TrCla4 promotes actin polymerization at the hyphal tip and mycelial growth in Trichophyton rubrum

IMPORTANCE

Superficial fungal infections, such as athlete's foot, affect more than 10% of the world's population and have a significant impact on quality of life. Despite the fact that treatment-resistant fungi are a concern, there are just a few antifungal drug targets accessible, as opposed to the wide range of therapeutic targets found in bacterial infections. As a result, additional alternatives are sought. In this study, we generated a PAK TrCla4 deletion strain (∆Trcla4) of Trichophyton rubrum. The ∆Trcla4 strain exhibited deficiencies in mycelial growth, hyphal morphology, and polarized actin localization at the hyphal tip. IPA-3 and FRAX486, small chemical inhibitors of mammalian PAK, were discovered to limit fungal mycelial proliferation. According to our findings, fungal PAKs are interesting therapeutic targets for the development of new antifungal medicines.

Ependymoma associated protein Zfta is expressed in immature ependymal cells but is not essential for ependymal development in mice

The fusion protein of uncharacterised zinc finger translocation associated (ZFTA) and effector transcription factor of tumorigenic NF-κB signalling, RELA (ZFTA-RELA), is expressed in more than two-thirds of supratentorial ependymoma (ST-EPN-RELA), but ZFTA’s expression profile and functional analysis in multiciliated ependymal (E1) cells have not been examined. Here, we showed the mRNA expression of mouse Zfta peaks on embryonic day (E) 17.5 in the wholemount of the lateral walls of the lateral ventricle. Zfta was expressed in the nuclei of FoxJ1-positive immature E1 (pre-E1) cells in E18.5 mouse embryonic brain. Interestingly, the transcription factors promoting ciliogenesis (ciliary TFs) (e.g., multicilin) and ZFTA-RELA upregulated luciferase activity using a 5′ upstream sequence of ZFTA in cultured cells. Zfta^tm1/tm1 knock-in mice did not show developmental defects or abnormal fertility. In the Zfta^tm1/tm1 E1 cells, morphology, gene expression, ciliary beating frequency and ependymal flow were unaffected. These results suggest that Zfta is expressed in pre-E1 cells, possibly under the control of ciliary TFs, but is not essential for ependymal development or flow. This study sheds light on the mechanism of the ZFTA-RELA expression in the pathogenesis of ST-EPN-RELA: Ciliary TFs initiate ZFTA-RELA expression in pre-E1 cells, and ZFTA-RELA enhances its own expression using positive feedback.

Caenorhabditis elegans PTR/PTCHD PTR-18 promotes the clearance of extracellular hedgehog-related protein via endocytosis

Spatiotemporal restriction of signaling plays a critical role in animal development and tissue homeostasis. All stem and progenitor cells in newly hatched C. elegans larvae are quiescent and capable of suspending their development until sufficient food is supplied. Here, we show that ptr-18, which encodes the evolutionarily conserved patched-related (PTR)/patched domain-containing (PTCHD) protein, temporally restricts the availability of extracellular hedgehog-related protein to establish the capacity of progenitor cells to maintain quiescence. We found that neural progenitor cells exit from quiescence in ptr-18 mutant larvae even when hatched under starved conditions. This unwanted reactivation depended on the activity of a specific set of hedgehog-related grl genes including grl-7. Unexpectedly, neither PTR-18 nor GRL-7 were expressed in newly hatched wild-type larvae. Instead, at the late embryonic stage, both PTR-18 and GRL-7 proteins were first localized around the apical membrane of hypodermal and neural progenitor cells and subsequently targeted for lysosomal degradation before hatching. Loss of ptr-18 caused a significant delay in GRL-7 clearance, causing this protein to be retained in the extracellular space in newly hatched ptr-18 mutant larvae. Furthermore, the putative transporter activity of PTR-18 was shown to be required for the appropriate function of the protein. These findings not only uncover a previously undescribed role of PTR/PTCHD in the clearance of extracellular hedgehog-related proteins via endocytosis-mediated degradation but also illustrate that failure to temporally restrict intercellular signaling during embryogenesis can subsequently compromise post-embryonic progenitor cell function.

Hedgehog-related genes regulate reactivation of quiescent neural progenitors in Caenorhabditis elegans

The animal body contains various types of stem and progenitor cells. These undifferentiated cells coordinate the balance between quiescence and proliferation with dynamics of various physiological conditions such as the developmental stage, food availability, and injury. Although regulation of such coordination plays a critical role in maintaining tissue homeostasis, controlling the growth rate and regeneration, much of its mechanism remains elusive. Newly hatched Caenorhabditis elegans larvae possess quiescent stem and progenitor cells in several tissues, and these cells are reactivated by the insulin/insulin-like growth factor (IGF) signaling (IIS) pathway only when sufficient food is supplied. Maintenance of the quiescence of neuronal and mesodermal progenitor cells requires microRNA (miRNA), miR-235, which is upregulated under the starvation. On the other hand, feeding ample food downregulates the miRNA via the activity of the IIS pathway. As miR-235 in the hypodermis can non-autonomously regulate quiescence of neuronal and mesodermal progenitor cells, a cell-cell signaling pathway has been hypothesized to act downstream of the miRNA. Here, we provide evidence that two hedgehog-related (hh-r) genes, grl-5 and grl-7, are targets of miR-235 that promote reactivation of quiescent neuroblasts. These grl genes possess an miR-235 binding site on 3'UTRs of their transcripts, and are upregulated in starved mir-235 mutant larvae. grl-5 and grl-7 promoters can continuously drive the expression of GFP-pest reporter protein in the hypodermis under the fed condition. However, expression of these reporters is strikingly downregulated under the starvation condition after hatching. We found that miR-235 can repress expression of reporter genes via the predicted miR-235 binding sites on the grl-5 and grl-7 3'UTRs. Furthermore, activity of grl-5 and grl-7 genes are required for reactivation of neural progenitor cells in starved mir-235 mutant larvae. These findings suggest that the IIS pathway-miR-235 signaling in the hypodermis non-autonomously regulates quiescence of neural progenitor cells, partly via grl-5 and grl-7.

Decreased conformational stability in the oncogenic N92I mutant of Ras-related C3 botulinum toxin substrate 1

Ras-related C3 botulinum toxin substrate 1 (Rac1) functions as a molecular switch by cycling between an inactive guanosine diphosphate (GDP)-bound state and an active guanosine triphosphate (GTP)-bound state. An oncogenic mutant of Rac1, an N92I mutant, strongly promotes cell proliferation and subsequent oncogenic activities by facilitating the intrinsic GDP dissociation in the inactive GDP-bound state. Here, we used solution nuclear magnetic resonance spectroscopy to investigate the activation mechanism of the N92I mutant. We found that the static structure of the GDP binding site is not markedly perturbed by the mutation, but the overall conformational stability decreases in the N92I mutant, which then facilitates GDP dissociation by lowering the activation energy for the dissociation reaction. On the basis of these results, we proposed the activation mechanism of the N92I mutant, in which the decreased conformational stability plays important roles in its activation process.

