Septins stabilize mitochondria in Tetrahymena thermophila

The septin modifier, forchlorfenuron, activates NLRP3 via a potassium-independent mitochondrial axis

The Nod-like receptor protein 3 (NLRP3) inflammasome is activated by stimuli that induce perturbations in cell homeostasis, which commonly converge on cellular potassium efflux. NLRP3 has thus emerged as a sensor for ionic flux. Here, we identify forchlorfenuron (FCF) as an inflammasome activator that triggers NLRP3 signaling independently of potassium efflux. FCF triggers the rearrangement of septins, key cytoskeletal proteins that regulate mitochondrial function. We report that FCF triggered the rearrangement of SEPT2 into tubular aggregates and stimulated SEPT2-independent NLRP3 inflammasome signaling. Similar to imiquimod, FCF induced the collapse of the mitochondrial membrane potential and mitochondrial respiration. FCF thereby joins the imidazoquinolines as a structurally distinct class of molecules that triggers NLRP3 inflammasome signaling independent of potassium efflux, likely by inducing mitochondrial damage.

The Evolutionary Origins and Ancestral Features of Septins

Septins are a family of membrane-associated cytoskeletal GTPases that play crucial roles in various cellular processes, such as cell division, phagocytosis, and organelle fission. Despite their importance, the evolutionary origins and ancestral function of septins remain unclear. In opisthokonts, septins form five distinct groups of orthologs, with subunits from multiple groups assembling into heteropolymers, thus supporting their diverse molecular functions. Recent studies have revealed that septins are also conserved in algae and protists, indicating an ancient origin from the last eukaryotic common ancestor. However, the phylogenetic relationships among septins across eukaryotes remained unclear. Here, we expanded the list of non-opisthokont septins, including previously unrecognized septins from rhodophyte red algae and glaucophyte algae. Constructing a rooted phylogenetic tree of 254 total septins, we observed a bifurcation between the major non-opisthokont and opisthokont septin clades. Within the non-opisthokont septins, we identified three major subclades: Group 6 representing chlorophyte green algae (6A mostly for species with single septins, 6B for species with multiple septins), Group 7 representing algae in chlorophytes, heterokonts, haptophytes, chrysophytes, and rhodophytes, and Group 8 representing ciliates. Glaucophyte and some ciliate septins formed orphan lineages in-between all other septins and the outgroup. Combining ancestral-sequence reconstruction and AlphaFold predictions, we tracked the structural evolution of septins across eukaryotes. In the GTPase domain, we identified a conserved GAP-like arginine finger within the G-interface of at least one septin in most algal and ciliate species. This residue is required for homodimerization of the single Chlamydomonas septin, and its loss coincided with septin duplication events in various lineages. The loss of the arginine finger is often accompanied by the emergence of the α0 helix, a known NC-interface interaction motif, potentially signifying the diversification of septin-septin interaction mechanisms from homo-dimerization to hetero-oligomerization. Lastly, we found amphipathic helices in all septin groups, suggesting that curvature-sensing is an ancestral trait of septin proteins. Coiled-coil domains were also broadly distributed, while transmembrane domains were found in some septins in Group 6A and 7. In summary, this study advances our understanding of septin distribution and phylogenetic groupings, shedding light on their ancestral features, potential function, and early evolution.

Septins throughout phylogeny are predicted to have a transmembrane domain, which in Caenorhabditis elegans is functionally important

Septins, a conserved family of filament-forming proteins, contribute to eukaryotic cell division, polarity, and membrane trafficking. Septins are thought to act in these processes by scaffolding other proteins to the plasma membrane. The mechanisms by which septins associate with the plasma membrane are not well understood but can involve two polybasic domains and/or an amphipathic helix. We discovered that the genomes of organisms throughout phylogeny, but not most commonly used model organisms, encode one or more septins predicted to have transmembrane domains. The nematode Caenorhabditis elegans, which was thought to express only two septin proteins, UNC-59 and UNC-61, translates multiple isoforms of UNC-61, and one isoform, UNC-61a, is predicted to contain a transmembrane domain. UNC-61a localizes specifically to the apical membrane of the C. elegans vulva and is important for maintaining vulval morphology. UNC-61a partially compensates for the loss of the other two UNC-61 isoforms, UNC-61b and UNC-61c. The UNC-61a transmembrane domain is sufficient to localize a fluorophore to membranes in mammalian cells, and its deletion from UNC-61a recapitulates the phenotypes of unc-61a null animals. The localization and loss-of-function phenotypes of UNC-61a and its transmembrane domain suggest roles in cell polarity and secretion and help explain the cellular and tissue biological underpinnings of C. elegans septin null alleles' enigmatically hypomorphic phenotypes. Together, our findings reveal a novel mechanism of septin-membrane association with profound implications for the dynamics and regulation of this association.

