Sec24b selectively sorts Vangl2 to regulate planar cell polarity during neural tube closure

Charting the secretory pathway in a simple eukaryote

George Palade, a founding father of cell biology and of the American Society for Cell Biology (ASCB), established the ultrastructural framework for an analysis of how proteins are secreted and membranes are assembled in eukaryotic cells. His vision inspired a generation of investigators to probe the molecular mechanisms of protein transport. My laboratory has dissected these pathways with complementary genetic and biochemical approaches. Peter Novick, one of my first graduate students, isolated secretion mutants of Saccharomyces cerevisiae, and through cytological analysis of single and double mutants and molecular cloning of the corresponding SEC genes, we established that yeast cells use a secretory pathway fundamentally conserved in all eukaryotes. A biochemical reaction that recapitulates the first half of the secretory pathway was used to characterize Sec proteins that comprise the polypeptide translocation channel in the endoplasmic reticulum (ER) membrane (Sec61) and the cytoplasmic coat protein complex (COPII) that captures cargo proteins into transport vesicles that bud from the ER.

Cargo loading at the ER

Trafficking of newly synthesized cargo through the early secretory pathway defines and maintains the intracellular organization of eukaryotic cells as well as the organization of tissues and organs. The importance of this pathway is underlined by the increasing number of mutations in key components of the ER export machinery that are causative of a diversity of human diseases. Here we discuss the molecular mechanisms that dictate cargo selection during vesicle budding. While, in vitro reconstitution assays, unicellular organisms such as budding yeast, and mammalian cell culture still have much to offer in terms of gaining a full understanding of the molecular basis for secretory cargo export, such assays have to date been limited to analysis of smaller, freely diffusible cargoes. The export of large macromolecular complexes from the ER such as collagens (up to 300 nm) or lipoproteins (~500 nm) presents a clear problem in terms of maintaining both selectivity and efficiency of export. It has also become clear that in order to translate our knowledge of the molecular basis for ER export to a full understanding of the implications for normal development and disease progression, the use of metazoan models is essential. Combined, these approaches are now starting to shed light not only on the mechanisms of macromolecular cargo export from the ER but also reveal the implications of failure of this process to human development and disease.

A novel hypomorphic Looptail allele at the planar cell polarity Vangl2 gene

Vangl2 forms part of the planar cell polarity signalling pathway and is the gene defective in the Looptail (Lp) mouse mutant. Two previously described alleles, Lp and Lp(m1Jus) , segregate in a semi-dominant fashion, with heterozygotes displaying the looped-tail appearance, while homozygotes show the neural tube defect called craniorachischisis. Here, we report a novel experimentally induced allele, Lp(m2Jus) , that carries a missense mutation, R259L, in Vangl2. This mutation was specific to the Lp phenotype and absent from both parental strains and 28 other inbred strains. Notably, this mutation segregates in a recessive manner with all heterozygotes appearing normal and 47% of homozygotes showing a looped-tail. Homozygous Lp(m2Jus) embryos showed spina bifida in 12%. Lp(m2Jus) genetically interacts with Lp with 77% of compound heterozygotes displaying craniorachischisis. Vangl2(R259L) behaved like the wild-type allele in overexpression and morpholino knockdown/rescue assays in zebrafish embryos. These data suggest that Lp(m2Jus) represents a new hypomorphic allele of Lp.

The planar cell polarity pathway in vertebrate development

Directing the orientation of cells in three dimensions is a fundamental aspect of many of the processes underlying the generation of the appropriate shape and function of tissues and organs during embryonic development. In an epithelium, this requires not only the establishment of apicobasal polarity, but also cell arrangement in a specific direction in the plane of the cell sheet. The molecular pathway central to regulating this planar cell polarity (PCP) was originally discovered in the fruit fly Drosophila melanogaster and has more recently been shown to act in a highly analogous way in vertebrates, involving a strongly overlapping set of genes. Mutant studies and molecular analyses have led to insights into the role of ordered planar cell polarity in the development of a wide variety of organs and tissues. In this review, we give an overview of recent developments in the study of planar polarity signaling in vertebrates.

