Septin-mediated RhoA activation engages the exocyst complex to recruit the cilium transition zone

Septins are filamentous GTPases that play important but poorly characterized roles in ciliogenesis. Here, we show that SEPTIN9 regulates RhoA signaling at the base of cilia by binding and activating the RhoA guanine nucleotide exchange factor, ARHGEF18. GTP-RhoA is known to activate the membrane targeting exocyst complex, and suppression of SEPTIN9 causes disruption of ciliogenesis and mislocalization of an exocyst subunit, SEC8. Using basal body-targeted proteins, we show that upregulating RhoA signaling at the cilium can rescue ciliary defects and mislocalization of SEC8 caused by global SEPTIN9 depletion. Moreover, we demonstrate that the transition zone components, RPGRIP1L and TCTN2, fail to accumulate at the transition zone in cells lacking SEPTIN9 or depleted of the exocyst complex. Thus, SEPTIN9 regulates the recruitment of transition zone proteins on Golgi-derived vesicles by activating the exocyst via RhoA to allow the formation of primary cilia.

The complex web of canonical and non-canonical Hedgehog signaling

Hedgehog (Hh) signaling is a widely studied signaling pathway because of its critical roles during development and in cell homeostasis. Vertebrate canonical and non-canonical Hh signaling are typically assumed to be distinct and occur in different cellular compartments. While research has primarily focused on the canonical form of Hh signaling and its dependency on primary cilia - microtubule-based signaling hubs - an extensive list of crucial functions mediated by non-canonical Hh signaling has emerged. Moreover, amounting evidence indicates that canonical and non-canonical modes of Hh signaling are interlinked, and that they can overlap spatially, and in many cases interact functionally. Here, we discuss some of the many cellular effects of non-canonical signaling and discuss new evidence indicating inter-relationships with canonical signaling. We discuss how Smoothened (Smo), a key component of the Hh pathway, might coordinate such diverse downstream effects. Collectively, pursuit of questions such as those proposed here will aid in elucidating the full extent of Smo function in development and advance its use as a target for cancer therapeutics.

A non-canonical Hedgehog pathway initiates ciliogenesis and autophagy

Primary cilia function as critical signaling hubs whose absence leads to severe disorders collectively known as ciliopathies; our knowledge of ciliogenesis remains limited. We show that Smo induces ciliogenesis through two distinct yet essential noncanonical Hh pathways in several cell types, including neurons. Surprisingly, ligand activation of Smo induces autophagy via an LKB1-AMPK axis to remove the satellite pool of OFD1. This is required, but not sufficient, for ciliogenesis. Additionally, Smo activates the Gαi-LGN-NuMA-dynein axis, causing accumulation of a portion of OFD1 at centrioles in early ciliogenesis. Both pathways are critical for redistribution of BBS4 from satellites to centrioles, which is also mediated by OFD1 centriolar translocation. Notably, different Smo agonists, which activate Smo distinctly, activate one or the other of these pathways; only in combination they recapitulate the activity of Hh ligand. These studies provide new insight into physiological stimuli (Hh) that activate autophagy and promote ciliogenesis and introduce a novel role for the Gαi-LGN-NuMA-dynein complex in this process.

Revised subunit order of mammalian septin complexes explains their in vitro polymerization properties

Septins are conserved GTP-binding cytoskeletal proteins that polymerize into filaments by end-to-end joining of hetero-oligomeric complexes. In human cells, both hexamers and octamers exist, and crystallography studies predicted the order of the hexamers to be SEPT7-SEPT6-SEPT2-SEPT2-SEPT6-SEPT7, while octamers are thought to have the same core, but with SEPT9 at the ends. However, based on this septin organization, octamers and hexamers would not be expected to copolymerize due to incompatible ends. Here we isolated hexamers and octamers of specific composition from human cells and show that hexamers and octamers polymerize individually and, surprisingly, with each other. Binding of the Borg homology domain 3 (BD3) domain of Borg3 results in distinctive clustering of each filament type. Moreover, we show that the organization of hexameric and octameric complexes is inverted compared with its original prediction. This revised septin organization is congruent with the organization and behavior of yeast septins suggesting that their properties are more conserved than was previously thought.

DIAPH1 regulates ciliogenesis and trafficking in primary cilia

Primary cilia are critical hubs for several signaling pathways, and defects in ciliogenesis or cilia maintenance produce a range of diseases collectively known as ciliopathies. Ciliogenesis requires vesicle trafficking along a network of microtubules and actin filaments to the basal body. The DIAPH1 (Diaphanous-related formin) family of formins promotes both actin polymerization and EB1-dependent microtubule (MT) stability. EB1 and EB3 have previously been implicated in cilia biogenesis to carry out centrosome-related functions. However, the role of DIAPH1 proteins had not been examined. Here we show that the depletion of DIAPH1 decreased ciliogenesis, cilia length, and reduced trafficking within cilia. Additionally, both actin nucleating and microtubule-stabilizing properties of DIAPH1 are important for their cilia functions. To assess their roles in ciliogenesis in isolation, we targeted DIAPH1 specifically to the basal body, which caused an increase in cilia length and increased trafficking within cilia. Intriguingly, expression of DIAPH1 mutants associated with human deafness and microcephaly impaired ciliation and caused cilia elongation and bulb formation. These results suggest that the actin and microtubule functions of DIAPH1 proteins regulate cilia maintenance in part by regulating vesicular trafficking to the base of the primary cilia.