The nuclear transport factor CSE1 drives macronuclear volume increase and macronuclear node coalescence in Stentor coeruleus

Stentor coeruleus provides a unique opportunity to study how cells regulate nuclear shape because its macronucleus undergoes a rapid, dramatic, and developmentally regulated shape change. We found that the volume of the macronucleus increases during coalescence, suggesting an inflation-based mechanism. When the nuclear transport factor, CSE1, is knocked down by RNAi, the shape and volume changes of the macronucleus are attenuated, and nuclear morphology is altered. CSE1 protein undergoes a dynamic relocalization correlated with nuclear shape changes, being mainly cytoplasmic prior to nuclear coalescence, and accumulating inside the macronucleus during coalescence. At the end of regeneration, CSE1 protein levels are reduced as the macronucleus returns to its pre-coalescence volume. We propose a model in which nuclear transport via CSE1 is required to increase the volume of the macronucleus, thereby decreasing the surface-to-volume ratio and driving coalescence of the nodes into a single mass.

Live-cell imaging of septins and cell polarity proteins in the growing dikaryotic vegetative hypha of the model mushroom Coprinopsis cinerea

The developmental biology underlying the morphogenesis of mushrooms remains poorly understood despite the essential role of fungi in the terrestrial environment and global carbon cycle. The mushroom Coprinopsis cinerea is a leading model system for the molecular and cellular basis of fungal morphogenesis. The dikaryotic vegetative hyphae of this fungus grow by tip growth with clamp cell formation, conjugate nuclear division, septation, subapical peg formation, and fusion of the clamp cell to the peg. Studying these processes provides many opportunities to gain insights into fungal cell morphogenesis. Here, we report the dynamics of five septins, as well as the regulators CcCla4, CcSpa2, and F-actin, visualized by tagging with fluorescent proteins, EGFP, PA-GFP or mCherry, in the growing dikaryotic vegetative hyphae. We also observed the nuclei using tagged Sumo proteins and histone H1. The five septins colocalized at the hyphal tip in the shape of a dome with a hole (DwH). CcSpa2-EGFP signals were observed in the hole, while CcCla4 signals were observed as the fluctuating dome at the hyphal tip. Before septation, CcCla4-EGFP was also occasionally recruited transiently around the future septum site. Fluorescent protein-tagged septins and F-actin together formed a contractile ring at the septum site. These distinct specialized growth machineries at different sites of dikaryotic vegetative hyphae provide a foundation to explore the differentiation program of various types of cells required for fruiting body formation.

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.

Septins From Protists to People

Septin GTPases form nonpolar heteropolymers that play important roles in cytokinesis and other cellular processes. The ability to form heteropolymers appears to be critical to many septin functions and to have been a major driver of the high conservation of many septin domains. Septins fall into five orthologous groups. Members of Groups 1–4 interact with each other to form heterooligomers and are known as the “core septins.” Representative core septins are present in all fungi and animals so far examined and show positional orthology with monomer location in the heteropolymer conserved within groups. In contrast, members of Group 5 are not part of canonical heteropolymers and appear to interact only transiently, if at all, with core septins. Group 5 septins have a spotty distribution, having been identified in specific fungi, ciliates, chlorophyte algae, and brown algae. In this review we compare the septins from nine well-studied model organisms that span the tree of life (Homo sapiens, Drosophila melanogaster, Schistosoma mansoni, Caenorhabditis elegans, Saccharomyces cerevisiae, Aspergillus nidulans, Magnaporthe oryzae, Tetrahymena thermophila, and Chlamydomonas reinhardtii). We focus on classification, evolutionary relationships, conserved motifs, interfaces between monomers, and positional orthology within heteropolymers. Understanding the relationships of septins across kingdoms can give new insight into their functions.

