Here come the septins: novel polymers that coordinate intracellular functions and organization

The Septin Gene StSep4 Contributes to the Pathogenicity of Setosphaeria turcica by Regulating the Morphology, Cell Wall Integrity, and Pathogenic Factor Biosynthesis

Septins are a conserved group of GTP-binding proteins found in all eukaryotes and are the fourth-most abundant cytoskeletal proteins. Septins of some pathogenic fungi are involved in morphological changes related to infection. Our previous studies have identified four core septins (StSep1-4) in Setosphaeria turcica, the causal agent of northern corn leaf blight, while only StSep4 is significantly upregulated during the invasive process. We therefore used forchlorfenuron (FCF), the specific inhibitor of septin, and ΔStSep4 knockout mutants to further clarify the role of septins in S. turcica pathogenicity. FCF treatment caused a dose-dependent reduction in S. turcica colony growth, delayed the formation of infection structures, and reduced the penetration ability. ΔStSep4 knockout mutants displayed abnormal mycelium morphology, slow mycelial growth, conidiation deficiency, delayed appressorium development, and weakened pathogenicity. StSep4 deletion also broke cell wall integrity, altered chitin distribution, decreased the melanin content, and disrupted normal nuclear localization. A transcriptomic comparison revealed that genes differentially expressed between ΔStSep4 and WT were enriched in terms of ribosomes, protein translation, membrane components, and transmembrane transport activities. Our results demonstrate that StSep4 is required for morphology and pathogenicity in S. turcica, making it a promising target for the development of novel fungicides.

The emerging role of septins in fungal pathogenesis

Fungal pathogens undergo specific morphogenetic transitions in order to breach the outer surfaces of plants and invade the underlying host tissue. The ability to change cell shape and switch between non-polarised and polarised growth habits is therefore critical to the lifestyle of plant pathogens. Infection-related development involves remodelling of the cytoskeleton, plasma membrane and cell wall at specific points during fungal pathogenesis. Septin GTPases are components of the cytoskeleton that play pivotal roles in actin remodelling, micron-scale plasma membrane curvature sensing and cell polarity. Septin assemblages, such as rings, collars and gauzes, are known to have important roles in cell shape changes and are implicated in formation of specialised infection structures to enter plant cells. Here, we review and compare the reported functions of septins of plant pathogenic fungi, with a special focus on invasive growth. Finally, we discuss septins as potential targets for broad-spectrum antifungal plant protection strategies.

Screening ANLN and ASPM as bladder urothelial carcinoma-related biomarkers based on weighted gene co-expression network analysis

Introduction: Bladder cancer (BLCA) is one of the most common malignancies in the urinary system with a poor prognosis and high treatment costs. Identifying potential prognostic biomarkers is significant for exploring new therapeutic and predictive targets of BLCA.

Methods: In this study, we screened differentially expressed genes using the GSE37815 dataset. We then performed a weighted gene co‐expression network analysis (WGCNA) to identify the genes correlated with the histologic grade and T stage of BLCA using the GSE32548 dataset. Subsequently, Kaplan Meier survival analysis and Cox regression were used to further identify prognosis‐related hub genes using the datasets GSE13507 and TCGA‐BLCA. Moreover, we detected the expression of the hub genes in 35 paired samples, including BLCA and paracancerous tissue, from the Shantou Central Hospital by qRT‐polymerase chain reaction.

Results: This study showed that Anillin (ANLN) and Abnormal spindle-like microcephaly-associated gene (ASPM) were prognostic biomarkers for BLCA. High expression of ANLN and ASPM was associated with poor overall survival.The qRT‐PCR results revealed that ANLN and ASPM genes were upregulated in BLCA, and there was a correlation between the expression of ANLN and ASPM in cancer tissues and paracancerous tissue. Additionally, the increasing multiples in the ANLN gene was obvious in high-grade BLCA.

Discussion: In summary, this preliminary exploration indicated a correlation between ANLN and ASPM expression. These two genes, serving as the risk factors for BLCA progression, might be promising targets to improve the occurrence and progression of BLCA.

Septin7 is indispensable for proper skeletal muscle architecture and function

Today septins are considered as the fourth component of the cytoskeleton, with the Septin7 isoform playing a critical role in the formation of higher-order structures. While its importance has already been confirmed in several intracellular processes of different organs, very little is known about its role in skeletal muscle. Here, using Septin7 conditional knockdown (KD) mouse model, the C2C12 cell line, and enzymatically isolated adult muscle fibers, the organization and localization of septin filaments are revealed, and an ontogenesis-dependent expression of Septin7 is demonstrated. KD mice displayed a characteristic hunchback phenotype with skeletal deformities, reduction in in vivo and in vitro force generation, and disorganized mitochondrial networks. Furthermore, knockout of Septin7 in C2C12 cells resulted in complete loss of cell division while KD cells provided evidence that Septin7 is essential for proper myotube differentiation. These and the transient increase in Septin7 expression following muscle injury suggest that it may be involved in muscle regeneration and development.

Contribution of septins to human platelet structure and function

Although septins have been well-studied in nucleated cells, their role in anucleate blood platelets remains obscure. Here, we elucidate the contribution of septins to human platelet structure and functionality. We show that Septin-2 and Septin-9 are predominantly distributed at the periphery of resting platelets and co-localize strongly with microtubules. Activation of platelets by thrombin causes clustering of septins and impairs their association with microtubules. Inhibition of septin dynamics with forchlorfenuron (FCF) reduces thrombin-induced densification of septins and lessens their colocalization with microtubules in resting and activated platelets. Exposure to FCF alters platelet shape, suggesting that septins stabilize platelet cytoskeleton. FCF suppresses platelet integrin αIIbβ3 activation, promotes phosphatidylserine exposure on activated platelets, and induces P-selectin expression on resting platelets, suggesting septin involvement in these processes. Inhibition of septin dynamics substantially reduces platelet contractility and abrogates their spreading on fibrinogen-coated surfaces. Overall, septins strongly contribute to platelet structure, activation and biomechanics.

PIAS1 Regulates Hepatitis C Virus-Induced Lipid Droplet Accumulation by Controlling Septin 9 and Microtubule Filament Assembly

Chronic hepatitis C virus (HCV) infection often leads to fibrosis and chronic hepatitis, then cirrhosis and ultimately hepatocellular carcinoma (HCC). The processes of the HVC life cycle involve intimate interactions between viral and host cell proteins and lipid metabolism. However, the molecules and mechanisms involved in this tripartite interaction remain poorly understood. Herein, we show that the infection of HCC-derived Huh7.5 cells with HCV promotes upregulation of the protein inhibitor of activated STAT1 (PIAS1). Reciprocally, PIAS1 regulated the expression of HCV core protein and HCV-induced LD accumulation and impaired HCV replication. Furthermore, PIAS1 controlled HCV-promoted septin 9 filament formation and microtubule polymerization. Subsequently, we found that PIAS1 interacted with septin 9 and controlled its assembly on filaments, which thus affected septin 9-induced lipid droplet accumulation. Taken together, these data reveal that PIAS1 regulates the accumulation of lipid droplets and offer a meaningful insight into how HCV interacts with host proteins.

Neuroproteomics in Epilepsy: What Do We Know so Far?