Phagolysosome resolution requires contacts with the endoplasmic reticulum and phosphatidylinositol-4-phosphate signalling

Phosphoinositides have a pivotal role in the maturation of nascent phagosomes into microbicidal phagolysosomes. Following degradation of their contents, mature phagolysosomes undergo resolution, a process that remains largely uninvestigated. Here we studied the role of phosphoinositides in phagolysosome resolution. Phosphatidylinositol-4-phosphate (PtdIns(4)P), which is abundant in maturing phagolysosomes, was depleted as they tubulated and resorbed. Depletion was caused, in part, by transfer of phagolysosomal PtdIns(4)P to the endoplasmic reticulum, a process mediated by oxysterol-binding protein-related protein 1L (ORP1L), a RAB7 effector. ORP1L formed discrete tethers between the phagolysosome and the endoplasmic reticulum, resulting in distinct regions with alternating PtdIns(4)P depletion and enrichment. Tubules emerged from PtdIns(4)P-rich regions, where ADP-ribosylation factor-like protein 8B (ARL8B) and SifA- and kinesin-interacting protein/pleckstrin homology domain-containing family M member 2 (SKIP/PLEKHM2) accumulated. SKIP binds preferentially to monophosphorylated phosphoinositides, of which PtdIns(4)P is most abundant in phagolysosomes, contributing to their tubulation. Accordingly, premature hydrolysis of PtdIns(4)P impaired SKIP recruitment and phagosome resolution. Thus, resolution involves phosphoinositides and tethering of phagolysosomes to the endoplasmic reticulum.

Septin-regulated actin dynamics promote Salmonella invasion of host cells

Actin nucleators and their binding partners play crucial roles during Salmonella invasion, but how these factors are dynamically coordinated remains unclear. Here, we show that septins, a conserved family of GTP binding proteins, play a role during the early stages of Salmonella invasion. We demonstrate that septins are rapidly enriched at sites of bacterial entry and contribute to the morphology of invasion ruffles. We found that SEPTIN2, SEPTIN7, and SEPTIN9 are required for efficient bacterial invasion. Septins contributed to the recruitment of ROCK2 kinase during Salmonella invasion, and the downstream activation of the actin nucleating protein FHOD1. In contrast, activation of the ROCK2 substrate myosin II, which is known to be required for Salmonella enterica serovar Typhimurium invasion, did not require septins. Collectively, our studies provide new insight into the mechanisms involved in Salmonella invasion of host cells.

Membrane dynamics and organelle biogenesis-lipid pipelines and vesicular carriers

Discoveries spanning several decades have pointed to vital membrane lipid trafficking pathways involving both vesicular and non-vesicular carriers. But the relative contributions for distinct membrane delivery pathways in cell growth and organelle biogenesis continue to be a puzzle. This is because lipids flow from many sources and across many paths via transport vesicles, non-vesicular transfer proteins, and dynamic interactions between organelles at membrane contact sites. This forum presents our latest understanding, appreciation, and queries regarding the lipid transport mechanisms necessary to drive membrane expansion during organelle biogenesis and cell growth.

Uncovering the Roles of Septins in Cilia

Septins are a family of GTP-binding proteins that associate with cellular membranes and the cytoskeleton. Their ability to polymerize into filamentous structures permits them to serve as diffusion barriers for membrane proteins and as multi-molecular scaffolds that recruit components of signaling pathways. At the cellular level, septins contribute to the regulation of numerous processes, including cytokinesis, cell polarity, cell migration, and many others. In this review, we discuss emerging evidence for roles of mammalian septins in the biogenesis and function of flagella and cilia, and how this may impact human diseases such as ciliopathies.

VAPs and ACBD5 tether peroxisomes to the ER for peroxisome maintenance and lipid homeostasis

Peroxisomes and the ER exchange lipids for various metabolic and anabolic reactions. In this study, Hua et al. show that the interaction between the ER-resident VAPs with the peroxisomal protein ACBD5 tethers peroxisomes to the ER. This tether is required for the exchange of lipids, including cholesterol, between the two organelles.

Lipid exchange between the endoplasmic reticulum (ER) and peroxisomes is necessary for the synthesis and catabolism of lipids, the trafficking of cholesterol, and peroxisome biogenesis in mammalian cells. However, how lipids are exchanged between these two organelles is not understood. In this study, we report that the ER-resident VAMP-associated proteins A and B (VAPA and VAPB) interact with the peroxisomal membrane protein acyl-CoA binding domain containing 5 (ACBD5) and that this interaction is required to tether the two organelles together, thereby facilitating the lipid exchange between them. Depletion of either ACBD5 or VAP expression results in increased peroxisome mobility, suggesting that VAP–ACBD5 complex acts as the primary ER–peroxisome tether. We also demonstrate that tethering of peroxisomes to the ER is necessary for peroxisome growth, the synthesis of plasmalogen phospholipids, and the maintenance of cellular cholesterol levels. Collectively, our data highlight the importance of VAP–ACBD5–mediated contact between the ER and peroxisomes for organelle maintenance and lipid homeostasis.

Immunofluorescent staining of septins in primary cilia

Primary cilia are cellular antennae that receive and transduce extracellular cues. These microtubule-rich structures are comprised of at least three distinct ciliary compartments: basal bodies, transition zone, and axoneme. Septins have been implicated in cilia function at the transition zone, but accumulating evidence suggests that they localize predominantly within the axoneme. Here, we describe three fixation conditions that preserve the substructure of primary cilia and demonstrate known ciliary proteins that localize to these distinct ciliary substructures. Finally, we show immunostaining and live microscopy methods to detect septins within the axoneme.