ESCRT Machinery Mediates Cytokinetic Abscission in the Unicellular Red Alga Cyanidioschyzon merolae

In many eukaryotes, cytokinesis proceeds in two successive steps: first, ingression of the cleavage furrow and second, abscission of the intercellular bridge. In animal cells, the actomyosin contractile ring is involved in the first step, while the endosomal sorting complex required for transport (ESCRT), which participates in various membrane fusion/fission events, mediates the second step. Intriguingly, in archaea, ESCRT is involved in cytokinesis, raising the hypothesis that the function of ESCRT in eukaryotic cytokinesis descended from the archaeal ancestor. In eukaryotes other than in animals, the roles of ESCRT in cytokinesis are poorly understood. To explore the primordial core mechanisms for eukaryotic cytokinesis, we investigated ESCRT functions in the unicellular red alga Cyanidioschyzon merolae that diverged early in eukaryotic evolution. C. merolae provides an excellent experimental system. The cell has a simple organelle composition. The genome (16.5 Mb, 5335 genes) has been completely sequenced, transformation methods are established, and the cell cycle is synchronized by a light and dark cycle. Similar to animal and fungal cells, C. merolae cells divide by furrowing at the division site followed by abscission of the intercellular bridge. However, they lack an actomyosin contractile ring. The proteins that comprise ESCRT-I-IV, the four subcomplexes of ESCRT, are partially conserved in C. merolae. Immunofluorescence of native or tagged proteins localized the homologs of the five ESCRT-III components [charged multivesicular body protein (CHMP) 1, 2, and 4-6], apoptosis-linked gene-2-interacting protein X (ALIX), the ESCRT-III adapter, and the main ESCRT-IV player vacuolar protein sorting (VPS) 4, to the intercellular bridge. In addition, ALIX was enriched around the cleavage furrow early in cytokinesis. When the ESCRT function was perturbed by expressing dominant-negative VPS4, cells with an elongated intercellular bridge accumulated-a phenotype resulting from abscission failure. Our results show that ESCRT mediates cytokinetic abscission in C. merolae. The fact that ESCRT plays a role in cytokinesis in archaea, animals, and early diverged alga C. merolae supports the hypothesis that the function of ESCRT in cytokinesis descended from archaea to a common ancestor of eukaryotes.

Comparative Studies on the Polymorphism and Copy Number Variation of mtSSU rDNA in Ciliates (Protista, Ciliophora): Implications for Phylogenetic, Environmental, and Ecological Research

While nuclear small subunit ribosomal DNA (nSSU rDNA) is the most commonly-used gene marker in studying phylogeny, ecology, abundance, and biodiversity of microbial eukaryotes, mitochondrial small subunit ribosomal DNA (mtSSU rDNA) provides an alternative. Recently, both copy number variation and sequence variation of nSSU rDNA have been demonstrated for diverse organisms, which can contribute to misinterpretation of microbiome data. Given this, we explore patterns for mtSSU rDNA among 13 selected ciliates (representing five classes), a major component of microbial eukaryotes, estimating copy number and sequence variation and comparing to that of nSSU rDNA. Our study reveals: (1) mtSSU rDNA copy number variation is substantially lower than that for nSSU rDNA; (2) mtSSU rDNA copy number ranges from 1.0 × 10⁴ to 8.1 × 10⁵; (3) a most common sequence of mtSSU rDNA is also found in each cell; (4) the sequence variation of mtSSU rDNA are mainly indels in poly A/T regions, and only half of species have sequence variation, which is fewer than that for nSSU rDNA; and (5) the polymorphisms between haplotypes of mtSSU rDNA would not influence the phylogenetic topology. Together, these data provide more insights into mtSSU rDNA as a powerful marker especially for microbial ecology studies.

Who Needs a Contractile Actomyosin Ring? The Plethora of Alternative Ways to Divide a Protozoan Parasite

Cytokinesis, or the division of the cytoplasm, following the end of mitosis or meiosis, is accomplished in animal cells, fungi, and amoebae, by the constriction of an actomyosin contractile ring, comprising filamentous actin, myosin II, and associated proteins. However, despite this being the best-studied mode of cytokinesis, it is restricted to the Opisthokonta and Amoebozoa, since members of other evolutionary supergroups lack myosin II and must, therefore, employ different mechanisms. In particular, parasitic protozoa, many of which cause significant morbidity and mortality in humans and animals as well as considerable economic losses, employ a wide diversity of mechanisms to divide, few, if any, of which involve myosin II. In some cases, cell division is not only myosin II-independent, but actin-independent too. Mechanisms employed range from primitive mechanical cell rupture (cytofission), to motility- and/or microtubule remodeling-dependent mechanisms, to budding involving the constriction of divergent contractile rings, to hijacking host cell division machinery, with some species able to utilize multiple mechanisms. Here, I review current knowledge of cytokinesis mechanisms and their molecular control in mammalian-infective parasitic protozoa from the Excavata, Alveolata, and Amoebozoa supergroups, highlighting their often-underappreciated diversity and complexity. Billions of people and animals across the world are at risk from these pathogens, for which vaccines and/or optimal treatments are often not available. Exploiting the divergent cell division machinery in these parasites may provide new avenues for the treatment of protozoal disease.

