with aldolase, indicating considerable plasticity in mechanisms that direct the functions of the aldolase as a scaffold protein.

Driving membrane curvature in clathrin-dependent and clathrin-independent endocytosis

Cellular activity depends to a large extent on membrane bilayer dynamics. Many processes, such as organelle biogenesis and vesicular transport, rely on alterations in membrane structure and shape. It is now widely accepted that intracellular membrane curvature generation and remodelling is mediated and regulated by protein action, and the mechanisms behind the processes are currently being revealed. Here, we will briefly discuss the key principles of membrane deformation and focus on different endocytic events that use various kinds of proteins to shape the plasma membrane into transport carriers. The entry routes are adopted to make sure that a vast variety of molecules on the cell surface can be regulated by endocytosis. The principles for membrane sculpting of endocytic carriers can be viewed either from a perspective of rigid coat budding or of flexible opportunistic budding. We will discuss these principles and their implications, focusing on clathrin-dependent and -independent carrier formation and the proteins involved in the respective pathways.

Mechanisms of endocytosis

Endocytic mechanisms control the lipid and protein composition of the plasma membrane, thereby regulating how cells interact with their environments. Here, we review what is known about mammalian endocytic mechanisms, with focus on the cellular proteins that control these events. We discuss the well-studied clathrin-mediated endocytic mechanisms and dissect endocytic pathways that proceed independently of clathrin. These clathrin-independent pathways include the CLIC/GEEC endocytic pathway, arf6-dependent endocytosis, flotillin-dependent endocytosis, macropinocytosis, circular doral ruffles, phagocytosis, and trans-endocytosis. We also critically review the role of caveolae and caveolin1 in endocytosis. We highlight the roles of lipids, membrane curvature-modulating proteins, small G proteins, actin, and dynamin in endocytic pathways. We discuss the functional relevance of distinct endocytic pathways and emphasize the importance of studying these pathways to understand human disease processes.

SNX9 - a prelude to vesicle release

The sorting nexin SNX9 has, in the past few years, been singled out as an important protein that participates in fundamental cellular activities. SNX9 binds strongly to dynamin and is partly responsible for the recruitment of this GTPase to sites of endocytosis. SNX9 also has a high capacity for modulation of the membrane and might therefore participate in the formation of the narrow neck of endocytic vesicles before scission occurs. Once assembled on the membrane, SNX9 stimulates the GTPase activity of dynamin to facilitate the scission reaction. It has also become clear that SNX9 has the ability to activate the actin regulator N-WASP in a membrane-dependent manner to coordinate actin polymerization with vesicle release. In this Commentary, we summarize several aspects of SNX9 structure and function in the context of membrane remodeling, discuss its interplay with various interaction partners and present a model of how SNX9 might work in endocytosis.

Out, in and back again: PtdIns(4,5)P(2) regulates cadherin trafficking in epithelial morphogenesis

The morphogenesis of epithelial cells in the tissue microenvironment depends on the regulation of the forces and structures that keep cells in contact with their neighbours. The formation of cell-cell contacts is integral to the establishment and maintenance of epithelial morphogenesis. In epithelial tissues, the misregulation of the signalling pathways that control epithelial polarization induces migratory and invasive cellular phenotypes. Many cellular processes influence cadherin targeting and function, including exocytosis, endocytosis and recycling. However, the localized generation of the lipid messenger PtdIns(4,5)P(2) is emerging as a fundamental signal controlling all of these processes. The PtdIns(4,5)P(2)-generating enzymes, PIPKs (phosphatidylinositol phosphate kinases) are therefore integral to these pathways. By the spatial and temporal targeting of PIPKs via the actions of its functional protein associates, PtdIns(4,5)P(2) is generated at discrete cellular locales to provide the cadherin-trafficking machinery with its required lipid messenger. In the present review, we discuss the involvement of PtdIns(4,5)P(2) and the PIPKs in the regulation of the E-cadherin (epithelial cadherin) exocytic and endocytic machinery, the modulation of actin structures at sites of adhesion, and the direction of cellular pathways which determine the fate of E-cadherin and cell-cell junctions. Recent work is also described that has defined phosphoinositide-mediated E-cadherin regulatory pathways by the use of organismal models.

Shaping membranes for endocytosis

Endocytosis is essential for virtually all eukaryotic cells to internalize nutrients, antigens, pathogens, and cell surface receptors from the plasma membrane into membrane-bounded, endocytic vesicles to regulate cell homeostasis, cell signaling, and development. Distinct mechanisms mediate endocytic uptake of a large variety of distinctly sized cargoes ranging from small molecules to viruses or bacteria. Common to all of these endocytic pathways is the deformation of the plasma membrane by intracellular factors including scaffolding proteins, amphipathic peripheral membrane proteins, and lipid-modifying enzymes. In this review we summarize how different cargoes exploit distinct pathways for cell entry, and how proteins assist the generation of curved membrane domains during internalization.

SNX9 regulates tubular invagination of the plasma membrane through interaction with actin cytoskeleton and dynamin 2

Dynamic membrane remodeling during intracellular trafficking is controlled by the intricate interplay between lipids and proteins. BAR domains are modules that participate in endocytic processes by binding and deforming the lipid bilayer. Sorting nexin 9 (SNX9), which functions in clathrin-mediated endocytosis, contains a BAR domain, however, the properties of this domain are not well understood. Here we show that SNX9 shares many properties with other BAR domain-containing proteins, such as amphiphysin and endophilin. SNX9 is able to deform the plasma membrane, as well as liposomes, into narrow tubules and recruit N-WASP and dynamin 2 to these tubules via its SH3 domain. SNX9-induced tubulation is antagonized by N-WASP and dynamin 2 while it is enhanced by perturbation of actin dynamics. However, SNX9 also has several unique properties. The tubulating activity requires the BAR and PX domains, as well as the low-complexity (LC) domain, which binds the Arp2/3 complex. SNX9 also binds to PtdIns(4)P-5-kinases via its PX domain and its tubulating activity is regulated by phosphoinositides. In addition, the kinase activity of PtdIns(4)P-5-kinases is stimulated by interaction with SNX9, suggesting a positive feedback interaction between SNX9 and PtdIns(4)P-5-kinases. These results suggest that SNX9 functions in the coordination of membrane remodeling and fission via interactions with actin-regulating proteins, endocytic proteins and PtdIns(4,5)P2-metabolizing enzymes.