Anita G. Fernandez

COBRA, an Arabidopsis Extracellular Glycosyl-Phosphatidyl Inositol-Anchored Protein, Specifically Controls Highly Anisotropic Expansion through Its Involvement in Cellulose Microfibril Orientation

The orientation of cell expansion is a process at the heart of plant morphogenesis. Cellulose microfibrils are the primary anisotropic material in the cell wall and thus are likely to be the main determinant of the orientation of cell expansion. COBRA (COB) has been identified previously as a potential regulator of cellulose biogenesis. In this study, characterization of a null allele, cob-4, establishes the key role of COB in controlling anisotropic expansion in most developing organs. Quantitative polarized-light and field-emission scanning electron microscopy reveal that loss of anisotropic expansion in cob mutants is accompanied by disorganization of the orientation of cellulose microfibrils and subsequent reduction of crystalline cellulose. Analyses of the conditional cob-1 allele suggested that COB is primarily implicated in microfibril deposition during rapid elongation. Immunodetection analysis in elongating root cells revealed that, in agreement with its substitution by a glycosylphosphatidylinositol anchor, COB was polarly targeted to both the plasma membrane and the longitudinal cell walls and was distributed in a banding pattern perpendicular to the longitudinal axis via a microtubule-dependent mechanism. Our observations suggest that COB, through its involvement in cellulose microfibril orientation, is an essential factor in highly anisotropic expansion during plant morphogenesis.

MEL-28 Is Downstream of the Ran Cycle and Is Required for Nuclear-Envelope Function and Chromatin Maintenance

Early embryonic development depends on the faithful execution of basic cell biological processes whose coordination remains largely unknown. With a global network analysis, we found MEL-28 to be associated with two types of complexes, one implicated in nuclear-envelope function and the other in chromatin organization. Here, we show that MEL-28, a protein that shuttles between the nucleus and the kinetochore during the cell cycle, is required for the structural and functional integrity of the nuclear envelope. In addition, mel-28(RNAi) embryos exhibit defects in chromosome condensation, pronuclear migration, kinetochore assembly, and spindle assembly. This combination of mel-28(RNAi) phenotypes resemble those caused by depleting members of the Ran cycle in C. elegans, a conserved cellular signaling pathway that is required for mitotic spindle assembly, nuclear-envelope reformation after mitosis, and nucleocytoplasmic exchange (reviewed in). Although MEL-28 localization to the nuclear periphery is not dependent on nuclear pore components, it is dependent on RAN-1 and other key components of the Ran cycle. Thus, MEL-28 is downstream of the Ran cycle and is required for both proper nuclear-envelope function and chromatin maintenance.

Automated sorting of live C. elegans using laFACS

To the Editor: A recent paper in Nature Methods describes the use of fluorescence-activated cell sorting (FACS) is to sort Caenorhabditis elegans embryos1. Here we report FACS-based method to sort live C. elegans larvae, which permits us to rapidly collect large quantities of live genotyped worms from a mixed population. Using GFP-marked balancer chromosomes (Fig. 1a), and live-animal FACS laFACS, we routinely collected >100,000 genotyped animals in less than one hour. To test laFACS, we combined it with large-scale RNA interference (RNAi) screening and identified genetic interactors of mel-28, an important regulator of nuclear envelope and chromatin functions2–5. Figure 1 laFACS of GFP-negative L1 larvae. (a) Genetic scheme. A mel-28 mutant allele is kept over a balancer chromosome containing a GFP marker and a recessive lethal allele. GFP-negative progeny (F1) are mel-28 homozygotes and grow up to produce only dead embryos. ... Although a FACS machine is designed to sort single cells, a few modifications enabled sorting of ~0.25 mm long L1 C. elegans. First, we used a reduced drop-drive frequency of ~16.4 kHz to keep the larvae undamaged. We also used a 100-µm nozzle and set a gate to capture events with a high forward-scatter signal (Fig. 1b), confining our collections to larger objects (Supplementary Methods). After worm sorting, the same FACS machine could be used for several other applications, including sorting of yeast, Drosophila melanogaster cells, mammalian cells, and plant protoplasts. The worm sort had no impact on these applications, and the worm-specific FACS modifications were easily reversed to accommodate single-cell applications. We used laFACS to collect mel-28 (maternal-effect-lethal-28) homozygous worms from a mixed population. mel-28 homozygous hermaphrodites derived from heterozygous mothers appear phenotypically indistinguishable from wild-type animals until they mature and produce only inviable progeny. This selection is usually performed manually and thus is not amenable to large-scale applications. We first generated a GFP-marked strain in which the mel-28(t1684) mutation is balanced over a chromosome bearing lag-2::GFP6 (Fig. 1a). We sorted L1 worms, collecting GFP-negative mel-28(t1684) homozygotes. We selected GFP-negative larvae on the basis of the ratio of green (GFP fluorescence; 530/30 nm) to red (red-spectrum autofluorescence; 610/20 nm) signal (Fig. 1c,d). We separated GFP-positive and GFP-negative worms (Supplementary Figure 1). Typically, from a population of 600,000 larvae we retrieved ~130,000 healthy animals after one sort. This first-pass population was ~95-98% homozygous (n > 1,000, verified by microscopy). A second sort recovered an essentially 100% pure population of about 100,000 homozygous larvae (n > 20,000, verified by microscopy and by genetic analysis). This scheme can be easily adapted for the majority of C. elegans genes using available balancers. We used the collected homozygous mel-28 animals to perform an RNAi-based synthetic interaction screen using clones representing chromosome I genes7. The ability to easily collect large amounts of mel-28 homozygous animals allowed us to perform the RNAi repeatedly (up to 16 times). From over 2,000 genes tested, 12 showed synthetic phenotypes with mel-28: npp-2, npp-4, npp-12, npp-14, npp-17, his-67, his-68, exos-3, pas-5, phi-56, rpa-0, and rpl-30 (Supplementary Fig. 2 and Supplementary Table 1). These results agreed well with mel-28’s role in coordinating chromatin and nuclear envelope functions2,3. Obtaining large quantities of pure mutant populations could also be useful for chemical screens, microarrays, or biochemical assays, expanding the arsenal of high-throughput tools available in C. elegans.