Epilepsies are chronic neurological diseases that affect approximately 2% of the world population. In addition to being one of the most frequent neurological disorders, treatment for patients with epilepsy remains a challenge, because a proportion of patients do not respond to the antiseizure medications that are currently available. This results in a severe economic and social burden for patients, families, and the healthcare system. A characteristic common to all forms of epilepsy is the occurrence of epileptic seizures that are caused by abnormal neuronal discharges, leading to a clinical manifestation that is dependent on the affected brain region. It is generally accepted that an imbalance between neuronal excitation and inhibition generates the synchronic electrical activity leading to seizures. However, it is still unclear how a normal neural circuit becomes susceptible to the generation of seizures or how epileptogenesis is induced. Herein, we review the results of recent proteomic studies applied to investigate the underlying mechanisms leading to epilepsies and how these findings may impact research and treatment for these disorders.

The Biology of Invasive Growth by the Rice Blast Fungus Magnaporthe oryzae

This introductory chapter describes the life cycle of Magnaporthe oryzae, the causal agent of rice blast disease. During plant infection, M. oryzae forms a specialized infection structure called an appressorium, which generates enormous turgor, applied as a mechanical force to breach the rice cuticle. Appressoria form in response to physical cues from the hydrophobic rice leaf cuticle and nutrient availability. The signaling pathways involved in perception of surface signals are described and the mechanism by which appressoria function is also introduced. Re-polarization of the appressorium requires a septin complex to organize a toroidal F-actin network at the base of the cell. Septin aggregation requires a turgor-dependent sensor kinase, Sln1, necessary for re-polarization of the appressorium and development of a rigid penetration hypha to rupture the leaf cuticle. Once inside the plant, the fungus undergoes secretion of a large set of effector proteins, many of which are directed into plant cells using a specific secretory pathway. Here they suppress plant immunity, but can also be perceived by rice immune receptors, triggering resistances. M. oryzae then manipulates pit field sites, containing plasmodesmata, to facilitate rapid spread from cell to cell in plant tissue, leading to disease symptom development.

Junctional Localization of Septin 2 Is Required for Organization of Junctional Proteins in Static Endothelial Monolayers

OBJECTIVE

Septin 2 is localized at junctions in human microvascular endothelial monolayers. The junctional localization of septin 2 is necessary for organization of cell-cell adhesion proteins of endothelial cells. Approach and Results: Septin 2 was depleted at junctions by suppression of expression using shRNA, treatment with inflammatory cytokine, TNF (tumor necrosis factor)-α, and ectopic overexpression of septin 2 phosphatidylinositol 4,5-bisphosphate binding mutant defect in interaction with plasma membrane. Under those conditions, organizations and expression levels of various junctional proteins were analyzed. Confocal images of immunofluorescence staining showed substantial disorganization of adherens junctional proteins, nectin-2 and afadin, TJP (tight junction protein), ZO (zonula occludens)-1, and intercellular adhesion protein, PECAM-1 (platelet-endothelial cell adhesion molecule-1). Immunoblots for those proteins did not show significant changes in expression except for nectin-2 that highly increased in expression. Significant differential gene expression profiles and biological pathway analysis by septin 2 suppression and by TNF-α treatment using RNA-seq showed common overlapping pathways. The commonalities in expression may be consistent with the similar effects on the overall organization of cell-cell adhesion proteins.

CONCLUSIONS

Localization of septin 2 at cell junctions are required for the arrangement of junctional proteins and the integrity of the barrier formed by endothelial monolayers.

Anillin is an emerging regulator of tumorigenesis, acting as a cortical cytoskeletal scaffold and a nuclear modulator of cancer cell differentiation

Remodeling of the intracellular cytoskeleton plays a key role in accelerating tumor growth and metastasis. Targeting different cytoskeletal elements is important for existing and future anticancer therapies. Anillin is a unique scaffolding protein that interacts with major cytoskeletal structures, e.g., actin filaments, microtubules and septin polymers. A well-studied function of this scaffolding protein is the regulation of cytokinesis at the completion of cell division. Emerging evidence suggest that anillin has other important activities in non-dividing cells, including control of intercellular adhesions and cell motility. Anillin is markedly overexpressed in different solid cancers and its high expression is commonly associated with poor prognosis of patient survival. This review article summarizes rapidly accumulating evidence that implicates anillin in the regulation of tumor growth and metastasis. We focus on molecular and cellular mechanisms of anillin-dependent tumorigenesis that include both canonical control of cytokinesis and novel poorly understood functions as a nuclear regulator of the transcriptional reprogramming and phenotypic plasticity of cancer cells.

A complete compendium of crystal structures for the human SEPT3 subgroup reveals functional plasticity at a specific septin interface

Human septins 3, 9 and 12 are the only members of a specific subgroup of septins that display several unusual features, including the absence of a C-terminal coiled coil. This particular subgroup (the SEPT3 septins) are present in rod-like octameric protofilaments but are lacking in similar hexameric assemblies, which only contain representatives of the three remaining subgroups. Both hexamers and octamers can self-assemble into mixed filaments by end-to-end association, implying that the SEPT3 septins may facilitate polymerization but not necessarily function. These filaments frequently associate into higher order complexes which associate with biological membranes, triggering a wide range of cellular events. In the present work, a complete compendium of crystal structures for the GTP-binding domains of all of the SEPT3 subgroup members when bound to either GDP or to a GTP analogue is provided. The structures reveal a unique degree of plasticity at one of the filamentous interfaces (dubbed NC). Specifically, structures of the GDP and GTPγS complexes of SEPT9 reveal a squeezing mechanism at the NC interface which would expel a polybasic region from its binding site and render it free to interact with negatively charged membranes. On the other hand, a polyacidic region associated with helix α5', the orientation of which is particular to this subgroup, provides a safe haven for the polybasic region when retracted within the interface. Together, these results suggest a mechanism which couples GTP binding and hydrolysis to membrane association and implies a unique role for the SEPT3 subgroup in this process. These observations can be accounted for by constellations of specific amino-acid residues that are found only in this subgroup and by the absence of the C-terminal coiled coil. Such conclusions can only be reached owing to the completeness of the structural studies presented here.

Septin 9 isoforms promote tumorigenesis in mammary epithelial cells by increasing migration and ECM degradation through metalloproteinase secretion at focal adhesions

The cytoskeletal interacting protein Septin 9 (SEPT9), a member of the septin gene family, has been proposed to have oncogenic functions. It is a known hot spot of retroviral tagging insertion and a fusion partner of both de novo and therapy-induced mixed lineage leukemia (MLL). Of all septins, SEPT9 holds the strongest link to cancer, especially breast cancer. Murine models of breast cancer frequently exhibit Sept9 amplification in the form of double minute chromosomes, and about 20% of human breast cancer display genomic amplification and protein over expression at the SEPT9 locus. Yet, a clear mechanism by which SEPT9 elicits tumor-promoting functions is lacking.

To obtain unbiased insights on molecular signatures of SEPT9 upregulation in breast tumors, we overexpressed several of its isoforms in breast cancer cell lines. Global transcriptomic profiling supports a role of SEPT9 in invasion. Functional studies reveal that SEPT9 upregulation is sufficient to increase degradation of the extracellular matrix, while SEPT9 downregulation inhibits this process. The degradation pattern is peripheral and associated with focal adhesions (FA), where it is coupled with increased expression of matrix metalloproteinases. SEPT9 overexpression induces MMP upregulation in human tumors and in culture models and promotes MMP3 secretion to the media at FAs. Downregulation of SEPT9 or chemical inhibition of septin filament assembly impairs recruitment of MMP3 to FAs. Our results indicate that SEPT9 promotes upregulation and both trafficking and secretion of MMPs near FAs, thus enhancing migration and invasion of breast cancer cells.