Grl1 Protein is a Candidate K Antigen in Tetrahymena thermophila

In Tetrahymena, K antigens associate only with mature basal bodies and are expected to play important roles in the morphogenesis and function of the membrane skeleton around basal bodies, but these proteins have not been identified and their functions are unknown. Commercially available anti-human Rho GDP-dissociation inhibitor α (RhoGDIα) antibody (sc-33201) was accidentally found to show very similar immunofluorescence staining patterns to those of anti-K antigen antibodies, such as 424A8 and 10D12 mouse monoclonal antibodies, in Tetrahymena. A 40kDa protein recognized by this antibody was partially purified and identified as granule lattice protein 1 (Grl1p) by matrix-assisted laser desorption/ionization-tandem time-of-flight mass spectrometry. In immunoblotting experiments this antibody was suggested to recognize endogenous Grl1p. The three-dimensional structure of proGrl1p protein predicted by I-TASSER was similar to a spectrin family protein. Grl1 may be a K antigen and a spectrin-like protein in Tetrahymena.

Intermediate filament protein evolution and protists

Metazoans evolved from a single protist lineage. While all eukaryotes share a conserved actin and tubulin-based cytoskeleton, it is commonly perceived that intermediate filaments (IFs), including lamin, vimentin or keratin among many others, are restricted to metazoans. Actin and tubulin proteins are conserved enough to be detectable across all eukaryotic genomes using standard phylogenetic methods, but IF proteins, in contrast, are notoriously difficult to identify by such means. Since the 1950s, dozens of cytoskeletal proteins in protists have been identified that seemingly do not belong to any of the IF families described for metazoans, yet, from a structural and functional perspective fit criteria that define metazoan IF proteins. Here, we briefly review IF protein discovery in metazoans and the implications this had for the definition of this protein family. We argue that the many cytoskeletal and filament-forming proteins of protists should be incorporated into a more comprehensive picture of IF evolution by aligning it with the recent identification of lamins across the phylogenetic diversity of eukaryotic supergroups. This then brings forth the question of how the diversity of IF proteins has unfolded. The evolution of IF proteins likely represents an example of convergent evolution, which, in combination with the speed with which these cytoskeletal proteins are evolving, generated their current diversity. IF proteins did not first emerge in metazoa, but in protists. Only the emergence of cytosolic IF proteins that appear to stem from a nuclear lamin is unique to animals and coincided with the emergence of true animal multicellularity.

Filaments and fingers: Novel structural aspects of the single septin from Chlamydomonas reinhardtii

Septins are filament-forming GTP-binding proteins involved in many essential cellular events related to cytoskeletal dynamics and maintenance. Septins can self-assemble into heterocomplexes, which polymerize into highly organized, cell membrane-interacting filaments. The number of septin genes varies among organisms, and although their structure and function have been thoroughly studied in opisthokonts (including animals and fungi), no structural studies have been reported for other organisms. This makes the single septin from Chlamydomonas (CrSEPT) a particularly attractive model for investigating whether functional homopolymeric septin filaments also exist. CrSEPT was detected at the base of the flagella in Chlamydomonas, suggesting that CrSEPT is involved in the formation of a membrane-diffusion barrier. Using transmission electron microscopy, we observed that recombinant CrSEPT forms long filaments with dimensions comparable with those of the canonical structure described for opisthokonts. The GTP-binding domain of CrSEPT purified as a nucleotide-free monomer that hydrolyzes GTP and readily binds its analog guanosine 5'-3-O-(thio)triphosphate. We also found that upon nucleotide binding, CrSEPT formed dimers that were stabilized by an interface involving the ligand (G-interface). Across this interface, one monomer supplied a catalytic arginine to the opposing subunit, greatly accelerating the rate of GTP hydrolysis. This is the first report of an arginine finger observed in a septin and suggests that CrSEPT may act as its own GTP-activating protein. The finger is conserved in all algal septin sequences, suggesting a possible correlation between the ability to form homopolymeric filaments and the accelerated rate of hydrolysis that it provides.