Additive effects of Kothala himbutu (Salacia reticulata) extract and a lactic acid bacterium (Enterococcus faecalis YM0831) for suppression of sucrose-induced hyperglycemia in an in vivo silkworm evaluation system

Using a silkworm evaluation system, we previously evaluated various substances that suppress postprandial hyperglycemia. Enterococcus faecalis YM0831, a lactic acid bacterium that inhibits glucose uptake by the human intestinal Caco-2 cell line, exhibited hyperglycemia-suppressing effects in the silkworm system. In the present study, we found that Kothala himbutu (Salacia reticulata) extract, a traditional medicine containing α-glucosidase inhibitors, suppressed sucrose-induced hyperglycemia in the silkworm system. Moreover, combined oral administration of lactic acid bacteria YM0831 with Kothala himbutu extract had stronger suppressive effects on sucrose-induced hyperglycemia than single administration of either component. These findings suggest that the silkworm system provides a simple way to evaluate the effects of supplements on the suppression of blood glucose level induced by sucrose ingestion.

COPII proteins exhibit distinct subdomains within each ER exit site for executing their functions

Secretory proteins are exported from special domains of the endoplasmic reticulum (ER) termed ER exit sites, via COPII-coated carriers. We recently showed that TANGO1 and Sec16 cooperatively organize mammalian ER exit sites for efficient secretion. However, the detailed spatial organization of mammalian ER exit sites is yet to be revealed. Here, we used super-resolution confocal live imaging microscopy (SCLIM) to investigate the localization of endogenous proteins, and we identified domains abundant in transmembrane complexes (TANGO1/cTAGE5/Sec12) juxtaposed to Sec16. Interestingly, this domain can be distinguished from the inner and the outer coats of COPII proteins within each mammalian ER exit site. Cargoes are partially concentrated in the domain for secretion. Our results suggest that mammalian ER exit sites compartmentalize proteins according to their function in COPII vesicle formation.

Loss of the small GTPase Arl8b results in abnormal development of the roof plate in mouse embryos

Lysosomes are acidic organelles responsible for degrading both exogenous and endogenous materials. The small GTPase Arl8 localizes primarily to lysosomes and is involved in lysosomal function. In the present study, using Arl8b gene-trapped mutant (Arl8b^-/- ) mice, we show that Arl8b is required for the development of dorsal structures of the neural tube, including the thalamus and hippocampus. In embryonic day (E) 10.5 Arl8b^-/- embryos, Sox1 (a neuroepithelium marker) was ectopically expressed in the roof plate, whereas the expression of Gdf7 and Msx1 (roof plate markers) was reduced in the dorsal midline of the midbrain. Ectopic expression of Sox1 in Arl8b^-/- embryos was detected also at E9.0 in the neural fold, which gives rise to the roof plate. In addition, the levels of Bmp receptor IA and phosphorylated Smad 1/5/8 (downstream of BMP signaling) were increased in the neural fold of E9.0 Arl8b^-/- embryos. These results suggest that Arl8b is involved in the development of the neural fold and the subsequently formed roof plate, possibly via control of BMP signaling.

Conformational landscape alternations promote oncogenic activities of Ras-related C3 botulinum toxin substrate 1 as revealed by NMR

Ras-related C3 botulinum toxin substrate 1 (Rac1) plays critical roles in the maintenance of cell morphology by cycling between inactive guanosine diphosphate (GDP)-bound and active guanosine triphosphate (GTP)-bound states. Rac1 P29S mutant is known to strongly promote oncogenesis by facilitating its intrinsic GDP dissociation and thereby increasing the level of the GTP-bound state. Here, we used solution nuclear magnetic resonance spectroscopy to investigate the activation mechanism of the oncogenic P29S mutant. We demonstrate that the conformational landscape is markedly altered in the mutant, and the preexisting equilibrium is shifted toward the conformation with reduced affinity for Mg²⁺ _, a cofactor that is critical for maintaining stable GDP binding. Our results suggest that the alternation of the preexisting conformational equilibrium of proteins is one of the fundamental mechanisms underlying their oncogenic activities.

Small GTPase R-Ras participates in neural tube formation in zebrafish embryonic spinal cord

Ras related (R-Ras), a small GTPase, is involved in the maintenance of apico-basal polarity in neuroepithelial cells of the zebrafish hindbrain, axonal collapse in cultured murine hippocampal neurons, and maturation of blood vessels in adult mice. However, the role of R-Ras in neural tube formation remains unknown. Using antisense morpholino oligonucleotides (AMOs), we found that in the spinal cord of zebrafish embryos, the lumen was formed bilaterally in rras morphants, whereas it was formed at the midline in control embryos. As AMO can cause off-target effects, we generated rras mutant zebrafish lines using CRISPR/Cas9 technology. Although these rras mutant embryos did not have a bilateral lumen in the spinal cord, the following findings suggest that the phenotype is unlikely due to an off-target effect of rras AMO: 1) The rras morphant phenotype was rescued by an injection of AMO-resistant rras mRNA, and 2) a bilaterally segregated spinal cord was not observed in rras mutant embryos injected with rras AMO. The results suggest that the function of other ras family genes may be redundant in rras mutants. Previous research reported a bilaterally formed lumen in the spinal cord of zebrafish embryos with a mutation in a planar cell polarity (PCP) gene, van gogh-like 2 (vangl2). In the present study, in cultured cells, R-Ras was co-immunoprecipitated with Vangl2 but not with another PCP regulator, Pricke1. Interestingly, the interaction between R-Ras and Vangl2 was stronger in guanine-nucleotide free point mutants of R-Ras than in wild-type or constitutively active (GTP-bound) forms of R-Ras. R-Ras may regulate neural tube formation in cooperation with Vangl2 in the developing zebrafish spinal cord.

Regulation of the Sar1 GTPase Cycle Is Necessary for Large Cargo Secretion from the Endoplasmic Reticulum

Proteins synthesized within the endoplasmic reticulum (ER) are transported to the Golgi via coat protein complex II (COPII)-coated vesicles. The formation of COPII-coated vesicles is regulated by the GTPase cycle of Sar1. Activated Sar1 is recruited to ER membranes and forms a pre-budding complex with cargoes and the inner-coat complex. The outer-coat complex then stimulates Sar1 inactivation and completes vesicle formation. The mechanisms of forming transport carriers are well-conserved among species; however, in mammalian cells, several cargo molecules such as collagen, and chylomicrons are too large to be accommodated in conventional COPII-coated vesicles. Thus, special cargo-receptor complexes are required for their export from the ER. cTAGE5/TANGO1 complexes and their isoforms have been identified as cargo receptors for these macromolecules. Recent reports suggest that the cTAGE5/TANGO1 complex interacts with the GEF and the GAP of Sar1 and tightly regulates its GTPase cycle to accomplish large cargo secretion.