Preliminary investigation of the effects of an FCF inhibitor interference with Septin in the early stage embryos in mice

The objective of the present study was to investigate the effects of for chlorfenuron (FCF) interference with the septin protein on early stage embryos in mice. The 1‑cell embryos were collected and divided into an FCF interference group and a control group. The FCF interference group was cultured in FCF media and the control group was cultured in dimethyl sulphoxide media at 37˚C with 5% CO2 until the desired phase was achieved. Septin2 protein expression was detected using immunofluorescence and western blot analysis. Blastocyst α‑tubulin was stained by immunofluorescence to observe the alterations in spindles and microtubules. The rate of early embryo development into blastocysts was significantly reduced following FCF treatment (P<0.05). In the control group, septin2 was observed with a confocal microscope; septin2 was expressed in embryos at all stages and mainly in the blastomeres from the 2‑cell stage onwards, with the expression concentrated in the nuclei of the blastomeres as identified by strong fluorescence. In the FCF interference group, septin2 was weakly expressed in the nuclei of blastomeres at the 2‑ and 4‑cell stages, and in the granulated blastomeres at the 4‑ and 8‑cell stages. Expression was barely observed in and following the morula. Granulation was observed starting from the 4‑ and 8‑cell stages. Compared with the control group, the FCF interference group exhibited irregular microtubules, abnormal spindle morphology and disordered chromosome arrangement in the blastocysts. The septin2 protein was expressed throughout the early stage embryo from the 2‑cell stage to the blastocyst and localized in the nuclei of blastomeres. When the septin protein experienced interference by the FCF inhibitor, septin2 protein expression was reduced, which simultaneously resulted in abnormal embryonic development, uneven cytoplasmic division, various sizes and a reduced number of blastomeres, granulation in the blastomeres, disordered blastocyst microtubule distribution, spindle shape alterations and an abnormality of chromosome arrangement.

Leukodystrophy caused by plasmalogen deficiency rescued by glyceryl 1-myristyl ether treatment

Plasmalogens are the most abundant form of ether phospholipids in myelin and their deficiency causes Rhizomelic Chondrodysplasia Punctata (RCDP), a severe developmental disorder. Using the Gnpat-knockout (KO) mouse as a model of RCDP, we determined the consequences of a plasmalogen deficiency during myelination and myelin homeostasis in the central nervous system (CNS). We unraveled that the lack of plasmalogens causes a generalized hypomyelination in several CNS regions including the optic nerve, corpus callosum and spinal cord. The defect in myelin content evolved to a progressive demyelination concomitant with generalized astrocytosis and white matter-selective microgliosis. Oligodendrocyte precursor cells (OPC) and mature oligodendrocytes were abundant in the CNS of Gnpat KO mice during the active period of demyelination. Axonal loss was minimal in plasmalogen-deficient mice, although axonal damage was observed in spinal cords from aged Gnpat KO mice. Characterization of the plasmalogen-deficient myelin identified myelin basic protein and septin 7 as early markers of dysmyelination, whereas myelin-associated glycoprotein was associated with the active demyelination phase. Using in vitro myelination assays, we unraveled that the intrinsic capacity of oligodendrocytes to ensheath and initiate membrane wrapping requires plasmalogens. The defect in plasmalogens was rescued with glyceryl 1-myristyl ether [1-O-tetradecyl glycerol (1-O-TDG)], a novel alternative precursor in the plasmalogen biosynthesis pathway. 1-O-TDG treatment rescued myelination in plasmalogen-deficient oligodendrocytes and in mutant mice. Our results demonstrate the importance of plasmalogens for oligodendrocyte function and myelin assembly, and identified a novel strategy to promote myelination in nervous tissue.

Septin 9 has Two Polybasic Domains Critical to Septin Filament Assembly and Golgi Integrity

Septins are GTP-binding proteins involved in several membrane remodeling mechanisms. They associate with membranes, presumably using a polybasic domain (PB1) that interacts with phosphoinositides (PIs). Membrane-bound septins assemble into microscopic structures that regulate membrane shape. How septins interact with PIs and then assemble and shape membranes is poorly understood. Here, we found that septin 9 has a second polybasic domain (PB2) conserved in the human septin family. Similar to PB1, PB2 binds specifically to PIs, and both domains are critical for septin filament formation. However, septin 9 membrane association is not dependent on these PB domains, but on putative PB-adjacent amphipathic helices. The presence of PB domains guarantees protein enrichment in PI-contained membranes, which is critical for PI-enriched organelles. In particular, we found that septin 9 PB domains control the assembly and functionality of the Golgi apparatus. Our findings offer further insight into the role of septins in organelle morphology.

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Two polybasic domains mediate septin 9 interactions with PIs

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Human septins have amphipathic helices suitable for membrane binding

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Septin 9 polybasic domains mediate the formation of septin higher-order structures

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The mutation or depletion of septin polybasic domains induces Golgi fragmentation

Reorganization of Septins Modulates Synaptic Transmission at Neuromuscular Junctions

Septins (Sept) are highly conserved Guanosine-5'-triphosphate (GTP)-binding cytoskeletal proteins involved in neuronal signaling in the central nervous system but their involvement in signal transmission in peripheral synapses remains unclear. Sept5 and Sept9 proteins were detected in mouse peripheral neuromuscular junctions by immunofluorescence with a greater degree of co-localization with presynaptic than postsynaptic membranes. Preincubation of neuromuscular junction preparations with the inhibitor of Sept dynamics, forchlorfenuron (FCF), decreased co-localization of Sept with presynaptic membranes. FCF introduced ex vivo or in vivo had no effect on the amplitude of the spontaneous endplate currents (EPCs), indicating the absence of postsynaptic effects of FCF. However, FCF decreased acetylcholine (ACh) quantal release in response to nerve stimulation, reduced the amplitude of evoked quantal currents and decreased the number of quanta with long synaptic delays, demonstrating the presynaptic action of FCF. Nevertheless, FCF had no effect on the amplitude of calcium transient in nerve terminals, as detected by calcium-sensitive dye, and slightly decreased the ratio of the second response amplitude to the first one in paired-pulse experiments. These results suggest that FCF-induced decrease in ACh quantal secretion is not due to a decrease in Ca²⁺ influx but is likely related to the impairment of later stages occurring after Ca²⁺ entry, such as trafficking, docking or membrane fusion of synaptic vesicles. Therefore, Sept9 and Sept5 are abundantly expressed in presynaptic membranes, and disruption of Sept dynamics suppresses the evoked synchronous and delayed asynchronous quantal release of ACh, strongly suggesting an important role of Sept in the regulation of neurotransmission in peripheral synapses.

Glucose starvation triggers filamentous septin assemblies in an S. pombe septin-2 deletion mutant

Using correlative light and electron microscopy (CLEM), we studied the intracellular organization by of glucose-starved fission yeast cells (Schizosaccharomyces pombe) with regards to the localization of septin proteins throughout the cytoplasm. Thereby, we found that for cells carrying a deletion of the gene encoding septin-2 (spn2Δ), starvation causes a GFP-tagged version of septin-3 (spn3-GFP) and family members, to assemble into a single, prominent filamentous structure. It was previously shown that during exponential growth, spn2Δ cells form septin-3 polymers. However, the polymers we observed during exponential growth are different from the spn3p-GFP structure we observed in starved cells. Using CLEM, in combination with anti-GFP immunolabeling on plastic-sections, we could assign spn3p-GFP to the filaments we have found in EM pictures. Besides septin-3, these filamentous assemblies most likely also contain septin-1 as an RFP-tagged version of this protein forms a very similar structure in starved spn2Δ cells. Our data correlate phase-contrast and fluorescence microscopy with electron micrographs of plastic-embedded cells, and further on with detailed views of tomographic 3D reconstructions. Cryo-electron microscopy of spn2Δ cells in vitrified sections revealed a very distinct overall morphology of the spn3p-GFP assembly. The fine-structured, regular density pattern suggests the presence of assembled septin-3 filaments that are clearly different from F-actin bundles. Furthermore, we found that starvation causes substantial mitochondria fission, together with massive decoration of their outer membrane by ribosomes.