The Hippo Pathway Maintains the Equatorial Division Plane in the Ciliate Tetrahymena

The mechanisms that govern pattern formation within the cell are poorly understood. Ciliates carry on their surface an elaborate pattern of cortical organelles that are arranged along the anteroposterior and circumferential axes by largely unknown mechanisms. Ciliates divide by tandem duplication: the cortex of the predivision cell is remodeled into two similarly sized and complete daughters. In the conditional cdaI-1 mutant of Tetrahymena thermophila, the division plane migrates from its initially correct equatorial position toward the cell's anterior, resulting in unequal cell division, and defects in nuclear divisions and cytokinesis. We used comparative whole genome sequencing to identify the cause of cdaI-1 as a mutation in a Hippo/Mst kinase. CdaI is a cortical protein with a cell cycle-dependent, highly polarized localization. Early in cell division, CdaI marks the anterior half of the cell, and later concentrates at the posterior end of the emerging anterior daughter. Despite the strong association of CdaI with the new posterior cell end, the cdaI-1 mutation does not affect the patterning of the new posterior cortical organelles. We conclude that, in Tetrahymena, the Hippo pathway maintains an equatorial position of the fission zone, and, by this activity, specifies the relative dimensions of the anterior and posterior daughter cell.

Intracellular symbiosis of algae with possible involvement of mitochondrial dynamics

Algal endosymbiosis is widely present among eukaryotes including many protists and metazoans. However, the mechanisms involved in their interactions between host and symbiont remain unclear. Here, we used electron microscopy and three-dimensional reconstruction analyses to examine the ultrastructural interactions between the symbiotic zoochlorella and the organelles in the host Paramecium bursaria, which is a model system of endosymbiosis. Although in chemically fixed samples the symbiotic algae show no direct structural interactions with the host organelles and the perialgal vacuole membrane (PVM), in cryofixed P. bursaria samples the intimate connections were identified between the host mitochondria and the symbiotic algae via the PVM. The PVM was closely apposed to the cell wall of the symbiotic algae and in some places it showed direct contacts to the host mitochondrial membrane and the cell wall of the symbiotic algae. Further, the PVM-associated mitochondria formed a mitochondrial network and were also connected to host ER. Our observations propose a new endosymbiotic systems between the host eukaryotes and the symbionts where the benefiting symbiosis is performed through intimate interactions and an active structural modification in the host organelles.

A role for septin 2 in Drp1-mediated mitochondrial fission

Mitochondria are essential eukaryotic organelles often forming intricate networks. The overall network morphology is determined by mitochondrial fusion and fission. Among the multiple mechanisms that appear to regulate mitochondrial fission, the ER and actin have recently been shown to play an important role by mediating mitochondrial constriction and promoting the action of a key fission factor, the dynamin‐like protein Drp1. Here, we report that the cytoskeletal component septin 2 is involved in Drp1‐dependent mitochondrial fission in mammalian cells. Septin 2 localizes to a subset of mitochondrial constrictions and directly binds Drp1, as shown by immunoprecipitation of the endogenous proteins and by pulldown assays with recombinant proteins. Depletion of septin 2 reduces Drp1 recruitment to mitochondria and results in hyperfused mitochondria and delayed FCCP‐induced fission. Strikingly, septin depletion also affects mitochondrial morphology in Caenorhabditis elegans, strongly suggesting that the role of septins in mitochondrial dynamics is evolutionarily conserved.