Novel Host Proteins and Signaling Pathways in Enteropathogenic E. coli Pathogenesis Identified by Global Phosphoproteome Analysis

Enteropathogenic Escherichia coli (EPEC) uses a type III secretion system (T3SS) to directly translocate effector proteins into host cells where they play a pivotal role in subverting host cell signaling needed for disease. However, our knowledge of how EPEC affects host protein phosphorylation is limited to a few individual protein studies. We employed a quantitative proteomics approach to globally map alterations in the host phosphoproteome during EPEC infection. By characterizing host phosphorylation events at various time points throughout infection, we examined how EPEC dynamically impacts the host phosphoproteome over time. This experimental setup also enabled identification of T3SS-dependent and -independent changes in host phosphorylation. Specifically, T3SS-regulated events affected various cellular processes that are known EPEC targets, including cytoskeletal organization, immune signaling, and intracellular trafficking. However, the involvement of phosphorylation in these events has thus far been poorly studied. We confirmed the MAPK family as an established key host player, showed its central role in signal transduction during EPEC infection, and extended the repertoire of known signaling hubs with previously unrecognized proteins, including TPD52, CIN85, EPHA2, and HSP27. We identified altered phosphorylation of known EPEC targets, such as cofilin, where the involvement of phosphorylation has so far been undefined, thus providing novel mechanistic insights into the roles of these proteins in EPEC infection. An overlap of regulated proteins, especially those that are cytoskeleton-associated, was observed when compared with the phosphoproteome of Shigella-infected cells. We determined the biological relevance of the phosphorylation of a novel protein in EPEC pathogenesis, septin-9 (SEPT9). Both siRNA knockdown and a phosphorylation-impaired SEPT9 mutant decreased bacterial adherence and EPEC-mediated cell death. In contrast, a phosphorylation-mimicking SEPT9 mutant rescued these effects. Collectively, this study provides the first global analysis of phosphorylation-mediated processes during infection with an extracellular, diarrheagenic bacterial pathogen.

Barriers to the free diffusion of proteins and lipids in the plasma membrane

Biological membranes segregate into specialized functional domains of distinct composition, which can persist for the entire life of the cell. How separation of their lipid and (glyco)protein components is generated and maintained is not well understood, but the existence of diffusional barriers has been proposed. Remarkably, the physical nature of such barriers and the manner whereby they impede the free diffusion of molecules in the plane of the membrane has rarely been studied in depth. Moreover, alternative mechanisms capable of generating membrane inhomogeneity are often disregarded. Here we describe prototypical biological systems where membrane segregation has been amply documented and discuss the role of diffusional barriers and other processes in the generation and maintenance of their structural and functional compartmentalization.

Cell and molecular biology of septins

Septins are a family of GTP-binding proteins that assemble into cytoskeletal filaments. Unlike other cytoskeletal components, septins form ordered arrays of defined stoichiometry that can polymerize into long filaments and bundle laterally. Septins associate directly with membranes and have been implicated in providing membrane stability and serving as diffusion barriers for membrane proteins. In addition, septins bind other proteins and have been shown to function as multimolecular scaffolds by recruiting components of signaling pathways. Remarkably, septins participate in a spectrum of cellular processes including cytokinesis, ciliogenesis, cell migration, polarity, and cell-pathogen interactions. Given their breadth of functions, it is not surprising that septin abnormalities have also been linked to human diseases. In this review, we discuss the current knowledge of septin structure, assembly and function, and discuss these in the context of human disease.

Mapping the cellular response to small molecules using chemogenomic fitness signatures

Genome-wide characterization of the in vivo cellular response to perturbation is fundamental to understanding how cells survive stress. Identifying the proteins and pathways perturbed by small molecules affects biology and medicine by revealing the mechanisms of drug action. We used a yeast chemogenomics platform that quantifies the requirement for each gene for resistance to a compound in vivo to profile 3250 small molecules in a systematic and unbiased manner. We identified 317 compounds that specifically perturb the function of 121 genes and characterized the mechanism of specific compounds. Global analysis revealed that the cellular response to small molecules is limited and described by a network of 45 major chemogenomic signatures. Our results provide a resource for the discovery of functional interactions among genes, chemicals, and biological processes.

Cytoskeleton: septins do the horizontal tango

Septins are a family of membrane-associated filamentous proteins that are essential in many biological processes, such as cell division. A recent study has provided the first visualization of septin filament formation in real time, leading to important new insights into their organization.

Drosophila SNAP-29 is an essential SNARE that binds multiple proteins involved in membrane traffic

Each membrane fusion event along the secretory and endocytic pathways requires a specific set of SNAREs to assemble into a 4-helical coiled-coil, the so-called trans-SNARE complex. Although most SNAREs contribute one helix to the trans-SNARE complex, members of the SNAP-25 family contribute two helixes. We report the characterization of the Drosophila homologue of SNAP-29 (dSNAP-29), which is expressed throughout development. Unlike the other SNAP-25 like proteins in fruit fly (i.e., dSNAP-25 and dSNAP-24), which form SDS-resistant SNARE complexes with their cognate SNAREs, dSNAP-29 does not participate in any SDS-resistant complexes, despite its interaction with dsyntaxin1 and dsyntaxin16 in vitro. Immunofluorescence studies indicated that dSNAP-29 is distributed in various tissues, locating in small intracellular puncta and on the plasma membrane, where it associates with EH domain-containing proteins implicated in the endocytic pathway. Overexpression and RNAi studies suggested that dSNAP-29 mediates an essential process in Drosophila development.

Structure of LIMP-2 provides functional insights with implications for SR-BI and CD36

Members of the CD36 superfamily of scavenger receptor proteins are important regulators of lipid metabolism and innate immunity. They recognize normal and modified lipoproteins, as well as pathogen-associated molecular patterns. The family consists of three members: SR-BI (which delivers cholesterol to the liver and steroidogenic organs and is a co-receptor for hepatitis C virus), LIMP-2/LGP85 (which mediates lysosomal delivery of β-glucocerebrosidase and serves as a receptor for enterovirus 71 and coxsackieviruses) and CD36 (a fatty-acid transporter and receptor for phagocytosis of effete cells and Plasmodium-infected erythrocytes). Notably, CD36 is also a receptor for modified lipoproteins and β-amyloid, and has been implicated in the pathogenesis of atherosclerosis and of Alzheimer's disease. Despite their prominent roles in health and disease, understanding the function and abnormalities of the CD36 family members has been hampered by the paucity of information about their structure. Here we determine the crystal structure of LIMP-2 and infer, by homology modelling, the structure of SR-BI and CD36. LIMP-2 shows a helical bundle where β-glucocerebrosidase binds, and where ligands are most likely to bind to SR-BI and CD36. Remarkably, the crystal structure also shows the existence of a large cavity that traverses the entire length of the molecule. Mutagenesis of SR-BI indicates that the cavity serves as a tunnel through which cholesterol(esters) are delivered from the bound lipoprotein to the outer leaflet of the plasma membrane. We provide evidence supporting a model whereby lipidic constituents of the ligands attached to the receptor surface are handed off to the membrane through the tunnel, accounting for the selective lipid transfer characteristic of SR-BI and CD36.