Septin 7 mediates high glucose-induced podocyte apoptosis

Podocyte depletion is a central pathological mechanism of diabetic nephropathy (DN). Hyperglycemia induced podocyte apoptosis, resulting in podocyte depletion. However, the crucial mechanism of hyperglycemia-induced podocyte apoptosis remains poorly understood. In this study, we evaluated the expression of septin 7, a GTP-binding protein, in glomerular podocytes of patients and mice with DN, and investigated the pro-apoptotic effect of septin 7 on high glucose (HG) induced podocyte apoptosis in vitro. We found septin 7 expression was markedly increased not only in glomerular podocytes of patients and db/db mice with DN but also in cultured podocytes with HG stimulation. Knocking down septin 7 with siRNA could attenuate HG induced podocytes apoptosis and excessive intracellular Ca²⁺ concentration. This study revealed septin7 may potentially play a proapoptotic role in podocyte under diabetic conditions and may provide a potential target for preventing podocyte apoptosis in DN.

CDC42 Negatively Regulates Testis-Specific SEPT12 Polymerization

Septin (SEPT) genes encode well-preserved polymerizing GTP-binding cytoskeletal proteins. The cellular functions of SEPTs consist of mitosis, cytoskeletal remodeling, cell polarity, and vesicle trafficking through interactions with various types of cytoskeletons. We discovered that mutated SEPTIN12 in different codons resulted in teratozoospermia or oligozoospermia. In mouse models with a defective Septin12 allele, sperm morphology was abnormal, sperm count decreased, and sperms were immotile. However, the regulators of SEPT12 are completely unknown. Some studies have indicated that CDC42 negatively regulates the polymerization of SEPT2/6/7 complexes in mammalian cell lines. In this study, we investigated whether CDC42 modulates SEPT12 polymerization and is involved in the terminal differentiation of male germ cells. First, through scanning electron microscopy analysis, we determined that the loss of Septin12 caused defective sperm heads. This indicated that Septin12 is critical for the formation of sperm heads. Second, CDC42 and SEPT12 were similarly localized in the perinuclear regions of the manchette at the head of elongating spermatids, neck region of elongated spermatids, and midpiece of mature spermatozoa. Third, wild-type CDC42 and CDC42Q61L (a constitutive-acting-mutant) substantially repressed SEPT12 polymerization, but CDC42T17N (a dominant-negative-acting mutant) did not, as evident through ectopic expression analysis. We concluded that CDC42 negatively regulates SEPT12 polymerization and is involved in terminal structure formation of sperm heads.

Cilium structure, assembly, and disassembly regulated by the cytoskeleton

The cilium, once considered a vestigial structure, is a conserved, microtubule-based organelle critical for transducing extracellular chemical and mechanical signals that control cell polarity, differentiation, and proliferation. The cilium undergoes cycles of assembly and disassembly that are controlled by complex inter-relationships with the cytoskeleton. Microtubules form the core of the cilium, the axoneme, and are regulated by post-translational modifications, associated proteins, and microtubule dynamics. Although actin and septin cytoskeletons are not major components of the axoneme, they also regulate cilium organization and assembly state. Here, we discuss recent advances on how these different cytoskeletal systems­ affect cilium function, structure, and organization.

Septins regulate junctional integrity of endothelial monolayers

Junctional integrity of endothelial monolayers is crucial to control movement of molecules and cells across the endothelium. Examining the structure and dynamics of cell junctions in endothelial monolayers, we discovered a role for septins. Contacts between adjacent endothelial cells were dynamic, with protrusions extending above or below neighboring cells. Vascular endothelial cadherin (VE-cadherin) was present at cell junctions, with a membrane-associated layer of F-actin. Septins localized at cell-junction membranes, in patterns distinct from VE-cadherin and F-actin. Septins assumed curved and scallop-shaped patterns at junctions, especially in regions of positive membrane curvature associated with actin-rich membrane protrusions. Depletion of septins led to disrupted morphology of VE-cadherin junctions and increased expression of VE-cadherin. In videos, septin-depleted cells displayed remodeling at cell junctions; regions with VE-cadherin were broader, and areas with membrane ruffling were wider. Septin depletion and junction disruption led to functional loss of junctional integrity, revealed by decreased transendothelial electric resistance and increased transmigration of immune cells. We conclude that septins, as cytoskeletal elements associated with the plasma membrane, are important for cell junctions and junctional integrity of endothelial monolayers, functioning at regions of positive curvature in support of actin-rich protrusions to promote cadherin-based cell junctions.

The role of septin 7 in physiology and pathological disease: A systematic review of current status

Septins are a conserved family of cytoskeletal GTPases present in different organisms, including yeast, drosophila, Caenorhabditis elegans and humans. In humans, septins are involved in various cellular processes, including exocytosis, apoptosis, leukemogenesis, carcinogenesis and neurodegeneration. Septin 7 is unique out of 13 human septins. Mammalian septin 6, septin 7, septin 2 and septin 9 coisolate together in complexes to form the core unit for the generation of the septin filaments. Physiological septin filaments are hetero-oligomeric complexes consisting of core septin hexamers and octamers. Furthermore, septin 7 plays a crucial role in cytokinesis and mitosis. Septin 7 is localized to the filopodia and branches of developing hippocampal neurons, and is the most abundant septin in the adult rat forebrain as well as a structural component of the human and mouse sperm annuli. Septin 7 is crucial to the spine morphogenesis and dendrite growth in neurons, and is also a structural constituent of the annulus in human and mouse sperm. It can suppress growth of some tumours such as glioma and papillary thyroid carcinoma. However, the molecular mechanisms of involvement of septin 7 in human disease, especially in the development of cancer, remain unclear. This review focuses on the structure, function and mechanism of septin 7 in vivo, and summarizes the role of septin 7 in cell proliferation, cytokinesis, nervous and reproductive systems, as well as the underlying molecular events linking septin 7 to various diseases, such as Alzheimer's disease, schizophrenia, neuropsychiatric systemic lupus erythematosus, tumour and so on.

Phosphorylation of Pnut in the Early Stages of Drosophila Embryo Development Affects Association of the Septin Complex with the Membrane and Is Important for Viability

Septin proteins are polymerizing GTPases that are found in most eukaryotic species. Septins are important for cytokinesis and participate in many processes involving spatial modifications of the cell cortex. In Drosophila, septin proteins Pnut, Sep1, and Sep2 form a hexameric septin complex. Here, we found that septin protein Pnut is phosphorylated during the first 2 hr of Drosophila embryo development. To study the effect of Pnut phosphorylation in a live organism, we created a new Drosophila pnut null mutant that allows for the analysis of Pnut mutations during embryogenesis. To understand the functional significance of Pnut phosphorylation, Drosophila strains carrying nonphosphorylatable and phospho-mimetic mutant pnut transgenes were established. The expression of the nonphosphorylatable Pnut protein resulted in semilethality and abnormal protein localization, whereas the expression of the phospho-mimetic mutant form of Pnut disrupted the assembly of a functional septin complex and septin filament formation in vitro. Overall, our findings indicate that the controlled phosphorylation of Pnut plays an important role in regulating septin complex functions during organism development.