Off-target effects of the septin drug forchlorfenuron on nonplant eukaryotes

The septins are a family of GTP-binding proteins that form cytoskeletal filaments. Septins are highly conserved and evolutionarily ancient but are absent from land plants. The synthetic plant cytokinin forchlorfenuron (FCF) was shown previously to inhibit budding yeast cell division and induce ectopic septin structures (M. Iwase, S. Okada, T. Oguchi, and A. Toh-e, Genes Genet. Syst. 79:199-206, 2004, http://dx.doi.org/10.1266/ggs.79.199). Subsequent studies in a wide range of eukaryotes have concluded that FCF exclusively inhibits septin function, yet the mechanism of FCF action in nonplant cells remains poorly understood. Here, we report that the cellular effects of FCF are far more complex than previously described. The reported growth arrest of budding yeast cells treated with 1 mM FCF partly reflects sensitization caused by a bud4 mutation present in the W303 strain background. In wild-type (BUD4(+)) budding yeast, growth was inhibited at FCF concentrations that had no detectable effect on septin structure or function. Moreover, FCF severely inhibited the proliferation of fission yeast cells, in which septin function is nonessential. FCF induced fragmentation of budding yeast mitochondrial reticula and the loss of mitochondrial membrane potential. Mitochondria also fragmented in cultured mammalian cells treated with concentrations of FCF that previously were assumed to target septins only. Finally, FCF potently inhibited ciliation and motility and induced mitochondrial disorganization in Tetrahymena thermophila without apparent alterations in septin structure. None of these effects was consistent with the inhibition of septin function. Our findings point to nonseptin targets as major concerns when using FCF.

Formation and ingression of division furrow can progress under the inhibitory condition of actin polymerization in ciliate Tetrahymena pyriformis

In eukaryotic cells that multiply by binary fission, the interaction of actin filaments with myosin II in the contractile ring is widely recognized to generate force for membrane ingression into the cleavage furrow; however, the expression of myosin II is restricted in animals, yeast, fungi, and amoeba (collectively, unikonts). No corresponding motor protein capable of forming mini-filaments that could exert sufficient tension to cleave the cell body is found in bikonts, consisting of planta, algae, and most protozoa; however, cells in some bikont lineages multiply by binary fission, as do animal cells. Of these, the ciliate Tetrahymena is known to form an actin ring beneath the division furrow in cytokinesis. Here, we investigated the role of filamentous actin in the cytokinesis of Tetrahymena pyriformis by treating synchronized dividing cells with an actin-inhibiting drug, Latrunculin-A. Video microscopic observation of live cells undergoing cytokinesis was performed, and contrary to expectation, we found that initiation of furrow ingression and its progress are not suppressed under the inhibitory condition of actin polymerization in Tetrahymena cells. We suggest that an actin filament-independent mechanism of binary fission may have been acquired during the evolution in this organism.

Septins of Platyhelminths: identification, phylogeny, expression and localization among developmental stages of Schistosoma mansoni

Septins are a family of eukaryotic GTP binding proteins conserved from yeasts to humans. Originally identified in mutants of budding yeast, septins participate in diverse cellular functions including cytokinesis, organization of actin networks, cell polarity, vesicle trafficking and many others. Septins assemble into heteroligomers to form filaments and rings. Here, four septins of Schistosoma mansoni are described, which appear to be conserved within the phylum Platyhelminthes. These orthologues were related to the SEPT5, SEPT10 and SEPT7 septins of humans, and hence we have termed the schistosome septins SmSEPT5, SmSEPT10, SmSEPT7.1 and SmSEPT7.2. Septin transcripts were detected throughout the developmental cycle of the schistosome and a similar expression profile was observed for septins in the stages examined, consistent with concerted production of these proteins to form heterocomplexes. Immunolocalization analyses undertaken with antibodies specific for SmSEPT5 and SmSEPT10 revealed a broad tissue distribution of septins in the schistosomulum and colocalization of septin and actin in the longitudinal and circular muscles of the sporocyst. Ciliated epidermal plates of the miracidium were rich in septins. Expression levels for these septins were elevated in germ cells in the miracidium and sporocyst. Intriguingly, septins colocalize with the protonephridial system of the cercaria, which extends laterally along the length of this larval stage. Together, the findings revealed that schistosomes expressed several septins which likely form filaments within the cells, as in other eukaryotes. Identification and localization demonstrating a broad distribution of septins across organs and tissues of schistosome contributes towards the understanding of septins in schistosomes and other flatworms.

Localization and evolution of septins in algae

Septins are a group of GTP-binding proteins that are multi-functional, with a well-known role in cytokinesis in animals and fungi. Although the functions of septins have been thoroughly studied in opisthokonts (fungi and animals), the function and evolution of plant/algal septins are not as well characterized. Here we describe septin localization and expression in the green algae Nannochloris bacillaris and Marvania geminata. The present data suggest that septins localize at the division site when cytokinesis occurs. In addition, we show that septin homologs may be found only in green algae, but not in other major plant lineages, such as land plants, red algae and glaucophytes. We also found other septin homolog-possessing organisms among the diatoms, Rhizaria and cryptomonad/haptophyte lineages. Our study reveals the potential role of algal septins in cytokinesis and/or cell elongation, and confirms that septin genes appear to have been lost in the Plantae lineage, except in some green algae.