Rack1 is required for Vangl2 membrane localization and planar cell polarity signaling while attenuating canonical Wnt activity

The vertebrate planar cell polarity (PCP) pathway shares molecular components with the β-catenin-mediated canonical Wnt pathway but acts through membrane complexes containing Vang or Frizzled to orient neighboring cells coordinately. The molecular interactions underlying the action of Vang in PCP signaling and specification, however, are yet to be delineated. Here, we report the identification of Rack1 as an interacting protein of a vertebrate Vang protein, Vangl2. We demonstrate that Rack1 is required in zebrafish for PCP-regulated processes, including oriented cell division, cellular polarization, and convergent extension during gastrulation. We further show that the knockdown of Rack1 affects membrane localization of Vangl2 and that the Vangl2-interacting domain of Rack1 has a dominant-negative effect on Vangl2 localization and gastrulation. Moreover, Rack1 antagonizes canonical Wnt signaling. Together, our data suggest that Rack1 regulates the localization of an essential PCP protein and acts as a molecular switch to promote PCP signaling.

Molecular analysis and intestinal expression of SAR1 genes and proteins in Anderson's disease (Chylomicron retention disease)

BACKGROUND

Anderson's disease (AD) or chylomicron retention disease (CMRD) is a very rare hereditary lipid malabsorption syndrome. In order to discover novel mutations in the SAR1B gene and to evaluate the expression, as compared to healthy subjects, of the Sar1 gene and protein paralogues in the intestine, we investigated three previously undescribed individuals with the disease.

METHODS

The SAR1B, SAR1A and PCSK9 genes were sequenced. The expression of the SAR1B and SAR1A genes in intestinal biopsies of both normal individuals and patients was measured by RTqPCR. Immunohistochemistry using antibodies to recombinant Sar1 protein was used to evaluate the expression and localization of the Sar1 paralogues in the duodenal biopsies.

RESULTS

Two patients had a novel SAR1B mutation (p.Asp48ThrfsX17). The third patient, who had a previously described SAR1B mutation (p.Leu28ArgfsX7), also had a p.Leu21dup variant of the PCSK9 gene. The expression of the SAR1B gene in duodenal biopsies from an AD/CMRD patient was significantly decreased whereas the expression of the SAR1A gene was significantly increased, as compared to healthy individuals. The Sar1 proteins were present in decreased amounts in enterocytes in duodenal biopsies from the patients as compared to those from healthy subjects.

CONCLUSIONS

Although the proteins encoded by the SAR1A and SAR1B genes are 90% identical, the increased expression of the SAR1A gene in AD/CMRD does not appear to compensate for the lack of the SAR1B protein. The PCSK9 variant, although reported to be associated with low levels of cholesterol, does not appear to exert any additional effect in this patient. The results provide further insight into the tissue-specific nature of AD/CMRD.

Loss of membrane targeting of Vangl proteins causes neural tube defects

In the mouse, the loop-tail mutation (Lp) causes a very severe neural tube defect, which is caused by mutations in the Vangl2 gene. In mammals, Vangl1 and Vangl2 code for integral membrane proteins that assemble into asymmetrically distributed membrane complexes that establish planar cell polarity in epithelial cells and that regulate convergent extension movements during embryogenesis. To date, VANGL are the only genes in which mutations cause neural tube defects in humans. Three independently arising Lp alleles have been described for Vangl2: D255E, S464N, and R259L. Here we report a common mechanism for both the naturally occurring Lp (S464N) and a novel ENU-induced mutation Lp(m2Jus)(R259L). We show that the S464N and R259L variants stably expressed in polarized MDCK kidney cells fail to reach the plasma membrane, their site for biological function. The mutant variants are retained intracellularly in the endoplasmic reticulum, colocalizing with ER chaperone calreticulin. Furthermore, the mutants also show a dramatically reduced half-life of ∼3 h, compared to ∼22 h for the wild-type protein, and are rapidly degraded in a proteasome-dependent and MG132-sensitive fashion. Coexpressing individually the three known allelic Lp variants with the wild-type protein does not influence the localization of the WT at the plasma membrane, suggesting that the codominant nature of the Lp trait in vivo is due to haploid insufficiency caused by a partial loss of function in a gene dosage-dependent pathway, as opposed to a dominant negative phenotype. Our study provides a biochemical framework for the study of recently identified mutations in hVANGL1 and hVANGL2 in sporadic or familial cases of neural tube defects.