Structure-based analysis of the guanine nucleotide exchange factor SmgGDS reveals armadillo-repeat motifs and key regions for activity and GTPase binding

Small GTPases are molecular switches that have critical biological roles and are controlled by GTPase-activating proteins and guanine nucleotide exchange factors (GEFs). The smg GDP dissociation stimulator (SmgGDS) protein functions as a GEF for the RhoA and RhoC small GTPases. SmgGDS has various regulatory roles, including small GTPase trafficking and localization and as a molecular chaperone, and interacts with many small GTPases possessing polybasic regions. Two SmgGDS splice variants, SmgGDS-558 and SmgGDS-607, differ in GEF activity and binding affinity for RhoA depending on the lipidation state, but the reasons for these differences are unclear. Here we determined the crystal structure of SmgGDS-558, revealing a fold containing tandem copies of armadillo repeats not present in other GEFs. We also observed that SmgGDS harbors distinct positively and negatively charged regions, both of which play critical roles in binding to RhoA and GEF activity. This is the first report demonstrating a relationship between the molecular function and atomic structure of SmgGDS. Our findings indicate that the two SmgGDS isoforms differ in GTPase binding and GEF activity, depending on the lipidation state, thus providing useful information about the cellular functions of SmgGDS in cells.

TANGO1 recruits Sec16 to coordinately organize ER exit sites for efficient secretion

Mammalian endoplasmic reticulum (ER) exit sites export a variety of cargo molecules including oversized cargoes such as collagens. However, the mechanisms of their assembly and organization are not fully understood. TANGO1L is characterized as a collagen receptor, but the function of TANGO1S remains to be investigated. Here, we show that direct interaction between both isoforms of TANGO1 and Sec16 is not only important for their correct localization but also critical for the organization of ER exit sites. The depletion of TANGO1 disassembles COPII components as well as membrane-bound ER-resident complexes, resulting in fewer functional ER exit sites and delayed secretion. The ectopically expressed TANGO1 C-terminal domain responsible for Sec16 binding in mitochondria is capable of recruiting Sec16 and other COPII components. Moreover, TANGO1 recruits membrane-bound macromolecular complexes consisting of cTAGE5 and Sec12 to the ER exit sites. These data suggest that mammalian ER exit sites are organized by TANGO1 acting as a scaffold, in cooperation with Sec16 for efficient secretion.

Arl8b is required for lysosomal degradation of maternal proteins in the visceral yolk sac endoderm of mouse embryos

The small GTPase Arl8b localizes primarily to lysosomes and is involved in lysosomal motility and fusion. Here, we show that Arl8b is required for lysosomal degradation of maternal proteins in the visceral yolk sac endoderm (VYSE), an apical cell layer of the visceral yolk sac, of mouse embryos. The VYSE actively takes up maternal materials from uterine fluid and degrades them in lysosomes to provide breakdown products to the embryo as energy sources. Arl8b gene-trap mice (Arl8b−/−) displayed decreased early embryo body size. The Arl8b−/− VYSE exhibited defective endocytic trafficking to the lysosome and accumulation of maternal proteins such as albumin and immunoglobulin G in late endocytic organelles. Furthermore, Transthyretin-Cre;Arl8bflox/flox mice in which Arl8b was ablated specifically in the VYSE also showed decreased embryo body size, defects in trafficking to the lysosome, and reduction of the free amino acid level in the embryos. Taken together, these results suggest that Arl8b mediates lysosomal degradation of maternal proteins in the VYSE, thereby contributing to mouse embryonic development.

Distinct isoform-specific complexes of TANGO1 cooperatively facilitate collagen secretion from the endoplasmic reticulum

The short isoform of TANGO1, termed TANGO1S, is necessary for collagen secretion from the ER independently of TANGO1L. TANGO1L and TANGO1S form individual complexes at ER exit sites and cooperatively participate in collagen export from the ER.

Collagens synthesized within the endoplasmic reticulum (ER) are too large to fit in conventional COPII-coated transport vesicles; thus their export from the ER requires specialized factors. TANGO1 (L) is an integral membrane protein that binds to collagen and the coatomer of vesicles and is necessary for collagen secretion from the ER. Here we characterized the short isoform of TANGO1 (TANGO1S), lacking the collagen-binding domain, and found that it was independently required for collagen export from the ER. Moreover, we found that each of the TANGO1 isoforms forms a stable protein complex with factors involved in collagen secretion: TANGO1L/cTAGE5/Sec12 (900 kDa) and TANGO1S/cTAGE5/Sec12 (700 kDa). Of interest, TANGO1S and TANGO1L seemed to be interchangeable in exporting collagen from the ER. Our results suggest that mammalian ER exit sites possess two different-sized membrane-bound macromolecular complexes that specifically function in large-cargo export from the ER.

Dual function of cTAGE5 in collagen export from the endoplasmic reticulum

Two functionally irreplaceable and molecularly separable modules in cTAGE5 are both required for collagen VII export from the ER. The concentration of Sec12 induced by cTAGE5 serves for efficient production of activated Sar1 around ER exit sites, and the GTPase cycle of Sar1 seems to be required for collagen VII export from the ER.

Two independent functions of cTAGE5 have been reported in collagen VII export from the endoplasmic reticulum (ER). cTAGE5 not only forms a cargo receptor complex with TANGO1, but it also acts as a scaffold to recruit Sec12, a guanine-nucleotide exchange factor for Sar1 GTPase, to ER exit sites. However, the relationship between the two functions remains unclear. Here we isolated point mutants of cTAGE5 that lost Sec12-binding ability but retained binding to TANGO1. Although expression of the mutant alone could not rescue the defects in collagen VII secretion mediated by cTAGE5 knockdown, coexpression with Sar1, but not with the GTPase-deficient mutant, recovered secretion. The expression of Sar1 alone failed to rescue collagen secretion in cTAGE5-depleted cells. Taken together, these results suggest that two functionally irreplaceable and molecularly separable modules in cTAGE5 are both required for collagen VII export from the ER. The recruitment of Sec12 by cTAGE5 contributes to efficient activation of Sar1 in the vicinity of ER exit sites. In addition, the GTPase cycle of Sar1 appears to be responsible for collagen VII exit from the ER.