Proteomic Dissection of Nanotopography-Sensitive Mechanotransductive Signaling Hubs that Foster Neuronal Differentiation in PC12 Cells

Neuronal cells are competent in precisely sensing nanotopographical features of their microenvironment. The perceived microenvironmental information will be “interpreted” by mechanotransductive processes and impacts on neuronal functioning and differentiation. Attempts to influence neuronal differentiation by engineering substrates that mimic appropriate extracellular matrix (ECM) topographies are hampered by the fact that profound details of mechanosensing/-transduction complexity remain elusive. Introducing omics methods into these biomaterial approaches has the potential to provide a deeper insight into the molecular processes and signaling cascades underlying mechanosensing/-transduction but their exigence in cellular material is often opposed by technical limitations of major substrate top-down fabrication methods. Supersonic cluster beam deposition (SCBD) allows instead the bottom-up fabrication of nanostructured substrates over large areas characterized by a quantitatively controllable ECM-like nanoroughness that has been recently shown to foster neuron differentiation and maturation. Exploiting this capacity of SCBD, we challenged mechanosensing/-transduction and differentiative behavior of neuron-like PC12 cells with diverse nanotopographies and/or changes of their biomechanical status, and analyzed their phosphoproteomic profiles in these settings. Versatile proteins that can be associated to significant processes along the mechanotransductive signal sequence, i.e., cell/cell interaction, glycocalyx and ECM, membrane/f-actin linkage and integrin activation, cell/substrate interaction, integrin adhesion complex, actomyosin organization/cellular mechanics, nuclear organization, and transcriptional regulation, were affected. The phosphoproteomic data suggested furthermore an involvement of ILK, mTOR, Wnt, and calcium signaling in these nanotopography- and/or cell mechanics-related processes. Altogether, potential nanotopography-sensitive mechanotransductive signaling hubs participating in neuronal differentiation were dissected.

Interaction of Recombinant Gallus gallus SEPT5 and Brain Proteins of H5N1-Avian Influenza Virus-Infected Chickens

Septin forms a conserved family of cytoskeletal guanosine triphosphate (GTP) binding proteins that have diverse roles in protein scaffolding, vesicle trafficking, and cytokinesis. The involvement of septins in infectious viral disease pathogenesis has been demonstrated by the upregulation of SEPT5 protein and its mRNA in brain tissues of H5N1-infected chickens, thus, providing evidence for the potential importance of this protein in the pathogenesis of neurovirulence caused by the avian influenza virus. In this study, cloning, expression, and purification of Gallus gallus SEPT5 protein was performed in Escherichia coli. The SEPT5 gene was inserted into the pRSETB expression vector, transformed in the E. coli BL21 (DE3) strain and the expression of SEPT5 protein was induced by IPTG. The SEPT5 protein was shown to be authentic as it was able to be pulled down by a commercial anti-SEPT5 antibody in a co-immunoprecipitation assay. In vivo aggregation of the recombinant protein was limited by cultivation at a reduced temperature of 16 °C. Using co-immunoprecipitation techniques, the purified recombinant SEPT5 protein was used to pull down host’s interacting or binding proteins, i.e., proteins of brains of chickens infected with the H5N1 influenza virus. Interacting proteins, such as CRMP2, tubulin proteins, heat-shock proteins and other classes of septins were identified using LCMS/MS. Results from this study suggest that the codon-optimized SEPT5 gene can be efficiently expressed in the E. coli bacterial system producing authentic SEPT5 protein, thus, enabling multiple host’s proteins to interact with the SEPT5 protein.

Tpd3-Pph21 phosphatase plays a direct role in Sep7 dephosphorylation in Candida albicans

Septins are a component of the cytoskeleton and play important roles in diverse cellular processes including cell cycle control, cytokinesis and polarized growth. In fungi, septin organization, dynamics and function are regulated by phosphorylation, and several kinases responsible for the phosphorylation of several septins have been identified. However, little is known about the phosphatases that dephosphorylate septins. Here, we report the characterization of Tpd3, a structural subunit of the PP2A family of phosphatases, in the pathogenic fungus Candida albicans. We found that tpd3Δ/Δ cells are defective in hyphal growth and grow as pseudohyphae under yeast growth conditions with aberrant septin organization. Western blotting detected hyperphosphorylation of the septin Sep7 in cells lacking Tpd3. Tpd3 and Sep7 colocalize at the bud neck and can coimmunoprecipitate. Furthermore, we discovered similar defects in cells lacking Pph21, a catalytic subunit of the PP2A family, and its physical association with Tpd3. Importantly, purified Tpd3-Pph21 complexes can dephosphorylate Sep7 in vitro. Together, our findings strongly support the idea that the Tpd3-Pph21 complex dephosphorylates Sep7 and regulates morphogenesis and cytokinesis. The tpd3Δ/Δ mutant is greatly reduced in virulence in mice, providing a potential antifungal target.

A Förster Resonance Energy Transfer (FRET)-based System Provides Insight into the Ordered Assembly of Yeast Septin Hetero-octamers

Prior studies in both budding yeast (Saccharomyces cerevisiae) and in human cells have established that septin protomers assemble into linear hetero-octameric rods with 2-fold rotational symmetry. In mitotically growing yeast cells, five septin subunits are expressed (Cdc3, Cdc10, Cdc11, Cdc12, and Shs1) and assemble into two types of rods that differ only in their terminal subunit: Cdc11-Cdc12-Cdc3-Cdc10-Cdc10-Cdc3-Cdc12-Cdc11 and Shs1-Cdc12-Cdc3-Cdc10-Cdc10-Cdc3-Cdc12-Shs1. EM analysis has shown that, under low salt conditions, the Cdc11-capped rods polymerize end to end to form long paired filaments, whereas Shs1-capped rods form arcs, spirals, and rings. To develop a facile method to study septin polymerization in vitro, we exploited our previous work in which we generated septin complexes in which all endogenous cysteine (Cys) residues were eliminated by site-directed mutagenesis, except an introduced E294C mutation in Cdc11 in these experiments. Mixing samples of a preparation of such single-Cys containing Cdc11-capped rods that have been separately derivatized with organic dyes that serve as donor and acceptor, respectively, for FRET provided a spectroscopic method to monitor filament assembly mediated by Cdc11-Cdc11 interaction and to measure its affinity under specified conditions. Modifications of this same FRET scheme also allow us to assess whether Shs1-capped rods are capable of end to end association either with themselves or with Cdc11-capped rods. This FRET approach also was used to follow the binding to septin filaments of a septin-interacting protein, the type II myosin-binding protein Bni5.

Nerve growth factor promotes reorganization of the axonal microtubule array at sites of axon collateral branching

The localized debundling of the axonal microtubule array and the entry of microtubules into axonal filopodia are two defining features of collateral branching. We report that nerve growth factor (NGF), a branch-inducing signal, increases the frequency of microtubule debundling along the axon shaft of chicken embryonic sensory neurons. Sites of debundling correlate strongly with the localized targeting of microtubules into filopodia. Platinum replica electron microscopy suggests physical interactions between debundled microtubules and axonal actin filaments. However, as evidenced by depolymerization of actin filaments and inhibition of myosin II, actomyosin force generation does not promote debundling. In contrast, loss of actin filaments or inhibition of myosin II activity promotes debundling, indicating that axonal actomyosin forces suppress debundling. MAP1B is a microtubule associated protein that represses axon branching. Following treatment with NGF, microtubules penetrating filopodia during the early stages of branching exhibited lower levels of associated MAP1B. NGF increased and decreased the levels of MAP1B phosphorylated at a GSK-3β site (pMAP1B) along the axon shaft and within axonal filopodia, respectively. The levels of MAP1B and pMAP1B were not altered at sites of debundling, relative to the rest of the axon. Unlike the previously determined effects of NGF on the axonal actin cytoskeleton, the effects of NGF on microtubule debundling were not affected by inhibition of protein synthesis. Collectively, these data indicate that NGF promotes localized axonal microtubule debundling, that actomyosin forces antagonize microtubule debundling, and that NGF regulates pMAP1B in axonal filopodia during the early stages of collateral branch formation.