An update to the list of mouse mutants with neural tube closure defects and advances toward a complete genetic perspective of neural tube closure

The number of mouse mutants and strains with neural tube defects (NTDs) now exceeds 240, including 205 representing specific genes, 30 for unidentified genes, and 9 multifactorial strains. These mutants identify genes needed for embryonic neural tube closure. Reports of 50 new NTD mutants since our 2007 review (Harris and Juriloff, 2007) were considered in relation to the previously reviewed mutants to obtain new insights into mechanisms of NTD etiology. In addition to null mutations, some are hypomorphs or conditional mutants. Some mutations do not cause NTDs on their own, but do so in digenic, trigenic, and oligogenic combinations, an etiology that likely parallels the nature of genetic etiology of human NTDs. Mutants that have only exencephaly are fourfold more frequent than those that have spina bifida aperta with or without exencephaly. Many diverse cellular functions and biochemical pathways are involved; the NTD mutants draw new attention to chromatin modification (epigenetics), the protease-activated receptor cascade, and the ciliopathies. Few mutants directly involve folate metabolism. Prevention of NTDs by maternal folate supplementation has been tested in 13 mutants and reduces NTD frequency in six diverse mutants. Inositol reduces spina bifida aperta frequency in the curly tail mutant, and three new mutants involve inositol metabolism. The many NTD mutants are the foundation for a future complete genetic understanding of the processes of neural fold elevation and fusion along mechanistically distinct cranial-caudal segments of the neural tube, and they point to several candidate processes for study in human NTD etiology.

Akt phosphorylates Sec24: new clues into the regulation of ER-to-Golgi trafficking

Regulation of protein transport within the early secretory pathway is a relatively unexplored area. Here, we propose a new player in the control of protein transport from the endoplasmic reticulum (ER) to the Golgi. Akt is an important signaling kinase whose functioning is perturbed in diseases such as cancer and diabetes. We discovered that Akt phosphorylates Sec24, an essential coat protein II (COPII) component involved in mediating cargo selection for ER-to-Golgi trafficking. We discuss how this finding may provide new insights into the regulation of protein transport.

Molecular basis of LMAN1 in coordinating LMAN1-MCFD2 cargo receptor formation and ER-to-Golgi transport of FV/FVIII

The LMAN1-MCFD2 (lectin, mannose binding 1/multiple coagulation factor deficiency protein 2) cargo receptor complex transports coagulation factors V (FV) and VIII (FVIII) from the endoplasmic reticulum (ER) to the ER-Golgi intermediate compartment (ERGIC). LMAN1 (ERGIC-53) is a hexameric transmembrane protein with a carbohydrate recognition domain (CRD) on the ER luminal side. Here, we show that mutations in the first beta sheet of the CRD abolish MCFD2 binding without affecting the mannose binding, suggesting that LMAN1 interacts with MCFD2 through its N-terminal beta sheet, consistent with recently reported crystal structures of the CRD-MCFD2 complex. Mutations in the Ca(2+)- and sugar-binding sites of the CRD disrupt FV and FVIII interactions, without affecting MCFD2 binding. This interaction is independent of MCFD2, as LMAN1 mutants defective in MCFD2 binding can still interact with FVIII. Thus, the CRD of LMAN1 contains distinct, separable binding sites for both its partner protein (MCFD2) and the cargo proteins (FV/FVIII). Monomeric LMAN1 mutants are defective in ER exit and unable to interact with MCFD2, suggesting that the oligomerization of LMAN1 is necessary for its cargo receptor function. These results point to a central role of LMAN1 in regulating the binding in the ER and the subsequent release in the ERGIC of FV and FVIII.

The novel mouse mutant, chuzhoi, has disruption of Ptk7 protein and exhibits defects in neural tube, heart and lung development and abnormal planar cell polarity in the ear

Background

The planar cell polarity (PCP) signalling pathway is fundamental to a number of key developmental events, including initiation of neural tube closure. Disruption of the PCP pathway causes the severe neural tube defect of craniorachischisis, in which almost the entire brain and spinal cord fails to close. Identification of mouse mutants with craniorachischisis has proven a powerful way of identifying molecules that are components or regulators of the PCP pathway. In addition, identification of an allelic series of mutants, including hypomorphs and neomorphs in addition to complete nulls, can provide novel genetic tools to help elucidate the function of the PCP proteins.