Di-Ras2 Protein Forms a Complex with SmgGDS Protein in Brain Cytosol in Order to Be in a Low Affinity State for Guanine Nucleotides

The Ras family of small GTPases function in a wide variety of biological processes as "molecular switches" by cycling between inactive GDP-bound and active GTP-bound forms. Di-Ras1 and Di-Ras2 were originally identified as small GTPases forming a distinct subgroup of the Ras family. Di-Ras1/Di-Ras2 mRNAs are detected predominantly in brain and heart tissues. Biochemical analysis of Di-Ras1/Di-Ras2 has revealed that they have little GTPase activity and that their intrinsic guanine-nucleotide exchange rates are much faster than that of H-Ras. Yet little is known about the biological role(s) of Di-Ras1/Di-Ras2 or of how their activities are regulated. In the present study we found that endogenous Di-Ras2 co-purifies with SmgGDS from rat brain cytosol. Size-exclusion chromatography of purified recombinant proteins showed that Di-Ras2 forms a high affinity complex with SmgGDS. SmgGDS is a guanine nucleotide exchange factor with multiple armadillo repeats and has recently been shown to specifically activate RhoA and RhoC. In contrast to the effect on RhoA, SmgGDS does not act as a guanine nucleotide exchange factor for Di-Ras2 but instead tightly associates with Di-Ras2 to reduce its binding affinity for guanine nucleotides. Finally, pulse-chase analysis revealed that Di-Ras2 binds, in a C-terminal CAAX motif-dependent manner, to SmgGDS immediately after its synthesis. This leads to increased Di-Ras2 stability. We thus propose that isoprenylated Di-Ras2 forms a tight complex with SmgGDS in cytosol immediately after its synthesis, which lowers its affinity for guanine nucleotides.

Mechanisms for exporting large-sized cargoes from the endoplasmic reticulum

Cargo proteins exported from the endoplasmic reticulum to the Golgi apparatus are typically transported in coat protein complex II (COPII)-coated vesicles of 60–90 nm diameter. Several cargo molecules including collagens and chylomicrons form structures that are too large to be accommodated by these vesicles, but their secretion still requires COPII proteins. Here, we first review recent progress on large cargo secretions derived especially from animal models and human diseases, which indicate the importance of COPII proteins. We then discuss the recent isolation of specialized factors that modulate the process of COPII-dependent cargo formation to facilitate the exit of large-sized cargoes from the endoplasmic reticulum. Based on these findings, we propose a model that describes the importance of the GTPase cycle for secretion of oversized cargoes. Next, we summarize reports that describe the structures of COPII proteins and how these results provide insight into the mechanism of assembly of the large cargo carriers. Finally, we discuss what issues remain to be solved in the future.

The C. elegans Hypodermis Couples Progenitor Cell Quiescence to the Dietary State

The nutritional status of an organism can greatly impact the function and behavior of stem and progenitor cells [1]. However, the regulatory circuits that inform these cells about the dietary environment remain to be elucidated. Newly hatched C. elegans larvae (L1s) halt development in "L1 arrest" or "L1 diapause" until ample food is encountered and triggers stem and progenitor cells to exit from quiescence [2]. The insulin/insulin-like growth factor signaling (IIS) pathway plays a key role in this reactivation [3, 4], but its site(s) of action have not been elucidated nor have the nutrient molecule(s) that stimulate the pathway been identified. By tissue-specifically modulating the activity of its components, we demonstrate that the IIS pathway acts in the hypodermis to regulate nutrition-responsive reactivation of neural and mesodermal progenitor cells. We identify ethanol, a likely component of the natural Caenorhabditis habitat, and amino acids as nutrients that synergistically reactivate somatic progenitor cells and upregulate expression of insulin-like genes in starved L1 larvae. The hypodermis likely senses the availability of amino acids because forced activation of the amino-acid-responsive Rag-TORC1 (target of rapamycin complex 1) pathway in this tissue can also release somatic progenitor cell quiescence in the presence of ethanol. Finally, there appears to be crosstalk between the IIS and Rag-TORC1 pathways because constitutive activation of the IIS pathway requires Rag to promote reactivation. This work demonstrates that ethanol and amino acids act as dietary cues via the IIS and Rag-TORC1 pathways in the hypodermis to coordinately control progenitor cell behavior.

Concentration of Sec12 at ER exit sites via interaction with cTAGE5 is required for collagen export

By interacting with the collagen cargo receptor component cTAGE5, Sec12 concentrates at ER exit sites and generates the high levels of GTP-bound Sar1 necessary for export of collagen to the Golgi.

Mechanisms for exporting variably sized cargo from the endoplasmic reticulum (ER) using the same machinery remain poorly understood. COPII-coated vesicles, which transport secretory proteins from the ER to the Golgi apparatus, are typically 60–90 nm in diameter. However, collagen, which forms a trimeric structure that is too large to be accommodated by conventional transport vesicles, is also known to be secreted via a COPII-dependent process. In this paper, we show that Sec12, a guanine-nucleotide exchange factor for Sar1 guanosine triphosphatase, is concentrated at ER exit sites and that this concentration of Sec12 is specifically required for the secretion of collagen VII but not other proteins. Furthermore, Sec12 recruitment to ER exit sites is organized by its direct interaction with cTAGE5, a previously characterized collagen cargo receptor component, which functions together with TANGO1 at ER exit sites. These findings suggest that the export of large cargo requires high levels of guanosine triphosphate–bound Sar1 generated by Sec12 localized at ER exit sites.

Active transport and diffusion barriers restrict Joubert Syndrome-associated ARL13B/ARL-13 to an Inv-like ciliary membrane subdomain