Daughter cell identity emerges from the interplay of Cdc42, septins, and exocytosis

Asymmetric cell division plays a crucial role in cell differentiation, unequal replicative senescence, and stem cell maintenance. In budding yeast, the identities of mother and daughter cells begin to diverge at bud emergence when distinct plasma-membrane domains are formed and separated by a septin ring. However, the mechanisms underlying this transformation remain unknown. Here, we show that septins recruited to the site of polarization by Cdc42-GTP inhibit Cdc42 activity in a negative feedback loop, and this inhibition depends on Cdc42 GTPase-activating proteins. Combining live-cell imaging and computational modeling, we demonstrate that the septin ring is sculpted by polarized exocytosis, which creates a hole in the accumulating septin density and relieves the inhibition of Cdc42. The nascent ring generates a sharp boundary that confines the Cdc42 activity and exocytosis strictly to its enclosure and thus clearly delineates the daughter cell identity. Our findings define a fundamental mechanism underlying eukaryotic cell fate differentiation.

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Septins provide negative feedback to Cdc42 activity that depends on Cdc42 GAPs

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Septin ring is formed by highly focused polarized exocytosis

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Suppression of exocytosis causes chasing behavior of septins

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Septin ring size is primarily determined by size of the underlying Cdc42 cluster

Minocycline, levodopa and MnTMPyP induced changes in the mitochondrial proteome profile of MPTP and maneb and paraquat mice models of Parkinson's disease

Mitochondrial dysfunction is the foremost perpetrator of the nigrostriatal dopaminergic neurodegeneration leading to Parkinson's disease (PD). However, the roles played by majority of the mitochondrial proteins in PD pathogenesis have not yet been deciphered. The present study investigated the effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and combined maneb and paraquat on the mitochondrial proteome of the nigrostriatal tissues in the presence or absence of minocycline, levodopa and manganese (III) tetrakis (1-methyl-4-pyridyl) porphyrin (MnTMPyP). The differentially expressed proteins were identified and proteome profiles were correlated with the pathological and biochemical anomalies induced by MPTP and maneb and paraquat. MPTP altered the expression of twelve while combined maneb and paraquat altered the expression of fourteen proteins. Minocycline, levodopa and MnTMPyP, respectively, restored the expression of three, seven and eight proteins in MPTP and seven, eight and eight proteins in maneb- and paraquat-treated groups. Although levodopa and MnTMPyP rescued from MPTP- and maneb- and paraquat-mediated increase in the microglial activation and decrease in manganese-superoxide dismutase expression and complex I activity, dopamine content and number of dopaminergic neurons, minocycline defended mainly against maneb- and paraquat-mediated alterations. The results demonstrate that MPTP and combined maneb and paraquat induce mitochondrial dysfunction and microglial activation and alter the expression of a bunch of mitochondrial proteins leading to the nigrostriatal dopaminergic neurodegeneration and minocycline, levodopa or MnTMPyP variably offset scores of such changes.

MgcRacGAP, a cytoskeleton regulator, inhibits HIF-1 transcriptional activity by blocking its dimerization

Hypoxia inducible factor-1 (HIF-1), a dimeric transcription factor of the bHLH-PAS family, is comprised of HIF-1α, which is inducible by hypoxia and ARNT or HIF-1β, which is constitutively expressed. HIF-1 is involved in cellular homeostasis under hypoxia, in development and in several diseases affected by oxygen availability, particularly cancer. Since its expression is positively correlated with poor outcome prognosis for cancer patients, HIF-1 is a target for pharmaceutical therapy. We have previously shown that male germ cell Rac GTPase activating protein (MgcRacGAP), a regulator of Rho proteins which are principally involved in cytoskeletal organization, binds to HIF-1α and inhibits its transcriptional activity. In this work, we have explored the mechanism of the MgcRacGAP-mediated HIF-1 inactivation. We show that the Myo domain of MgcRacGAP, which is both necessary and sufficient for HIF-1 repression, binds to the PAS-B domain of HIF-1α. Furthermore MgcRacGAP competes with ARNT for binding to the HIF-1α PAS-B domain, as shown by in vitro binding pull down assays. In mammalian cells, ARNT overexpression can overcome the MgcRacGAP-mediated inhibition and MgcRacGAP binding to HIF-1α in vivo inhibits its dimerization with ARNT. We additionally present results indicating that MgcRacGAP binding to HIF-1α is specific, since it does not affect the transcriptional activity of HIF-2, a close evolutionary relative of HIF-1 also involved in hypoxia regulation and cancer. Our results reveal a new mechanism for HIF-1 transcriptional activity regulation, suggest a novel hypoxia-cytoskeleton link and provide new tools for selective HIF-1 inhibition.

Comparative phosphoproteomics analysis of the effects of L-methionine on dairy cow mammary epithelial cells

Lu, L., Gao, X., Li, Q., Huang, J., Liu, R. and Li, H. 2012. Comparative phosphoproteomics analysis of the effects of L-methionine on dairy cow mammary epithelial cells. Can. J. Anim. Sci. 92: 433–442. L-methionine is an essential amino acid that plays fundamental roles in protein synthesis. Many nuclear phosphorylated proteins such as Stat5 (signal transducer and activator of transcription 5) and mTOR (mammalian target of rapamycin) regulate milk protein synthesis. But a comprehensive understanding of transcriptional and posttranscriptional regulation of milk protein synthesis is lacking. In the current study, two-dimensional gel electrophoresis (2-DE)/MS-based proteomics analysis was used to identify phosphoproteins responsible for milk protein synthesis in dairy cow mammary epithelial cells (DCMECs). The effects of L-methionine on DCMECs were analyzed by CASY (Counter Analyser System) technique, reversed phase high performance liquid chromatography. The results showed that rate of cell proliferation and expression of β-casein were increased in DCMECs treated with 0.6 mM L-methionine for 24 h. Five proteins for which expression was significantly increased in DCMECs were selected, and their expression changes were verified by quantitative real-time PCR and Western blot analysis. The five up-regulated expressed phosphoproteins included Staphylococcal nuclease domain-containing protein 1(SND1), Septin-6, Glycyl-tRNA synthetase (GARS), Twinfilin-1 and eukaryotic elongation factor1-beta (eEF1B). This study revealed that availability of L-methionine influences the levels of nuclear phosphorylated proteins of DCMECs which opens a new avenue for the study of the molecular mechanism linking to milk protein synthesis.