Results

We report the identification of a new N-ethyl-N-nitrosourea (ENU)-induced mutant with craniorachischisis, which we have named chuzhoi (chz). We demonstrate that chuzhoi mutant embryos fail to undergo initiation of neural tube closure, and have characteristics consistent with defective convergent extension. These characteristics include a broadened midline and reduced rate of increase of their length-to-width ratio. In addition, we demonstrate disruption in the orientation of outer hair cells in the inner ear, and defects in heart and lung development in chuzhoi mutants. We demonstrate a genetic interaction between chuzhoi mutants and both Vangl2^Lp and Celsr1^Crsh mutants, strengthening the hypothesis that chuzhoi is involved in regulating the PCP pathway. We demonstrate that chuzhoi maps to Chromosome 17 and carries a splice site mutation in Ptk7. This mutation results in the insertion of three amino acids into the Ptk7 protein and causes disruption of Ptk7 protein expression in chuzhoi mutants.

Conclusions

The chuzhoi mutant provides an additional genetic resource to help investigate the developmental basis of several congenital abnormalities including neural tube, heart and lung defects and their relationship to disruption of PCP. The chuzhoi mutation differentially affects the expression levels of the two Ptk7 protein isoforms and, while some Ptk7 protein can still be detected at the membrane, chuzhoi mutants demonstrate a significant reduction in membrane localization of Ptk7 protein. This mutant provides a useful tool to allow future studies aimed at understanding the molecular function of Ptk7.

The Palade symposium: celebrating cell biology at its best

A symposium was held at the University of California, San Diego, to honor the contributions of Nobel Laureate, George Palade, to cell biology. The speakers included Günter Blobel, on the structure and function of nuclear pore complexes; Peter Walter, on the unfolded protein response in health and disease; Randy Schekman, on human disease-linked mutations in the COPII machinery; Scott Emr, on the regulation of plasma membrane composition by selective endocytosis; Roger Kornberg, on the structure and function of the transcription machinery; Peter Novick, on the regulation of rab GTPases along the secretory pathway; Jim Spudich, on the mechanism of the enigmatic myosin VI motor; and Joe Goldstein, on the function of the Niemann-Pick C (NPC)-linked gene products, NPC1 and NPC2, in cholesterol transport. Their work showcased the multidisciplinary nature, diversity, and vitality of cell biology. In the words of George Palade, their talks also illustrated "how cell biology could be used to understand disease and how disease could be used to discover normal cell biology." An integrated understanding of the cellular machinery will be essential in tackling the plethora of questions and challenges posed by completion of the human genome and for understanding the molecular mechanisms underlying human disease.

Molecular and cellular mechanisms underlying neural tube defects in the loop-tail mutant mouse

Loop-tail (Lp) mice show a very severe neural tube defect (craniorachischisis) caused by mutations in the Vangl2 gene (D255E, S464N). Mammalian Vangl1 and Vangl2 are membrane proteins that play critical roles in development such as establishment of planar cell polarity (PCP) in epithelial layers and convergent extension movements during neurogenesis and cardiogenesis. Vangl proteins are thought to assemble with other PCP proteins (Dvl, Pk) to form membrane-bound PCP signaling complexes that provide polarity information to the cell. In the present study, we show that Vangl1 is expressed exclusively at the plasma membrane of transfected MDCK cells, where it is targeted to the basolateral membrane. Experiments with an inserted exofacial HA epitope indicate that the segment delimited by the predicted transmembrane domains 1 and 2 is exposed to the extracellular milieu. Comparative studies of the Lp-associated pathogenic mutation D255E indicate that the targeting of the mutant variant at the plasma membrane is greatly reduced; the mutant variant is predominantly retained intracellularly in endoplasmic reticulum (ER) vesicles colocalizing with the ER marker calreticulin. In addition, the D255E variant shows drastically reduced stability with a half-life of approximately 2 h, compared to >9 h for its wild type counterpart and is rapidly degraded in a proteasome-dependent and MG132 sensitive pathway. These findings highlight a critical role for D255 for normal folding and processing of Vangl proteins, with highly conservative substitutions not tolerated at that site. Our study provide an experimental framework for the analysis of human VANGL mutations recently identified in familial and sporadic cases of spina bifida.