Cilia are microtubule-based cell appendages, serving motility, chemo-/mechano-/photo- sensation, and developmental signaling functions. Cilia are comprised of distinct structural and functional subregions including the basal body, transition zone (TZ) and inversin (Inv) compartments, and defects in this organelle are associated with an expanding spectrum of inherited disorders including Bardet-Biedl syndrome (BBS), Meckel-Gruber Syndrome (MKS), Joubert Syndrome (JS) and Nephronophthisis (NPHP). Despite major advances in understanding ciliary trafficking pathways such as intraflagellar transport (IFT), how proteins are transported to subciliary membranes remains poorly understood. Using Caenorhabditis elegans and mammalian cells, we investigated the transport mechanisms underlying compartmentalization of JS-associated ARL13B/ARL-13, which we previously found is restricted at proximal ciliary membranes. We now show evolutionary conservation of ARL13B/ARL-13 localisation to an Inv-like subciliary membrane compartment, excluding the TZ, in many C. elegans ciliated neurons and in a subset of mammalian ciliary subtypes. Compartmentalisation of C. elegans ARL-13 requires a C-terminal RVVP motif and membrane anchoring to prevent distal cilium and nuclear targeting, respectively. Quantitative imaging in more than 20 mutants revealed differential contributions for IFT and ciliopathy modules in defining the ARL-13 compartment; IFT-A/B, IFT-dynein and BBS genes prevent ARL-13 accumulation at periciliary membranes, whereas MKS/NPHP modules additionally inhibit ARL-13 association with TZ membranes. Furthermore, in vivo FRAP analyses revealed distinct roles for IFT and MKS/NPHP genes in regulating a TZ barrier to ARL-13 diffusion, and intraciliary ARL-13 diffusion. Finally, C. elegans ARL-13 undergoes IFT-like motility and quantitative protein complex analysis of human ARL13B identified functional associations with IFT-B complexes, mapped to IFT46 and IFT74 interactions. Together, these findings reveal distinct requirements for sequence motifs, IFT and ciliopathy modules in defining an ARL-13 subciliary membrane compartment. We conclude that MKS/NPHP modules comprise a TZ barrier to ARL-13 diffusion, whereas IFT genes predominantly facilitate ARL-13 ciliary entry and/or retention via active transport mechanisms.

The inhibitory G protein G(i) identified as pertussis toxin-catalyzed ADP-ribosylation

Pertussis toxin (PTX) produced by Bordetella pertussis was first introduced by Ui and his colleagues in research on signal transduction under the name islet-activating protein in 1979, when the mechanism of toxin-induced stimulation of insulin release from pancreatic islets was reported in the rat. The stimulatory effect of PTX in vivo results from the blockage of α(2)-adrenergic receptor-mediated inhibition of insulin release. The receptor-induced inhibition of cAMP formation was also abolished in pancreatic islets isolated from PTX-treated rats, suggesting that the toxin caused uncoupling of adenylyl cyclase inhibition from receptor stimulation. The action of PTX on isolated membranes required a cytosolic factor, nicotinamide adenine dinucleotide (NAD), and the uncoupling induced by PTX was shown to be due to the toxin-catalyzed ADP-ribosylation of a 41-kDa protein with NAD as another substrate. The 41-kDa PTX substrate was soon identified and purified as the α-subunit of the inhibitory G protein that transmits an inhibitory signal from membrane receptors to adenylyl cyclase. After demonstration of the molecular mechanism of PTX, the toxin was widely utilized as a probe for identifying and analyzing major αβγ-trimeric G proteins. Thus, PTX-sensitive G proteins appeared to carry positive and negative signals from many membrane receptors to a variety of effectors other than adenylyl cyclase.

The microRNA miR-235 couples blast-cell quiescence to the nutritional state

The coordination of stem- and blast-cell behaviours, such as self-renewal, differentiation and quiescence, with physiological changes underlies growth, regeneration and tissue homeostasis. Germline stem and somatic blast cells in newly hatched Caenorhabditis elegans larvae can suspend postembryonic development, which consists of diverse cellular events such as migration, proliferation and differentiation, until the nutritional state becomes favourable (termed L1 diapause). Although previous studies showed that the insulin/insulin-like growth factor (IGF) signalling (IIS) pathway regulates this developmental quiescence, the detailed mechanism by which the IIS pathway enables these multipotent cells to respond to nutrient availability is unknown. Here we show in C. elegans that the microRNA (miRNA) miR-235, a sole orthologue of mammalian miR-92 from the oncogenic miR-17-92 cluster, acts in the hypodermis and glial cells to arrest postembryonic developmental events in both neuroblasts and mesoblasts. Expression of mir-235 persists during L1 diapause, and decreases upon feeding in a manner dependent on the IIS pathway. Upregulation of one of the miR-235 targets, nhr-91, which encodes an orthologue of mammalian germ cell nuclear factor, is responsible for defects caused by loss of the miRNA. Our findings establish a novel role of a miR-92 orthologue in coupling blast-cell behaviours to the nutritional state.

Arl8/ARL-8 functions in apoptotic cell removal by mediating phagolysosome formation in Caenorhabditis elegans

Efficient clearance of apoptotic cells by phagocytes is important for development, tissue homeostasis, and the prevention of autoimmune responses. Phagosomes containing apoptotic cells undergo acidification and mature from Rab5-positive early to Rab7-positive late stages. Phagosomes finally fuse with lysosomes to form phagolysosomes, which degrade apoptotic cells; however, the molecular mechanism underlying phagosome-lysosome fusion is not fully understood. Here we show that the Caenorhabditis elegans Arf-like small GTPase Arl8 (ARL-8) is involved in phagolysosome formation and is required for the efficient removal of apoptotic cells. Loss of function of arl-8 results in the accumulation of apoptotic germ cells. Both the engulfment of the apoptotic cells by surrounding somatic sheath cells and the phagosomal maturation from RAB-5- to RAB-7-positive stages occur in arl-8 mutants. However, the phagosomes fail to fuse with lysosomes in the arl-8 mutants, leading to the accumulation of RAB-7-positive phagosomes and the delayed degradation of apoptotic cells. ARL-8 localizes primarily to lysosomes and physically interacts with the homotypic fusion and protein sorting complex component VPS-41. Collectively our findings reveal that ARL-8 facilitates apoptotic cell removal in vivo by mediating phagosome-lysosome fusion during phagocytosis.

Biological role of the two overlapping poly(A)-binding protein interacting motifs 2 (PAM2) of eukaryotic releasing factor eRF3 in mRNA decay

Eukaryotic releasing factor GSPT/eRF3 mediates translation termination-coupled mRNA decay via interaction with a cytosolic poly(A)-binding protein (PABPC1). A region of eRF3 containing two overlapping PAM2 (PABPC1-interacting motif 2) motifs is assumed to bind to the PABC domain of PABPC1, on the poly(A) tail of mRNA. PAM2 motifs are also found in the major deadenylases Caf1-Ccr4 and Pan2-Pan3, whose activities are enhanced upon PABPC1 binding to these motifs. Their deadenylase activities are regulated by eRF3, in which two overlapping PAM2 motifs competitively prevent interaction with PABPC1. However, it is unclear how these overlapping motifs recognize PABC and regulate deadenylase activity in a translation termination-coupled manner. We used a dominant-negative approach to demonstrate that the N-terminal PAM2 motif is critical for eRF3 binding to PABPC1 and that both motifs are required for function. Isothermal titration calorimetry (ITC) and NMR analyses revealed that the interaction is in equilibrium between the two PAM2-PABC complexes, where only one of the two overlapping PAM2 motifs is PABC-bound and the other is PABC-unbound and partially accessible to the other PABC. Based on these results, we proposed a biological role for the overlapping PAM2 motifs in the regulation of deadenylase accessibility to PABPC1 at the 3' end of poly(A).