Matrix stiffness regulates endothelial cell proliferation through septin 9

Endothelial proliferation, which is an important process in vascular homeostasis, can be regulated by the extracellular microenvironment. In this study we demonstrated that proliferation of endothelial cells (ECs) was enhanced on hydrogels with high stiffness (HSG, 21.5 kPa) in comparison to those with low stiffness (LSG, 1.72 kPa). ECs on HSG showed markedly prominent stress fibers and a higher RhoA activity than ECs on LSG. Blockade of RhoA attenuated stress fiber formation and proliferation of ECs on HSG, but had little effect on ECs on LSG; enhancement of RhoA had opposite effects. The phosphorylations of Src and Vav2, which are positive RhoA upstream effectors, were higher in ECs on HSG. The inhibition of Src/Vav2 attenuated the HSG-mediated RhoA activation and EC proliferation but exhibited nominal effects on ECs on LSG. Septin 9 (SEPT9), the negative upstream effector for RhoA, was significantly higher in ECs on LSG. The inhibition of SEPT9 increased RhoA activation, Src/Vav2 phosphorylations, and EC proliferation on LSG, but showed minor effects on ECs on HSG. We further demonstrated that the inactivation of integrin α(v)β(3) caused an increase of SEPT9 expression in ECs on HSG to attenuate Src/Vav2 phosphorylations and inhibit RhoA-dependent EC proliferation. These results demonstrate that the SEPT9/Src/Vav2/RhoA pathway constitutes an important molecular mechanism for the mechanical regulation of EC proliferation.

Actin recruitment to the Chlamydia inclusion is spatiotemporally regulated by a mechanism that requires host and bacterial factors

The ability to exit host cells at the end of their developmental growth is a critical step for the intracellular bacterium Chlamydia. One exit strategy, extrusion, is mediated by host signaling pathways involved with actin polymerization. Here, we show that actin is recruited to the chlamydial inclusion as a late event, occurring after 20 hours post-infection (hpi) and only within a subpopulation of cells. This event increases significantly in prevalence and extent from 20 to 68 hpi, and actin coats strongly correlated with extrusions. In contrast to what has been reported for other intracellular pathogens, actin nucleation on Chlamydia inclusions did not 'flash', but rather exhibited moderate depolymerization dynamics. By using small molecule agents to selectively disrupt host signaling pathways involved with actin nucleation, modulate actin polymerization dynamics and also to disable the synthesis and secretion of chlamydial proteins, we further show that host and bacterial proteins are required for actin coat formation. Transient disruption of either host or bacterial signaling pathways resulted in rapid loss of coats in all infected cells and a reduction in extrusion formation. Inhibition of Chlamydia type III secretion also resulted in rapid loss of actin association on inclusions, thus implicating chlamydial effector proteins(s) as being central factors for engaging with host actin nucleating factors, such as formins. In conclusion, our data illuminate the host and bacterial driven process by which a dense actin matrix is dynamically nucleated and maintained on the Chlamydia inclusion. This late stage event is not ubiquitous for all infected cells in a population, and escalates in prevalence and extent throughout the developmental cycle of Chlamydia, culminating with their exit from the host cell by extrusion. The initiation of actin recruitment by Chlamydia appears to be novel, and may serve as an upstream determinant of the extrusion mechanism.

Septin 7 is required for orderly meiosis in mouse oocytes

Septin 7 is a conserved GTP-binding protein. In this study, we examined the localization and functions of Septin 7 during mouse oocyte meiotic maturation. Immunofluorescent analysis showed that intrinsic Septin 7 localized to the spindles from the pro-MI stage to the MII stage. Knockdown of Septin 7 by siRNA microinjection caused abnormal spindles and affected extrusion of the first polar body. Septin 7 mRNA tagged with myc was injected into GV stage oocytes to overexpress Septin 7. Overexpressed Myc-Septin 7 localized to the spindle and beneath the plasma membrane displaying long filaments. Fluorescence intensity of spindle α-tubulin in myc-Septin 7-injected oocytes was weaker than that of the control group, demonstrating that Septin 7 may influence recruitment of α-tubulin to spindles. MII oocytes injected with myc-Septin 7 exhibited abnormal chromosome alignment, and parthenogenetic activation failed to allow extrusion of the second polar body, suggesting that overexpression of Septin 7 may affect extrusion of the polar body by disturbing the alignment of chromosomes and regulating α-tubulin recruitment to spindles. In summary, Septin 7 may regulate meiotic cell cycle progression by affecting microtubule cytoskeletal dynamics in mouse oocytes.

Septin-mediated plant cell invasion by the rice blast fungus, Magnaporthe oryzae

To cause rice blast disease, the fungus Magnaporthe oryzae develops a pressurized dome-shaped cell called an appressorium, which physically ruptures the leaf cuticle to gain entry to plant tissue. Here, we report that a toroidal F-actin network assembles in the appressorium by means of four septin guanosine triphosphatases, which polymerize into a dynamic, hetero-oligomeric ring. Septins scaffold F-actin, via the ezrin-radixin-moesin protein Tea1, and phosphatidylinositide interactions at the appressorium plasma membrane. The septin ring assembles in a Cdc42- and Chm1-dependent manner and forms a diffusion barrier to localize the inverse-bin-amphiphysin-RVS-domain protein Rvs167 and the Wiskott-Aldrich syndrome protein Las17 at the point of penetration. Septins thereby provide the cortical rigidity and membrane curvature necessary for protrusion of a rigid penetration peg to breach the leaf surface.

Time course proteomic profiling of cellular responses to immunological challenge in the sea urchin, Heliocidaris erythrogramma

Genome sequences and high diversity cDNA arrays have provided a detailed molecular understanding of immune responses in a number of invertebrates, including sea urchins. However, complementary analyses have not been undertaken at the level of proteins. Here, we use shotgun proteomics to describe changes in the abundance of proteins from coelomocytes of sea urchins after immunological challenge and wounding. The relative abundance of 345 reproducibly identified proteins were measured 6, 24 and 48 h after injection. Significant changes in the relative abundance of 188 proteins were detected. These included pathogen-binding proteins, such as the complement component C3 and scavenger receptor cysteine rich proteins, as well as proteins responsible for cytoskeletal remodeling, endocytosis and intracellular signaling. An initial systemic reaction to wounding was followed by a more specific response to immunological challenge involving proteins such as apolipophorin, dual oxidase, fibrocystin L, aminopeptidase N and α-2-macroglobulin.

The extracellular signal-regulated kinase 3 (mitogen-activated protein kinase 6 [MAPK6])-MAPK-activated protein kinase 5 signaling complex regulates septin function and dendrite morphology

Mitogen-activated protein kinase-activated protein (MAPKAP) kinase 5 (MK5) deficiency is associated with reduced extracellular signal-regulated kinase 3 (ERK3) (mitogen-activated protein kinase 6) levels, hence we utilized the MK5 knockout mouse model to analyze the physiological functions of the ERK3/MK5 signaling module. MK5-deficient mice displayed impaired dendritic spine formation in mouse hippocampal neurons in vivo. We performed large-scale interaction screens to understand the neuronal functions of the ERK3/MK5 pathway and identified septin7 (Sept7) as a novel interacting partner of ERK3. ERK3/MK5/Sept7 form a ternary complex, which can phosphorylate the Sept7 regulators Binders of Rho GTPases (Borgs). In addition, the brain-specific nucleotide exchange factor kalirin-7 (Kal7) was identified as an MK5 interaction partner and substrate protein. In transfected primary neurons, Sept7-dependent dendrite development and spine formation are stimulated by the ERK3/MK5 module. Thus, the regulation of neuronal morphogenesis is proposed as the first physiological function of the ERK3/MK5 signaling module.

Application of in utero electroporation and live imaging in the analyses of neuronal migration during mouse brain development

Correct neuronal migration is crucial for brain architecture and function. During cerebral cortex development (corticogenesis), excitatory neurons generated in the proliferative zone of the dorsal telencephalon (mainly ventricular zone) move through the intermediate zone and migrate past the neurons previously located in the cortical plate and come to rest just beneath the marginal zone. The in utero electroporation technique is a powerful method for rapid gain- and loss-of-function studies of neuronal development, especially neuronal migration. This method enabled us to introduce genes of interest into ventricular zone progenitor cells of mouse embryos and to observe resulting phenotypes such as proliferation, migration, and cell morphology at later stages. In this Award Lecture Review, we focus on the application of the in utero electroporation method to functional analyses of cytoskeleton-related protein septin. We then refer to, as an advanced technique, the in utero electroporation-based real-time imaging method for analyses of cell signaling regulating neuronal migration. The in utero electroporation method and its application would contribute to medical molecular morphology through identification and characterization of the signaling pathways disorganized in various neurological and psychiatric disorders.