Identification of Conserved MEL-28/ELYS Domains with Essential Roles in Nuclear Assembly and Chromosome Segregation

Nucleoporins are the constituents of nuclear pore complexes (NPCs) and are essential regulators of nucleocytoplasmic transport, gene expression and genome stability. The nucleoporin MEL-28/ELYS plays a critical role in post-mitotic NPC reassembly through recruitment of the NUP107-160 subcomplex, and is required for correct segregation of mitotic chromosomes. Here we present a systematic functional and structural analysis of MEL-28 in C. elegans early development and human ELYS in cultured cells. We have identified functional domains responsible for nuclear envelope and kinetochore localization, chromatin binding, mitotic spindle matrix association and chromosome segregation. Surprisingly, we found that perturbations to MEL-28’s conserved AT-hook domain do not affect MEL-28 localization although they disrupt MEL-28 function and delay cell cycle progression in a DNA damage checkpoint-dependent manner. Our analyses also uncover a novel meiotic role of MEL-28. Together, these results show that MEL-28 has conserved structural domains that are essential for its fundamental roles in NPC assembly and chromosome segregation.

High-throughput fluorescence-based isolation of live C. elegans larvae

For the nematode Caenorhabditis elegans, automated selection of animals of specific genotypes from a mixed pool has become essential for genetic interaction or chemical screens. To date, such selection has been accomplished using specialized instruments. However, access to such dedicated equipment is not common. Here we describe live animal fluorescence-activated cell sorting (laFACS), a protocol for automatic selection of live first larval stage (L1) animals using a standard FACS system. We show that FACS can be used for the precise identification of GFP-expressing and non-GFP-expressing subpopulations and can accomplish high-speed sorting of live animals. We have routinely collected 100,000 or more homozygotes from a mixed starting population within 2 h, and with greater than 99% purity. The sorted animals continue to develop normally, making this protocol ideally suited for the isolation of terminal mutants for use in genetic interaction or chemical genetic screens.

Uncovering Buffered Pleiotropy: A Genome-Scale Screen for mel- 28 Genetic Interactors in Caenorhabditis elegans

mel-28 (maternal-effect-lethal-28) encodes a conserved protein required for nuclear envelope function and chromosome segregation in Caenorhabditis elegans. Because mel-28 is a strict maternal-effect lethal gene, its function is required in the early embryo but appears to be dispensable for larval development. We wanted to test the idea that mel-28 has postembryonic roles that are buffered by the contributions of other genes. To find genes that act coordinately with mel-28, we did an RNA interference−based genetic interaction screen using mel-28 and wild-type larvae. We screened 18,364 clones and identified 65 genes that cause sterility in mel-28 but not wild-type worms. Some of these genes encode components of the nuclear pore. In addition we identified genes involved in dynein and dynactin function, vesicle transport, and cell-matrix attachments. By screening mel-28 larvae we have bypassed the requirement for mel-28 in the embryo, uncovering pleiotropic functions for mel-28 later in development that are normally provided by other genes. This work contributes toward revealing the gene networks that underlie cellular processes and reveals roles for a maternal-effect lethal gene later in development.

A novel small molecule that disrupts a key event during the oocyte-to-embryo transition in C. elegans

The complex cellular events that occur in response to fertilization are essential for mediating the oocyte-to-embryo transition. Here, we describe a comprehensive small-molecule screen focused on identifying compounds that affect early embryonic events in Caenorhabditis elegans. We identify a single novel compound that disrupts early embryogenesis with remarkable stage and species specificity. The compound, named C22, primarily impairs eggshell integrity, leading to osmotic sensitivity and embryonic lethality. The C22-induced phenotype is dependent upon the upregulation of the LET-607/CREBH transcription factor and its candidate target genes, which primarily encode factors involved in diverse aspects of protein trafficking. Together, our data suggest that in the presence of C22, one or more key components of the eggshell are inappropriately processed, leading to permeable, inviable embryos. The remarkable specificity and reversibility of this compound will facilitate further investigation into the role and regulation of protein trafficking in the early embryo, as well as serve as a tool for manipulating the life cycle for other studies such as those involving aging. Summary: The small molecule C22 induces expression of the LET-607 transcription factor, leading to mis-regulation of protein trafficking and thus impairing eggshell formation and the oocyte-to-embryo transition.