ARTS, the unusual septin: structural and functional aspects

The human Septin 4 gene (Sept4) encodes two major protein isoforms; Sept4_i1 (H5/PNUTL2) and Sept4_i2/ARTS. Septins have been traditionally studied for their role in cytokinesis and their filament-forming abilities, but subsequently have been implicated in diverse functions, including membrane dynamics, cytoskeletal reorganization, vesicle trafficking, and tumorigenesis. ARTS is localized at mitochondria and promotes programmed cell death (apoptosis). These features distinguish ARTS from any other known human septin family member. This review compares the structural and functional properties of ARTS with other septins. In addition, it describes how a combination of two distinct promoters, differential splicing, and intron retention leads to the generation of two different Sept4 variants with diverse biological activity.

Conservation and innovation in Tetrahymena membrane traffic: proteins, lipids, and compartments

The past decade has seen a significant expansion in our understanding of membrane traffic in Tetrahymena thermophila, facilitated by the development of new experimental tools and by the availability of the macronuclear genome sequence. Here we review studies on multiple pathways of uptake and secretion, as well as work on metabolism of membrane lipids. We discuss evidence for conservation versus innovation in the mechanisms used in ciliates compared with those in other eukaryotic lineages, and raise the possibility that existing gene expression databases can be exploited to analyze specific pathways of membrane traffic in these cells.

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.

Septin genomics: a road less travelled

The human septins are part of a gene family, that is a group of genes with similar sequences and usually but not invariably share similar functions that are descended from a common ancestor. Here we review our current knowledge of the human septin gene family and highlight areas of uncertainty. Currently 13 human septin genes are known (SEPT1 to SEPT12 and SEPT14). What was known as SEPT13 is now defined as one of many SEPT7 related pseudogenes. The family is characterized by complex genomics and extensive (but not universal) splicing, giving rise to a plethora of septin isoforms. For only a few members of the family do we have a comprehensive insight into these transcripts and isoforms. Given the formation of countless septin homotypic and heterotypic interactions our understanding of the biology and pathobiology of the septin family will require a detailed understanding of the genomics, transcriptomics and regulation of all members of this diverse and complex family.

New insights into the phylogenetic distribution and evolutionary origins of the septins

Until recently, it had appeared that the septin family of proteins was restricted to the opisthokont eukaryotes (the fungi and animals and their close relatives the microsporidia and choanoflagellates). It has now become apparent that septins are also present in several other widely divergent eukaryotic lineages (chlorophyte algae, brown algae, and ciliates). This distribution and the details of the non-opisthokont septin sequences appear to require major revisions to hypotheses about the origins and early evolution of the septins.

Autophagy in protists

Autophagy is the degradative process by which eukaryotic cells digest their own components using acid hydrolases within the lysosome. Originally thought to function almost exclusively in providing starving cells with nutrients taken from their own cellular constituents, autophagy is in fact involved in numerous cellular events including differentiation, turnover of macromolecules and organelles, and defense against parasitic invaders. During the last 10-20 years, molecular components of the autophagic machinery have been discovered, revealing a complex interactome of proteins and lipids, which, in a concerted way, induce membrane formation to engulf cellular material and target it for lysosomal degradation. Here, our emphasis is autophagy in protists. We discuss experimental and genomic data indicating that the canonical autophagy machinery characterized in animals and fungi appeared prior to the radiation of major eukaryotic lineages. Moreover, we describe how comparative bioinformatics revealed that this canonical machinery has been subject to moderation, outright loss or elaboration on multiple occasions in protist lineages, most probably as a consequence of diverse lifestyle adaptations. We also review experimental studies illustrating how several pathogenic protists either utilize autophagy mechanisms or manipulate host-cell autophagy in order to establish or maintain infection within a host. The essentiality of autophagy for the pathogenicity of many parasites, and the unique features of some of the autophagy-related proteins involved, suggest possible new targets for drug discovery. Further studies of the molecular details of autophagy in protists will undoubtedly enhance our understanding of the diversity and complexity of this cellular phenomenon and the opportunities it offers as a drug target.