Neuronally expressed Ras-family GTPase Di-Ras modulates synaptic activity in Caenorhabditis elegans

Ras-family GTPases regulate a wide variety of cellular functions including cell growth and differentiation. Di-Ras, which belongs to a distinct subfamily of Ras-family GTPases, is expressed predominantly in brain, but the role of Di-Ras in nervous systems remains totally unknown. Here, we report that the Caenorhabditis elegans Di-Ras homologue drn-1 is expressed specifically in neuronal cells and involved in synaptic function at neuromuscular junctions. Loss of function of drn-1 conferred resistance to the acetylcholinesterase inhibitor aldicarb and partially suppressed the aldicarb-hypersensitive phenotypes of heterotrimeric G-protein mutants, in which acetylcholine release is up-regulated. drn-1 mutants displayed no apparent defects in the axonal distribution of the membrane-bound second messenger diacylglycerol (DAG), which is a key stimulator of acetylcholine release. Finally, we have identified EPAC-1, a C. elegans Epac homologue, as a binding partner for DRN-1. Deletion mutants of epac-1 displayed an aldicarb-resistant phenotype as drn-1 mutants. Genetic analysis of drn-1 and epac-1 showed that they acted in the same pathway to control acetylcholine release. Furthermore, DRN-1 and EPAC-1 were co-immunoprecipitated. These findings suggest that DRN-1 may function cooperatively with EPAC-1 to modulate synaptic activity in C. elegans.

Tetrameric interaction of the ectoenzyme CD38 on the cell surface enables its catalytic and raft-association activities

The leukocyte cell-surface antigen CD38 is the major nicotinamide adenide dinucleotide glycohydrolase in mammals, and its ectoenzyme activity is involved in calcium mobilization. CD38 is also a raft-dependent signaling molecule. CD38 forms a tetramer on the cell surface, but the structural basis and the functional significance of tetramerization have remained unexplored. We identified the interfaces contributing to the homophilic interaction of mouse CD38 by site-specific crosslinking on the cell surface with an expanded genetic code, based on a crystallographic analysis. A combination of the three interfaces enables CD38 to tetramerize: one interface involving the juxtamembrane α-helix is responsible for the formation of the core dimer, which is further dimerized via the other two interfaces. This dimerization of dimers is required for the catalytic activity and the localization of CD38 in membrane rafts. The glycosylation prevents further self-association of the tetramer. Accordingly, the tetrameric interaction underlies the multifaceted actions of CD38.

C. elegans AMPKs promote survival and arrest germline development during nutrient stress

Mechanisms controlling development, growth, and metabolism are coordinated in response to changes in environmental conditions, enhancing the likelihood of survival to reproductive maturity. Much remains to be learned about the molecular basis underlying environmental influences on these processes. C. elegans larvae enter a developmentally dormant state called L1 diapause when hatched into nutrient-poor conditions. The nematode pten homologue daf-18 is essential for maintenance of survival and germline stem cell quiescence during this period (Fukuyama et al., 2006; Sigmond et al., 2008), but the details of the signaling network(s) in which it functions remain to be elucidated. Here, we report that animals lacking both aak-1 and aak-2, which encode the two catalytic α subunits of AMP-activated protein kinase (AMPK), show reduced viability and failure to maintain mitotic quiescence in germline stem cells during L1 diapause. Furthermore, failure to arrest germline proliferation has a long term consequence; aak double mutants that have experienced L1 diapause develop into sterile adults when returned to food, whereas their continuously fed siblings are fertile. Both aak and daf-18 appear to maintain germline quiescence by inhibiting activity of the common downstream target, TORC1 (TOR Complex 1). In contrast, rescue of the lethality phenotype indicates that aak-2 acts not only in the intestine, as does daf-18, but also in neurons, likely promoting survival by preventing energy deprivation during L1 diapause. These results not only provide evidence that AMPK contributes to survival during L1 diapause in a manner distinct from that by which it controls dauer diapause, but they also suggest that AMPK suppresses TORC1 activity to maintain stem cell quiescence.

A novel acetylation cycle of transcription co-activator Yes-associated protein that is downstream of Hippo pathway is triggered in response to SN2 alkylating agents

Yes-associated protein (YAP) is a transcriptional co-activator that acts downstream of the Hippo signaling pathway and regulates multiple cellular processes. Although cytoplasmic retention of YAP is known to be mediated by Hippo pathway-dependent phosphorylation, post-translational modifications that regulate YAP in the nucleus remain unclear. Here we report the discovery of a novel cycle of acetylation/deacetylation of nuclear YAP induced in response to S(N)2 alkylating agents. We show that after treatment of cells with the S(N)2 alkylating agent methyl methanesulfonate, YAP phosphorylation mediated by the Hippo pathway is markedly reduced, leading to nuclear translocation of YAP and its acetylation. This YAP acetylation occurs on specific and highly conserved C-terminal lysine residues and is mediated by the nuclear acetyltransferases CBP (CREB binding protein) and p300. Conversely, the nuclear deacetylase SIRT1 is responsible for YAP deacetylation. Intriguingly, we found that YAP acetylation is induced specifically by S(N)2 alkylating agents and not by other DNA-damaging stimuli. These results identify a novel YAP acetylation cycle that occurs in the nucleus downstream of the Hippo pathway. Intriguingly, our findings also indicate that YAP acetylation is involved in responses to a specific type of DNA damage.

RINL, guanine nucleotide exchange factor Rab5-subfamily, is involved in the EphA8-degradation pathway with odin

The Rab family of small guanosine triphosphatases (GTPases) plays a vital role in membrane trafficking. Its active GTP-bound state is driven by guanine nucleotide-exchange factors (GEFs). Ras and Rab interactor (or Ras interaction/interference)-like (RINL), which contains a conserved VPS9 domain critical for GEF action, was recently identified as a new Rab5 subfamily GEF in vitro. However, its detailed function and interacting molecules have not yet been fully elucidated. Here we found that RINL has GEF activity for the Rab5 subfamily proteins by measuring their GTP-bound forms in cultured cells. We also found that RINL interacts with odin, a member of the ankyrin-repeat and sterile-alpha motif (SAM) domain-containing (Anks) protein family. In addition, the Eph tyrosine kinase receptor EphA8 formed a ternary complex with both RINL and odin. Interestingly, RINL expression in cultured cells reduced EphA8 levels in a manner dependent on both its GEF activity and interaction with odin. In addition, knockdown of RINL increased EphA8 level in HeLa cells. Our findings suggest that RINL, as a GEF for Rab5 subfamily, is implicated in the EphA8-degradation pathway via its interaction with odin.