The ZIP5 ectodomain co-localizes with PrP and may acquire a PrP-like fold that assembles into a dimer

The cellular prion protein (PrP(C)) was recently observed to co-purify with members of the LIV-1 subfamily of ZIP zinc transporters (LZTs), precipitating the surprising discovery that the prion gene family descended from an ancestral LZT gene. Here, we compared the subcellular distribution and biophysical characteristics of LZTs and their PrP-like ectodomains. When expressed in neuroblastoma cells, the ZIP5 member of the LZT subfamily was observed to be largely directed to the same subcellular locations as PrP(C) and both proteins were seen to be endocytosed through vesicles decorated with the Rab5 marker protein. When recombinantly expressed, the PrP-like domain of ZIP5 could be obtained with yields and levels of purity sufficient for structural analyses but it tended to aggregate, thereby precluding attempts to study its structure. These obstacles were overcome by moving to a mammalian cell expression system. The subsequent biophysical characterization of a homogeneous preparation of the ZIP5 PrP-like ectodomain shows that this protein acquires a dimeric, largely globular fold with an α-helical content similar to that of mammalian PrP(C). The use of a mammalian cell expression system also allowed for the expression and purification of stable preparations of Takifugu rubripes PrP-1, thereby overcoming a key hindrance to high-resolution work on a fish PrP(C).

Mitotic regulation of SEPT9 protein by cyclin-dependent kinase 1 (Cdk1) and Pin1 protein is important for the completion of cytokinesis

Precise cell division is essential for multicellular development, and defects in this process have been linked to cancer. Septins are a family of proteins that are required for mammalian cell division, but their function and mode of regulation during this process are poorly understood. Here, we demonstrate that cyclin-dependent kinase 1 (Cdk1) phosphorylates septin 9 (SEPT9) upon mitotic entry, and this phosphorylation controls association with the proline isomerase, Pin1. Both SEPT9 and Pin1 are critical for mediating the final separation of daughter cells. Expression of mutant SEPT9 that is defective in Pin1 binding was unable to rescue cytokinesis defects caused by SEPT9 depletion but rather induced dominant-negative defects in cytokinesis. However, unlike SEPT9 depletion, Pin1 was not required for the accumulation of the exocyst complex at the midbody. These results suggest that SEPT9 plays multiple roles in abscission, one of which is regulated by the action of Cdk1 and Pin1.

Uncovering principles that control septin-septin interactions

Septins comprise a conserved family of GTPases important in cytokinesis. These proteins polymerize into filaments from rod-shaped heteromeric septin complexes. Septins interact with one another at two interfaces (NC and G) that alternate within the complex. Here, we show that small mutations at the N terminus greatly enhance the formation of SEPT2 homopolymers. Taking advantage of this mutation to examine polymer formation using SEPT2 alone, we show that both NC and G interfaces are required for filament formation. However, co-expression of wild type SEPT2 with SEPT2 containing mutations at either NC or G interfaces revealed that only the NC mutant suppressed filament formation. NC mutants are able to interact with one another at putative G interfaces, whereas G mutants fail to interact at NC interfaces. In addition, all promiscuous septin pairwise interactions occur at the G interface. These findings suggest that G interface interactions must occur before NC interactions during polymer formation.

LIV-1 ZIP ectodomain shedding in prion-infected mice resembles cellular response to transition metal starvation

We recently documented the co-purification of members of the LIV-1 subfamily of ZIP (Zrt-, Irt-like Protein) zinc transporters (LZTs) with the cellular prion protein (PrP(C)) and, subsequently, established that the prion gene family descended from an ancestral LZT gene. Here, we begin to address whether the study of LZTs can shed light on the biology of prion proteins in health and disease. Starting from an observation of an abnormal LZT immunoreactive band in prion-infected mice, subsequent cell biological analyses uncovered a surprisingly coordinated biology of ZIP10 (an LZT member) and prion proteins that involves alterations to N-glycosylation and endoproteolysis in response to manipulations to the extracellular divalent cation milieu. Starving cells of manganese or zinc, but not copper, causes shedding of the N1 fragment of PrP(C) and of the ectodomain of ZIP10. For ZIP10, this posttranslational biology is influenced by an interaction between its PrP-like ectodomain and a conserved metal coordination site within its C-terminal multi-spanning transmembrane domain. The transition metal starvation-induced cleavage of ZIP10 can be differentiated by an immature N-glycosylation signature from a constitutive cleavage targeting the same site. Data from this work provide a first glimpse into a hitherto neglected molecular biology that ties PrP to its LZT cousins and suggest that manganese or zinc starvation may contribute to the etiology of prion disease in mice.