An emerging role for IQGAP1 in regulating protein traffic

IQGAP1, an effector of CDC42p GTPase, is a widely conserved, multifunctional protein that bundles F-actin through its N-terminus and binds microtubules through its C-terminus to modulate the cell architecture. It has emerged as a potential oncogene associated with diverse human cancers. Therefore, IQGAP1 has been heavily investigated; regardless, its precise cellular function remains unclear. Work from yeast suggests that IQGAP1 plays an important role in directed cell growth, which is a conserved feature crucial to morphogenesis, division axis, and body plan determination. New evidence suggests a conserved role for IQGAP1 in protein synthesis and membrane traffic, which may help to explain the diversity of its cellular functions. Membrane traffic mediates infections by intracellular pathogens and a range of degenerative human diseases arise from dysfunctions in intracellular traffic; thus, elucidating the mechanisms of cellular traffic will be important in order to understand the basis of a wide range of inherited and acquired human diseases. Recent evidence suggests that IQGAP1 plays its role in cell growth through regulating the conserved mTOR pathway. The mTOR signaling cascade has been implicated in membrane traffic and is activated in nearly all human cancers, but clinical response to the mTOR-specific inhibitor rapamycin has been disappointing. Thus, understanding the regulators of this pathway will be crucial in order to identify predictors of rapamycin sensitivity. In this review, I discuss emerging evidence that supports a potential role of IQGAP1 in regulating membrane traffic via regulating the mTOR pathway.

Regulation of coat assembly--sorting things out at the ER

The small GTPase Sar1 resides at the core of a regulatory cycle that controls protein export from the ER in COPII vesicles. Recent advances in minimally reconstituted systems indicate continual flux of Sar1 through GTPase cycles facilitates cargo concentration into forming vesicles that ultimately bud from membranes. During export from ER membranes, this GTPase cycle is harnessed through the combinatorial power of multiple coat subunits and cargo adaptors to sort an expanding array of proteins into ER-derived vesicles. The COPII budding machinery is further organized into higher-order structures at transitional zones on the ER surface where the large multi-domain Sec16 protein appears to perform a central function.

Planar cell polarity defects and defective Vangl2 trafficking in mutants for the COPII gene Sec24b

Among the cellular properties that are essential for the organization of tissues during animal development, the importance of cell polarity in the plane of epithelial sheets has become increasingly clear in the past decades. Planar cell polarity (PCP) signaling in vertebrates has indispensable roles in many aspects of their development, in particular, controlling alignment of various types of epithelial cells. Disrupted PCP has been linked to developmental defects in animals and to human pathology. Neural tube closure defects (NTD) and disorganization of the mechanosensory cells of the organ of Corti are commonly known consequences of disturbed PCP signaling in mammals. We report here a typical PCP phenotype in a mouse mutant for the Sec24b gene, including the severe NTD craniorachischisis, abnormal arrangement of outflow tract vessels and disturbed development of the cochlea. In addition, we observed genetic interaction between Sec24b and the known PCP gene, scribble. Sec24b is a component of the COPII coat protein complex that is part of the endoplasmic reticulum (ER)-derived transport vesicles. Sec24 isoforms are thought to be directly involved in cargo selection, and we present evidence that Sec24b deficiency specifically affects transport of the PCP core protein Vangl2, based on experiments in embryos and in cultured primary cells.

The trafficking protein Tmed2/p24beta(1) is required for morphogenesis of the mouse embryo and placenta

During vesicular transport between the endoplasmic reticulum and the Golgi, members of the TMED/p24 protein family form hetero-oligomeric complexes that facilitate protein-cargo recognition as well as vesicle budding. In addition, they regulate each other's level of expression. Despite analyses of TMED/p24 protein distribution in mammalian cells, yeast, and C. elegans, little is known about the role of this family in vertebrate embryogenesis. We report the presence of a single point mutation in Tmed2/p24beta(1) in a mutant mouse line, 99J, identified in an ENU mutagenesis screen for recessive developmental abnormalities. This mutation does not affect Tmed2/p24beta(1) mRNA levels but results in loss of TMED2/p24beta(1) protein. Prior to death at mid-gestation, 99J homozygous mutant embryos exhibit developmental delay, abnormal rostral-caudal elongation, randomized heart looping, and absence of the labyrinth layer of the placenta. We find that Tmed2/p24beta(1) is normally expressed in tissues showing morphological defects in 99J mutant embryos and that these affected tissues lack the TMED2/p24beta(1) oligomerization partners, TMED7/p24gamma(3) and TMED10/p24delta(1). Our data reveal a requirement for TMED2/p24beta(1) protein in the morphogenesis of the mouse embryo and placenta.