Characterization of RIN3 as a guanine nucleotide exchange factor for the Rab5 subfamily GTPase Rab31

The small GTPase Rab5, which cycles between GDP-bound inactive and GTP-bound active forms, plays essential roles in membrane budding and trafficking in the early endocytic pathway. Rab5 is activated by various vacuolar protein sorting 9 (VPS9) domain-containing guanine nucleotide exchange factors. Rab21, Rab22, and Rab31 (members of the Rab5 subfamily) are also involved in the trafficking of early endosomes. Mechanisms controlling the activation Rab5 subfamily members remain unclear. RIN (Ras and Rab interactor) represents a family of multifunctional proteins that have a VPS9 domain in addition to Src homology 2 (SH2) and Ras association domains. We investigated whether RIN family members act as guanine nucleotide exchange factors (GEFs) for the Rab5 subfamily on biochemical and cell morphological levels. RIN3 stimulated the formation of GTP-bound Rab31 in cell-free and in cell GEF activity assays. RIN3 also formed enlarged vesicles and tubular structures, where it colocalized with Rab31 in HeLa cells. In contrast, RIN3 did not exhibit any apparent effects on Rab21. We also found that serine to alanine substitutions in the sequences between SH2 and RIN family homology domain of RIN3 specifically abolished its GEF action on Rab31 but not Rab5. We examined whether RIN3 affects localization of the cation-dependent mannose 6-phosphate receptor (CD-MPR), which is transported between trans-Golgi network and endocytic compartments. We found that RIN3 partially translocates CD-MPR from the trans-Golgi network to peripheral vesicles and that this is dependent on its Rab31-GEF activity. These results indicate that RIN3 specifically acts as a GEF for Rab31.

cTAGE5 mediates collagen secretion through interaction with TANGO1 at endoplasmic reticulum exit sites

The mechanism of collagen secretion is not completely understood. It is found that cTAGE5 binds to TANGO1, and it is suggested that collagen VII export from the ER is driven by a cTAGE5/TANGO1 complex.

Cutaneous T-cell lymphoma-–associated antigen 5 (cTAGE5), an originally identified tumor antigen, is overexpressed in various cancer cell lines. The cDNA encodes an integral membrane protein containing two coiled-coil motifs and a proline-rich domain. We show that cTAGE5 specifically localizes to the endoplasmic reticulum (ER) exit sites. In addition, cTAGE5 forms a complex with TANGO1 (MIA3), a previously characterized cargo receptor for collagen VII, by the interaction of their coiled-coil motifs. Of interest, cTAGE5, as well as TANGO1, is capable of interacting with the inner-layer coatomer of COPII Sec23/24 complex through their C-terminal proline-rich domains and required for collagen VII secretion. We propose that cTAGE5 acts as a coreceptor of TANGO1 for collagen VII export from the ER.

RhoA activation participates in rearrangement of processing bodies and release of nucleated AU-rich mRNAs

Cytoplasmic ribonucleoprotein granules, known as processing bodies (P-bodies), contain a common set of conserved RNA-processing enzymes, and mRNAs with AU-rich elements (AREs) are delivered to P-bodies for translational silencing. Although the dynamics of P-bodies is physically linked to cytoskeletal network, it is unclear how small GTPases are involved in the P-body regulation and the ARE-mRNA metabolism. We found here that glucose depletion activates RhoA GTPase and alters the P-body dynamics in HeLa cells. These glucose-depleted effects are reproduced by the overexpression of the RhoA-subfamily GTPases and conversely abolished by the inhibition of RhoA activation. Interestingly, both RhoA activation and glucose depletion inhibit the mRNA accumulation and degradation. These findings indicate that RhoA participates in the stress-induced rearrangement of P-bodies and the release of nucleated ARE-mRNAs for their stabilization.

Negative regulation of wnt11 expression by Jnk signaling during zebrafish gastrulation

Stress-induced Sapk/Jnk signaling is involved in cell survival and apoptosis. Recent studies have increased our understanding of the physiological roles of Jnk signaling in embryonic development. However, still unclear is the precise function of Jnk signaling during gastrulation, a critical step in the establishment of the vertebrate body plan. Here we use morpholino-mediated knockdown of the zebrafish orthologs of the Jnk activators Mkk4 and Mkk7 to examine the effect of Jnk signaling abrogation on early vertebrate embryogenesis. Depletion of zebrafish Mkk4b led to abnormal convergent extension (CE) during gastrulation, whereas Mkk7 morphants exhibited defective somitogenesis. Surprisingly, Mkk4b morphants displayed marked upregulation of wnt11, which is the triggering ligand of CE and stimulates Jnk activation via the non-canonical Wnt pathway. Conversely, ectopic activation of Jnk signaling by overexpression of an active form of Mkk4b led to wnt11 downregulation. Mosaic lineage tracing studies revealed that Mkk4b-Jnk signaling suppressed wnt11 expression in a non-cell-autonomous manner. These findings provide the first evidence that wnt11 itself is a downstream target of the Jnk cascade in the non-canonical Wnt pathway. Our work demonstrates that Jnk activation is indispensable for multiple steps during vertebrate body plan formation. Furthermore, non-canonical Wnt signaling may coordinate vertebrate CE movements by triggering Jnk activation that represses the expression of the CE-triggering ligand wnt11.

An Arf-like small G protein, ARL-8, promotes the axonal transport of presynaptic cargoes by suppressing vesicle aggregation

Presynaptic assembly requires the packaging of requisite proteins into vesicular cargoes in the cell soma, their long-distance microtubule-dependent transport down the axon, and, finally, their reconstitution into functional complexes at prespecified sites. Despite the identification of several molecules that contribute to these events, the regulatory mechanisms defining such discrete states remain elusive. We report the characterization of an Arf-like small G protein, ARL-8, required during this process. arl-8 mutants prematurely accumulate presynaptic cargoes within the proximal axon of several neuronal classes, with a corresponding failure to assemble presynapses distally. This proximal accumulation requires the activity of several molecules known to catalyze presynaptic assembly. Dynamic imaging studies reveal that arl-8 mutant vesicles exhibit an increased tendency to form immotile aggregates during transport. Together, these results suggest that arl-8 promotes a trafficking identity for presynaptic cargoes, facilitating their efficient transport by repressing premature self-association.