Localization of septin proteins in the mouse cochlea

Septins are a family of GTP binding proteins that are well conserved in eukaryotic species except plants. Septins contribute to the lateral compartmentalization of membranes, cortical rigidity, and the regulation of membrane trafficking by associating with membrane lipids, actin, and microtubules. The organ of Corti in the cochlea has pivotal roles in auditory perception and includes two kinds of highly polarized cells, hair and supporting cells, both of which are rich in actin and microtubules. To identify the roles of septins in the cochlea, we analyzed the localization of three septin proteins, septin 4 (SEPT4), septin 5 (SEPT5), and septin 7 (SEPT7) that are abundantly expressed in brain tissues, and also examined auditory functions of Sept4 and Sept5 null mice. SEPT4, SEPT5, and SEPT7 were expressed in inner and outer pillar cells and Deiters' cells but the distribution patterns of each protein in Deiters' cells were different. SEPT4 and SEPT7 were expressed in the phalangeal process where SEPT5 was not detected. In addition to these cells SEPT5 and SEPT7 were co-localized with presynaptic vesicles of efferent nerve terminals. Only SEPT7 was expressed in the cochlea at embryonic stages. Although expression patterns of septin proteins suggested their important roles in the function of the cochlea, both Sept4 and Sept5 null mice had similar auditory functions to their wild type littermates. Immunohistochemical analysis of Sept4 null mice showed that compensatory expression of SEPT5 in the phalangeal process of Deiters' cells may have caused functional compensation of hearing ability in Sept4 null mice.

SEPT9 occupies the terminal positions in septin octamers and mediates polymerization-dependent functions in abscission

Septins are filamentous guanosine triphosphatase-binding proteins that are required for cytokinesis in a wide range of organisms from yeast to man. Several septins, including SEPT9, have been found to be altered in cancers, but their roles in malignancy and cytokinesis remain unclear. It is known that they assemble into rod-shaped oligomeric complexes that join end-on-end to form filaments, but whether SEPT9 incorporates into these complexes and how it does so are unanswered questions. We used tandem affinity purification of mammalian septin complexes to show that SEPT9 occupies a terminal position in an octameric septin complex. A mutant SEPT9, which cannot self-associate, disrupted septin filament formation and resulted in late abscission defects during cytokinesis but did not affect septin-dependent steps earlier in mitosis. These data suggest that mammalian SEPT9 holds a terminal position in the septin octamers, mediating abscission-specific polymerization during cytokinesis.

Cytoskeletal control of CD36 diffusion promotes its receptor and signaling function

The mechanisms that govern receptor coalescence into functional clusters--often a critical step in their stimulation by ligand--are poorly understood. We used single-molecule tracking to investigate the dynamics of CD36, a clustering-responsive receptor that mediates oxidized LDL uptake by macrophages. We found that CD36 motion in the membrane was spatially structured by the cortical cytoskeleton. A subpopulation of receptors diffused within linear confinement regions whose unique geometry simultaneously facilitated freedom of movement along one axis while increasing the effective receptor density. Co-confinement within troughs enhanced the probability of collisions between unligated receptors and promoted their clustering. Cytoskeleton perturbations that inhibited diffusion in linear confinement regions reduced receptor clustering in the absence of ligand and, following ligand addition, suppressed CD36-mediated signaling and internalization. These observations demonstrate a role for the cytoskeleton in controlling signal transduction by structuring receptor diffusion within membrane regions that increase their collision frequency.

Characterization of presynaptic septin complexes in mammalian hippocampal neurons

Septins are GTPases that form heteromeric complexes and are linked to neurological disorders. Although several septin subcomplexes have been reported in various mammalian tissues, the cellular and subcellular distribution of these complexes is largely unexplored. Using antibodies against ten mammalian septins, we show that septins diverge with respect to their tissue distribution implying that septin complexes in various tissues have unique composition. Although all ten septins examined were expressed in brain tissue, we describe septin complex(es) including SEPT3, SEPT5, SEPT6, SEPT7 and SEPT11 that could be functional within the presynapse because, unlike other septins they: (1) showed an increase in expression from embryonic day 15 to post-natal day 70, (2) were abundantly expressed in axons and growth cones of developing hippocampal neurons, (3) were found in presynaptic terminals of mature synapses, (4) were enriched in a preparation of synaptic vesicles and (5) immunoprecipitated together from brain tissue and cultured nerve cells. Knockdown of SEPT5 or SEPT7 in developing hippocampal neurons impaired axon growth. Because septins are functionally linked to the cytoskeleton and vesicle traffic, presynaptic neuronal septin complexes could be important for ensuring proper axon development and neurotransmitter release.

Distinct roles of septins in cytokinesis: SEPT9 mediates midbody abscission

Septins are a family of GTP-binding proteins implicated in mammalian cell division. Most studies examining the role of septins in this process have treated the family as a whole, thus neglecting the possibility that individual members may have diverse functions. To address this, we individually depleted each septin family member expressed in HeLa cells by siRNA and assayed for defects in cell division by immunofluorescence and time-lapse microscopy. Depletion of SEPT2, SEPT7, and SEPT11 causes defects in the early stages of cytokinesis, ultimately resulting in binucleation. In sharp contrast, SEPT9 is dispensable for the early stages of cell division, but is critical for the final separation of daughter cells. Rescue experiments indicate that SEPT9 isoforms containing the N-terminal region are sufficient to drive cytokinesis. We demonstrate that SEPT9 mediates the localization of the vesicle-tethering exocyst complex to the midbody, providing mechanistic insight into the role of SEPT9 during abscission.

Septins regulate developmental switching from microdomain to nanodomain coupling of Ca(2+) influx to neurotransmitter release at a central synapse

Neurotransmitter release depends critically on close spatial coupling of Ca(2+) entry to synaptic vesicles at the nerve terminal; however, the molecular substrates determining their physical proximity are unknown. Using the calyx of Held synapse, where "microdomain" coupling predominates at immature stages and developmentally switches to "nanodomain" coupling, we demonstrate that deletion of the filamentous protein Septin 5 imparts immature synapses with striking morphological and functional features reminiscent of mature synapses. This includes synaptic vesicles tightly localized to active zones, resistance to the slow Ca(2+) buffer EGTA and a reduced number of Ca(2+) channels required to trigger single fusion events. Disrupting Septin 5 organization acutely transforms microdomain to nanodomain coupling and potentiates quantal output in immature wild-type terminals. These observations suggest that Septin 5 is a core molecular substrate that differentiates distinct release modalities at the central synapse.