Expression of the SEPT9_i4 isoform confers resistance to microtubule-interacting drugs

BACKGROUND

The evolutionarily conserved septin family of genes encode GTP binding proteins involved in a variety of cellular functions including cytokinesis, apoptosis, membrane dynamics and vesicle trafficking. Septin proteins can form hetero-oligomeric complexes and interact with other proteins including actin and tubulin. The human SEPT9 gene on chromosome 17q25.3 has a complex genomic architecture with 18 different transcripts that can encode 15 distinct polypeptides. Two distinct transcripts with unique 5' ends (SEPT9_v4 and SEPT9_v4*) encode the same protein. In tumours the ratio of these transcripts changes with elevated levels of SEPT9_v4* mRNA, a transcript that is translated with enhanced efficiency leading to increased SEPT9_i4 protein.

METHODS

We have examined the effect of over-expression of SEPT9_i4 on the dynamics of microtubule polymer mass in cultured cells.

RESULTS

We show that the microtubule network in SEPT9_i4 over-expressing cells resists disruption by paclitaxel or cold incubation but also repolymerises tubulin more slowly after microtubule depolymerisation. Finally we show that SEPT9_i4 over-expressing cells have enhanced survival in the presence of clinically relevant microtubule acting drugs but not after treatment with DNAinteracting agents.

CONCLUSIONS

Given that SEPT9 over-expression is seen in diverse tumours and in particular ovarian and breast cancer, such data indicate that SEPT9_v4 expression may be clinically relevant and contribute to some forms of drug resistance.

Aberrant septin 11 is associated with sporadic frontotemporal lobar degeneration

Background

Detergent-insoluble protein accumulation and aggregation in the brain is one of the pathological hallmarks of neurodegenerative diseases. Here, we describe the identification of septin 11 (SEPT11), an enriched component of detergent-resistant fractions in frontotemporal lobar degeneration with ubiquitin-immunoreactive inclusions (FTLD-U), using large-scale unbiased proteomics approaches.

Results

We developed and applied orthogonal quantitative proteomic strategies for the unbiased identification of disease-associated proteins in FTLD-U. Using these approaches, we proteomically profiled detergent-insoluble protein extracts prepared from frontal cortex of FTLD-U cases, unaffected controls, or neurologic controls (i.e. Alzheimer's disease; AD). Among the proteins altered specifically in FTLD-U, we identified TAR DNA binding protein-43 (TDP-43), a known component of ubiquitinated inclusions. Moreover, we identified additional proteins enriched in detergent-resistant fractions in FTLD-U, and characterized one of them, SEPT11, in detail. Using independent highly sensitive targeted proteomics approaches, we confirmed the enrichment of SEPT11 in FTLD-U extracts. We further showed that SEPT11 is proteolytically cleaved into N-terminal fragments and, in addition to its prominent glial localization in normal brain, accumulates in thread-like pathology in affected cortex of FTLD-U patients.

Conclusions

The proteomic discovery of insoluble SEPT11 accumulation in FTLD-U, along with novel pathological associations, highlights a role for this cytoskeleton-associated protein in the pathogenesis of this complex disorder.

Mammalian septins: dynamic heteromers with roles in cellular morphogenesis and compartmentalization

The septins are a family of GTP-binding proteins, evolutionarily conserved from yeast through to mammals, with roles in multiple core cellular functions. Here we provide an overview of our current knowledge of septin structure and function and focus mainly on mammalian septins, but gain much insight by drawing on knowledge of septins in other organisms. We describe their genomic and transcriptional complexity: a complexity manifest also in the diversity of scaffold structures that septins can form. Septin complexes can act to localize interacting proteins at specific intracellular locales and can also define membrane compartments by defining diffusion barriers. By such activities, septins can contribute to the definition of spatial asymmetry and cell polarity and we suggest a potential role in stem cell biology. Finally, we review the evidence that septins contribute to various disease states and argue that it is a breakdown in the tight regulation of their expression (particularly of individual isoforms), and also their inherent ability to oligomerize, which is pathogenic. Study of the perturbation of septin complex formation in disease will provide valuable insights into septin biology and will be a fertile ground for study.

Promiscuous interactions of human septins: the GTP binding domain of SEPT7 forms filaments within the crystal

We describe the purification, crystallization and structure for the GTP-binding domain of human septin 7 (SEPT7G). We show that it forms filaments within the crystal lattice which employ both the G and NC interfaces, similar to those seen in the hetero-filament of SEPT2/6/7. The NC interface is considered promiscuous as it is absent from the hetero-filament. Such promiscuity could provide the potential for permuting monomers along a filament in order to generate diversity in hetero-polymers. On the other hand, our results suggest that the G and NC interfaces may be necessary but insufficient for determining correct hetero-filament assembly.

Septin filaments exhibit a dynamic, paired organization that is conserved from yeast to mammals

Polarized fluorescence microscopy reveals that septins across diverse species assemble into similar higher-order structures consisting of dynamic, paired filaments.

The septins are conserved, GTP-binding proteins important for cytokinesis, membrane compartmentalization, and exocytosis. However, it is unknown how septins are arranged within higher-order structures in cells. To determine the organization of septins in live cells, we developed a polarized fluorescence microscopy system to monitor the orientation of GFP dipole moments with high spatial and temporal resolution. When GFP was fused to septins, the arrangement of GFP dipoles reflected the underlying septin organization. We demonstrated in a filamentous fungus, a budding yeast, and a mammalian epithelial cell line that septin proteins were organized in an identical highly ordered fashion. Fluorescence anisotropy measurements indicated that septin filaments organized into pairs within live cells, just as has been observed in vitro. Additional support for the formation of pairs came from the observation of paired filaments at the cortex of cells using electron microscopy. Furthermore, we found that highly ordered septin structures exchanged subunits and rapidly rearranged. We conclude that septins assemble into dynamic, paired filaments in vivo and that this organization is conserved from yeast to mammals.

The fission yeast pleckstrin homology domain protein Spo7 is essential for initiation of forespore membrane assembly and spore morphogenesis

Sporulation in fission yeast represents a unique mode of cell division in which a new cell is formed within the cytoplasm of a mother cell. This event is accompanied by formation of the forespore membrane (FSM), which becomes the plasma membrane of spores. At prophase II, the spindle pole body (SPB) forms an outer plaque, from which formation of the FSM is initiated. Several components of the SPB play an indispensable role in SPB modification, and therefore in sporulation. In this paper, we report the identification of a novel SPB component, Spo7, which has a pleckstrin homology (PH) domain. We found that Spo7 was essential for initiation of FSM assembly, but not for SPB modification. Spo7 directly bound to Meu14, a component of the leading edge of the FSM, and was essential for proper localization of Meu14. The PH domain of Spo7 had affinity for phosphatidylinositol 3-phosphate (PI3P). spo7 mutants lacking the PH domain showed aberrant spore morphology, similar to that of meu14 and phosphatidylinositol 3-kinase (pik3) mutants. Our study suggests that Spo7 coordinates formation of the leading edge and initiation of FSM assembly, thereby accomplishing accurate formation of the FSM.