The arf-like GTPase Arl8 mediates delivery of endocytosed macromolecules to lysosomes in Caenorhabditis elegans

Late endocytic organelles including lysosomes are highly dynamic acidic organelles. Late endosomes and lysosomes directly fuse for content mixing to form hybrid organelles, from which lysosomes are reformed. It is not fully understood how these processes are regulated and maintained. Here we show that the Caenorhabditis elegans ARL-8 GTPase is localized primarily to lysosomes and involved in late endosome-lysosome fusion in the macrophage-like coelomocytes. Loss of arl-8 results in an increase in the number of late endosomal/lysosomal compartments, which are smaller than wild type. In arl-8 mutants, late endosomal compartments containing endocytosed macromolecules fail to fuse with lysosomal compartments enriched in the aspartic protease ASP-1. Furthermore, loss of arl-8 strongly suppresses formation of enlarged late endosome-lysosome hybrid organelles caused by mutations of cup-5, which is the orthologue of human mucolipin-1. These findings suggest that ARL-8 mediates delivery of endocytosed macromolecules to lysosomes by facilitating late endosome-lysosome fusion.

Quantitative characterization of Tob interactions provides the thermodynamic basis for translation termination-coupled deadenylase regulation

Translation termination-coupled deadenylation is the first and often the rate-limiting step of eukaryotic mRNA decay in which two deadenylases, Ccr4-Caf1 and Pan2, play key roles. One of the deadenylases, Caf1, associates with Tob, which recruits Caf1 to the poly(A) tail through interactions with a cytoplasmic poly(A)-binding protein 1 (PABPC1). We previously proposed that the competition between Tob and eRF3 (a translation termination factor that interacts with PABPC1) is responsible for the regulation of deadenylase activity. However, the molecular mechanism of the regulation should be addressed by investigating the binding affinity and the cellular levels of these proteins. In this work, we characterized the human Tob interactions with Caf1 and a C-terminal domain of PABPC1 (PABC). Nuclear magnetic resonance (NMR) and Western blot analyses revealed that Tob consists of a structured N-terminal BTG-Tob domain and an unstructured C-terminal region with two conserved PAM2 (PABPC1-interacting motif 2) motifs. The BTG-TOB domain associates with Caf1, whereas the C-terminal PAM2 motif binds to PABC, with a K(d) value of 20 microM. Furthermore, we demonstrated that the levels of eRF3 and Tob in HeLa cells are 4-5 microM and less than 0.2 microM, respectively. On the basis of these results, we propose a thermodynamic mechanism for the translation termination-coupled deadenylation mediated by the Tob-Caf1 complex.

The LIM protein Ajuba is required for ciliogenesis and left-right axis determination in medaka

Cilia are microtubule-based organelles that are present on the surfaces of almost all vertebrate cells. Most cilia function as sensory or molecular transport structures. Malfunctions of cilia have been implicated in several diseases of human development. The assembly of cilia is initiated by the centriole (or basal body), and several centrosomal proteins are involved in this process. The mammalian LIM protein Ajuba is a well-studied centrosomal protein that regulates cell division but its role in ciliogenesis is unknown. In this study, we isolated the medaka homolog of Ajuba and showed that Ajuba localizes to basal bodies of cilia in growth-arrested cells. Knockdown of Ajuba resulted in randomized left-right organ asymmetries and altered expression of early genes responsible for left-right body axis determination. At the cellular level, we found that Ajuba function was essential for ciliogenesis in the cells lining Kupffer's vesicle; it is these cells that induce the asymmetric fluid flow required for left-right axis determination. Taken together, our findings identify a novel role for Ajuba in the regulation of vertebrate ciliogenesis and left-right axis determination.

Joubert syndrome Arl13b functions at ciliary membranes and stabilizes protein transport in Caenorhabditis elegans

The small GTPase Arl13b regulates ciliary transmembrane protein localizations and anterograde IFT assembly stability.

The small ciliary G protein Arl13b is required for cilium biogenesis and sonic hedgehog signaling and is mutated in patients with Joubert syndrome (JS). In this study, using Caenorhabditis elegans and mammalian cell culture systems, we investigated the poorly understood ciliary and molecular basis of Arl13b function. First, we show that Arl13b/ARL-13 localization is frequently restricted to a proximal ciliary compartment, where it associates with ciliary membranes via palmitoylation modification motifs. Next, we find that loss-of-function C. elegans arl-13 mutants possess defects in cilium morphology and ultrastructure, as well as defects in ciliary protein localization and transport; ciliary transmembrane proteins abnormally accumulate, PKD-2 ciliary abundance is elevated, and anterograde intraflagellar transport (IFT) is destabilized. Finally, we show that arl-13 interacts genetically with other ciliogenic and ciliary transport–associated genes in maintaining cilium structure/morphology and anterograde IFT stability. Together, these data implicate a role for JS-associated Arl13b at ciliary membranes, where it regulates ciliary transmembrane protein localizations and anterograde IFT assembly stability.

Retinoic acid signaling positively regulates liver specification by inducing wnt2bb gene expression in medaka

During vertebrate embryogenesis, the liver develops at a precise location along the endodermal primitive gut tube because of signaling delivered by adjacent mesodermal tissues. Although several signaling molecules have been associated with liver formation, the molecular mechanism that regulates liver specification is still unclear. We previously performed a screen in medaka to isolate mutants with impaired liver development. The medaka hio mutants exhibit a profound (but transient) defect in liver specification that resembles the liver formation defect found in zebrafish prometheus (prt) mutants, whose mutation occurs in the wnt2bb gene. In addition to their liver abnormality, hio mutants lack pectoral fins and die after hatching. Positional cloning indicated that the hio mutation affects the raldh2 gene encoding retinaldehyde dehydrogenase type2 (RALDH2), the enzyme principally responsible for retinoic acid (RA) biosynthesis. Mutations of raldh2 in zebrafish preclude the development of pectoral fins. Interestingly, in hio mutants, expression of wnt2bb in the lateral plate mesoderm (LPM) directly adjacent to the liver-forming endoderm was completely lost.

CONCLUSION

Our data reveal the unexpected finding that RA signaling positively regulates the wnt2bb gene expression required for liver specification in medaka. These results suggest that a common molecular mechanism may underlie liver and pectoral fin specification during piscine embryogenesis.

Biochemical characterization of missense mutations in the Arf/Arl-family small GTPase Arl6 causing Bardet-Biedl syndrome

Bardet-Biedl syndrome (BBS) is a pleiotropically genetic disorder, whose etiology is linked to cilia. Mutations in the Arf/Arl-family GTPase Arl6 have been recently shown to be responsible for BBS type 3. Here we show that BBS mutations alter the guanine nucleotide-binding properties of Arl6. Specifically, substitution of 31st Threonine to Arginine selectively abrogates the GTP-binding ability of Arl6 without affecting GDP-binding/dissociating properties. Furthermore, all the BBS mutations in Arl6 result in low expression of the mutant proteins, which can be restored by the inhibition of the proteasome. These findings implicate that Arl6 mutants are destabilized and eliminated by the proteasome in cells, probably due to the altered nucleotide-binding properties.