Stabilization of the actomyosin ring enables spermatocyte cytokinesis in Drosophila

The scaffolding protein anillin recruits septins to the cleavage furrow and constrains actomyosin contractility. Expression of E-cadherin suppresses the cytokinesis defects caused by anillin knockdown and stabilizes F-actin in the furrow, thereby providing an alternate means of coupling the actomyosin ring to the plasma membrane during cytokinesis.

The scaffolding protein anillin is required for completion of cytokinesis. Anillin binds filamentous (F) actin, nonmuscle myosin II, and septins and in cell culture models has been shown to restrict actomyosin contractility to the cleavage furrow. Whether anillin also serves this function during the incomplete cytokinesis that occurs in developing germ cells has remained unclear. Here, we show that anillin is required for cytokinesis in dividing Drosophila melanogaster spermatocytes and that anillin, septins, and myosin II stably associate with the cleavage furrow in wild-type cells. Anillin is necessary for recruitment of septins to the cleavage furrow and for maintenance of F-actin and myosin II at the equator in late stages of cytokinesis. Remarkably, expression of DE-cadherin suppresses the cytokinesis defect of anillin-depleted spermatocytes. DE-cadherin recruits β-catenin (armadillo) and α-catenin to the cleavage furrow and stabilizes F-actin at the equator. Similarly, E-cadherin expression suppresses the cytokinesis defect caused by anillin knockdown in mouse L-fibroblast cells. Our results show that the anillin-septin and cadherin–catenin complexes can serve as alternative cassettes to promote tight physical coupling of F-actin and myosin II to the cleavage furrow and successful completion of cytokinesis.

Uptake of oxidized low density lipoprotein by CD36 occurs by an actin-dependent pathway distinct from macropinocytosis

The class B scavenger receptor CD36 has numerous ligands that include modified forms of low density lipoprotein, fibrillar amyloid, apoptotic cells, and Plasmodium falciparum-infected red blood cells, linking this molecule to atherosclerosis, Alzheimer disease, malaria, and other diseases. We studied the signaling events that follow receptor engagement and lead to CD36 and ligand internalization. We show that oxidized low density lipoprotein or antibody-induced clustering of CD36 triggers macropinocytosis and internalization of the receptor-ligand complex. Remarkably, however, CD36 internalization is independent of macropinocytosis and occurs by a novel endocytic mechanism that depends on actin, but not dynamin. This actin-driven endocytosis requires the activation Src family kinases, JNK, and Rho family GTPases, but, unlike macropinocytosis, it is not affected by inhibitors of phosphatidylinositol 3-kinase or Na/H exchange. Manipulation of this unique mode of internalization may prove helpful in the prevention and management of the wide range of diseases in which CD36 is implicated.

Sept5 deficiency exerts pleiotropic influence on affective behaviors and cognitive functions in mice

Deletion or duplication of the human chromosome 22q11.2 is associated with many behavioral traits and neuropsychiatric disorders, including autism spectrum disorders and schizophrenia. However, why phenotypes vary widely among individuals with identical deletions or duplications of 22q11.2 and which specific 22q11.2 genes contribute to these phenotypes are still poorly understood. Previous studies have identified a approximately 200 kb 22q11.2 region that contributes to behavioral phenotypes in mice. We tested the role of Septin 5 (Sept5), a gene encoded in the approximately 200 kb region, in affective behaviors, cognitive capacities and motor activity. To evaluate the impact of genetic backgrounds on behavioral phenotypes of Sept5 deficiency, we used mice on two genetic backgrounds. Our data show that Sept5 deficiency decreased affiliative active social interaction, but this phenotypic expression was influenced by genetic backgrounds. In contrast, Sept5 deficiency decreased anxiety-related behavior, increased prepulse inhibition and delayed acquisition of rewarded goal approach, independent of genetic background. These data suggest that Sept5 deficiency exerts pleiotropic effects on a select set of affective behaviors and cognitive processes and that genetic backgrounds could provide an epistatic influence on phenotypic expression.

Mammalian septins are required for phagosome formation

Septins are members of a highly conserved family of filamentous proteins that are required in many organisms for the completion of cytokinesis. In addition, septins have been implicated in a number of important cellular processes and have been suggested to have roles in regulating membrane traffic. Given the proposed role of septins in cell membrane dynamics, we investigated the function of septins during FcgammaR-mediated phagocytosis. We show that several septins are expressed in RAW264.7 and J774 mouse macrophage cell lines and that SEPT2 and SEPT11 are colocalized with submembranous actin-rich structures during the early stages of FcgammaR-mediated phagocytosis. In addition, SEPT2 accumulation is seen in primary human neutrophils and in nonprofessional phagocytes. The time course of septin accumulation mirrors actin accumulation and is inhibited by latrunculin and genistein, but not other inhibitors of phagocytosis. Inhibition of septin function by transient expression of the BD3 domain of BORG3, known to cause septin aggregation, or depletion of SEPT2 or SEPT11 by RNAi, significantly inhibited FcgammaR-mediated phagocytosis of IgG-coated latex beads. Interestingly, this occurred without affecting the accumulation of actin or the actin-associated protein coronin-1. These observations show that, although not necessary for actin recruitment, septins are required for efficient FcgammaR-mediated phagocytosis.

Sept12 is a component of the mammalian sperm tail annulus

Since their essential role in cytokinesis was first shown in yeast, the septins have been described to function in diverse cellular contexts. The members of this unique class of GTPases are capable of binding and hydrolyzing GTP, associating with membranes and oligomerizing into higher order structures. Here we describe Sept12, a novel septin, identified in a yeast two hybrid screen using Sept5 as the bait. Sept12 contains the primary sequence elements of a septin and is capable of interacting with other septins. In addition, Sept12 purifies with bound nucleotide and binds to phosphoinositides, confirming its identity as a septin. RT-PCR and Northern blots reveal that Sept12 mRNA is expressed predominantly in testis, and this is supported by tissue Western blots. In rats, Sept12 protein levels rise upon sexual maturity and the Sept12 protein colocalizes with the annulus in isolated mature spermatozoa. Further, coexpression of Sept12 with Sept4, an essential annulus component, results in complete colocalization of both proteins into robust and highly curved filaments in CHO cells. This study suggests Sept12 may be involved in mammalian fertility.

TB or not TB: calcium regulation in mycobacterial survival

Mycobacterium tuberculosis (Mtb)-the bacterium that causes tuberculosis-resides in phagosomes inside macrophages. This bacterium evades destruction by preventing phagosome maturation, which involves the fusion of phagosomes with lysosomes. In this issue of Cell, Jayachandran et al. (2007) suggest that mycobacteria co-opt the actin-binding protein coronin 1 to activate the phosphatase calcineurin, thereby preventing phagosomal maturation.

Coronin function is required for chemotaxis and phagocytosis in human neutrophils

Coronins are a family of conserved actin-associated proteins that have been implicated in a variety of cellular processes dependent on actin rearrangements. In this study, we show that in primary human neutrophils, coronins-1-4 and -7 are expressed. Coronin-1 accumulates at the leading edge of migrating neutrophils and at the nascent phagosome. Inhibition of coronin function by transduction of a dominant-negative form of the protein leads to inhibition of chemotaxis and a reduction in neutrophil spreading and adhesion. This inhibition appears to correlate with changes in the distribution of F-actin structures within the cell. In addition, phagocytosis is inhibited, but neither secretion nor activation of the NADPH oxidase appears to be affected. Together, these results show that coronins are required for actin-dependent changes in cell morphology that lead to migration and phagocytosis.

Resolving polarized views: septin filaments do the twist

Septins are a highly conserved family of filamentous GTPases required for cell division in a wide range of eukaryotic organisms. In a recent issue of Nature, Vrabioiu and Mitchison use polarized fluorescence microscopy to show that septin filaments undergo a highly orchestrated rotation, from a longitudinal to circumferential orientation, coincident with splitting of the septin ring during cytokinesis (Vrabioiu and Mitchison, 2006).

Proteomic identification of a role for the von Hippel Lindau tumour suppressor in changes in the expression of mitochondrial proteins and septin 2 in renal cell carcinoma

The von Hippel Lindau (VHL) tumour suppressor gene, VHL, plays a central role in development of sporadic conventional renal cell carcinomas (RCCs). Studying VHL function may, therefore, increase understanding of the pathogenesis of RCC and identify markers/therapeutic targets. Comparison of 2-DE protein profiles of VHL-defective RCC cells (UMRC2) transfected with control vector or wild-type VHL showed differences in 30 proteins, including several novel changes. One of the findings confirmed by Western blotting was up-regulation of the mitochondrial protein ubiquinol cytochrome c reductase complex core protein 2 following VHL transfection, a change that was also observed in two other cell line backgrounds. A marked decrease in expression of this and several other mitochondrial proteins was demonstrated in RCC tissues and using VHL-transfectants, several were shown to exhibit VHL-dependent regulation. Thus, VHL may contribute to the decreased mitochondrial function seen in RCC. A form of septin 2 down-regulated following VHL transfection was also identified. Septin 2 was up-regulated in 12/16 RCCs, while alteration of the form present was also observed in 1/3 tumours analysed. Thus, increased expression of septin 2 is a common event in RCC and protein modification may also alter septin 2 function in a subset of tumours.

GTP binding and hydrolysis kinetics of human septin 2

Septins are a family of conserved proteins that are essential for cytokinesis in a wide range of organisms including fungi, Drosophila and mammals. In budding yeast, where they were first discovered, they are thought to form a filamentous ring at the bridge between the mother and bud cells. What regulates the assembly and function of septins, however, has remained obscure. All septins share a highly conserved domain related to those found in small GTPases, and septins have been shown to bind and hydrolyze GTP, although the properties of this domain and the relationship between polymerization and GTP binding/hydrolysis is unclear. Here we show that human septin 2 is phosphorylated in vivo at Ser218 by casein kinase II. In addition, we show that recombinant septin 2 binds guanine nucleotides with a Kd of 0.28 microm for GTPgammaS and 1.75 microm for GDP. It has a slow exchange rate of 7 x 10(-5) s(-1) for GTPgammaS and 5 x 10(-4) s(-1) for GDP, and an apparent kcat value of 2.7 x 10(-4) s(-1), similar to those of the Ras superfamily of GTPases. Interestingly, the nucleotide binding affinity appears to be altered by phosphorylation at Ser218. Finally, we show that a single septin protein can form homotypic filaments in vitro, whether bound to GDP or GTP.

Septins: traffic control at the cytokinesis intersection

The physical division of one cell into two requires the highly orchestrated separation of genetic and cytoplasmic contents during M phase of the cell cycle. Mitosis, the physical segregation of the genetic material of a cell into two daughter cells, has traditionally received more attention than cytokinesis, the partitioning of the cytoplasmic contents, yet clearly the two processes must be intimately co-ordinated and tightly regulated. While plant cells divide by the formation of a membranous cell barrier called the phragmoplast, animal cell division is largely driven by contraction of an actomyosin ring. However, recent evidence has suggested that membranes derived from one or more intracellular compartments are also required to break the cytoplasmic bridge connecting two dividing cells during late telophase. In this review, we focus on studies of animal cell cytokinesis that support a requirement for specific endomembrane fusion during fission, define molecular components of the membrane fusion apparatus that may be involved and point to possible roles for an emerging family of cytoskeletal proteins, the septins, in this process.

Coronin-1 function is required for phagosome formation

Coronin-1 is an actin-associated protein whose function in actin dynamics has remained obscure. All coronin proteins have a variable N-terminal domain, followed by WD repeats and a C-terminal coiled-coil dimerization domain. Transfection of coronin-1-GFP into RAW 264.7 cells revealed that coronin rapidly and transiently associates with the phagosome. To determine if coronin is involved in mammalian phagocytosis we used a dominant-negative approach by expressing only the central WD domains. However, this caused cell rounding and dissociation from the substratum, hampering analysis of their phenotype. We therefore developed TAT-fusion constructs of coronin-1 WD domains to acutely introduce the recombinant protein fragment into live cells. We show that although TAT-WD has no effect on binding of opsonized RBCs to RAW 264.7 cells, receptor clustering or several downstream signaling events, lamellipodial extensions, and actin accumulation at the base of the bound particle were diminished. Furthermore, Arp3 accumulation at the phagosome was impaired after TAT-WD treatment. Interestingly, whereas coronin-1 also accumulates at the sites of actin remodeling associated with Salmonella invasion, TAT-WD had no effect on this process. Together, our data demonstrates that coronin-1 is required for an early step in phagosome formation, consistent with a role in actin polymerization.

Septin 3 (G-septin) is a developmentally regulated phosphoprotein enriched in presynaptic nerve terminals

The septins are GTPase enzymes with multiple roles in cytokinesis, cell polarity or exocytosis. The proteins from the mammalian septin genes are called Sept1-10. Most are expressed in multiple tissues, but the mRNA for Sept5 (CDCrel-1) and Sept3 (G-septin) appear to be primarily expressed in brain. Sept3 is phosphorylated by cGMP-dependent protein kinase I (PKG-I) and the cGMP/PKG pathway is involved in presynaptic plasticity. Therefore to determine whether Sept3 specifically associates with neurones and nerve terminals we investigated its distribution in rat brain and neuronal cultures. Sept3 protein was detected only in brain by immunoblot, but not in 12 other tissues examined. Levels were high in all adult brain regions, and reduced in those enriched in white matter. Expression was developmentally regulated, being absent in the early embryo, low in late embryonic rat brain and increasing after birth. Like dynamin I, Sept3 was specifically enriched in synaptosomes compared with whole brain, and was only found in a peripheral membrane extract and not in the soluble or membrane extracts. Sept3 was particularly abundant in mossy fibre nerve terminals in the hippocampus. In primary cultured hippocampal neurones Sept3 immunoreactivity was punctate in neurites and predominantly localized to presynaptic terminals, strongly colocalizing with synaptophysin and dynamin I. The specific nerve terminal localization was confirmed by immunogold electron microscopy. Together this shows that Sept3 is a neurone-specific protein highly enriched in nerve terminals which supports a secretory role in synaptic vesicle recycling.

Abnormalities of presynaptic protein CDCrel-1 in striatum of rats reared in social isolation: relevance to neural connectivity in schizophrenia

Post-weaning social isolation-rearing of rats leads to behavioural and neurochemical sequelae that model aspects of schizophrenia, and it may be useful to test hypotheses related to putative molecular mechanisms of the illness. In humans, the presynaptic protein CDCrel-1 represents an interesting candidate molecule for the mechanism and aetiology of schizophrenia. CDCrel-1 modulates dopamine neurotransmission, binds to the SNARE protein syntaxin and maps onto a region of chromosome 22q11 deleted in velo-cardio-facial and DiGeorge syndromes, which are associated with increased prevalence of schizophrenia. Using the isolation-rearing model, we measured immunoreactivity of the synaptic proteins CDCrel-1, synaptophysin and syntaxin. Male, Sprague-Dawley rats were raised in groups or in isolation for 12 weeks from weaning. Synaptic protein immunoreactivities were measured in striatal and hippocampal homogenates, using a sensitive enzyme-linked immunoadsorbent assay with monoclonal antibodies. Isolation-rearing produced region- and protein-specific effects. CDCrel-1 immunoreactivity was significantly lower in the striatum and marginally higher in the hippocampus of isolation-reared compared with socially reared animals. There were no statistically significant differences in synaptophysin immunoreactivity in either region. Confocal microscopy demonstrated a high degree of colocalization between the two presynaptic proteins. In striatum, a robust relationship between CDCrel-1 and syntaxin immunoreactivities was observed in socially reared rats, this was lost in the isolation-reared animals. Altered levels of the septin CDCrel-1 in isolation-reared rats may contribute to changes in neuronal connectivity and neurotransmission, and suggest a potential role for CDCrel-1 in schizophrenia related to chromosome 22q11 deletion syndrome.

Septin 2 phosphorylation: theoretical and mass spectrometric evidence for the existence of a single phosphorylation site in vivo

Septins constitute a family of conserved proteins that are required for cytokinesis in a wide range of organisms. Most cells express a set of septin proteins and these are found to assemble into hetero-oligomeric septin complexes that appear filamentous. However, the mechanisms controlling the function and polymerization of septins are not known. We therefore examined the possibility that septins could be post-translationally modified by phosphorylation. We present herein a combined theoretical and experimental approach for the analysis of Septin 2 (Sept2) monophosphorylation in vivo. We purified and characterized the human recombinant Sept2, a 45-kDa protein, expressed from Sf21 insect cells. Analysis by matrix-assisted laser desorption/ionization quadrupole time-of-flight mass spectrometry on the full-length protein sequence of wild-type Sept2 revealed a unique phosphorylation site at residue Ser248 in vivo, which is consistent with one of the twelve phosphorylation sites in the protein sequence theoretically predicted by the Netphos program. Additional predictions with the motif scan programs Scansite and Prosite suggest that the phosphorylation of wild-type Sept2 might be a potential substrate for casein kinase 2. Site-directed mutagenesis of residue 248 from serine to alanine abrogated this phosphorylation. The location of phosphorylation in Sept2 differs from the sites predicted for cGMP-dependent protein kinase (PKG) phosphorylation in Septin 3, raising the possibility that different septins may undergo distinct phosphorylation events that could control their functions in important cellular processes such as neurotransmission or cytokinesis.