Light-dependent N-end rule-mediated disruption of protein function in Saccharomyces cerevisiae and Drosophila melanogaster.

Here we describe the development and characterization of the photo-N-degron, a peptide tag that can be used in optogenetic studies of protein function in vivo. The photo-N-degron can be expressed as a genetic fusion to the amino termini of other proteins, where it undergoes a blue light-dependent conformational change that exposes a signal for the class of ubiquitin ligases, the N-recognins, which mediate the N-end rule mechanism of proteasomal degradation. We demonstrate that the photo-N-degron can be used to direct light-mediated degradation of proteins in Saccharomyces cerevisiae and Drosophila melanogaster with fine temporal control. In addition, we compare the effectiveness of the photo-N-degron with that of two other light-dependent degrons that have been developed in their abilities to mediate the loss of function of Cactus, a component of the dorsal-ventral patterning system in the Drosophila embryo. We find that like the photo-N-degron, the blue light-inducible degradation (B-LID) domain, a light-activated degron that must be placed at the carboxy terminus of targeted proteins, is also effective in eliciting light-dependent loss of Cactus function, as determined by embryonic dorsal-ventral patterning phenotypes. In contrast, another previously described photosensitive degron (psd), which also must be located at the carboxy terminus of associated proteins, has little effect on Cactus-dependent phenotypes in response to illumination of developing embryos. These and other observations indicate that care must be taken in the selection and application of light-dependent and other inducible degrons for use in studies of protein function in vivo, but importantly demonstrate that N- and C-terminal fusions to the photo-N-degron and the B-LID domain, respectively, support light-dependent degradation in vivo.

Reconstitution of Torso signaling in cultured cells suggests a role for both Trunk and Torso-like in receptor activation.

Formation of the Drosophila embryonic termini is controlled by the localized activation of the receptor tyrosine kinase Torso. Both Torso and Torso's presumed ligand, Trunk, are expressed uniformly in the early embryo. Polar activation of Torso requires Torso-like, which is expressed by follicle cells adjacent to the ends of the developing oocyte. We find that Torso expressed at high levels in cultured Drosophila cells is activated by individual application of Trunk, Torso-like or another known Torso ligand, Prothoracicotropic Hormone. In addition to assays of downstream signaling activity, Torso dimerization was detected using bimolecular fluorescence complementation. Trunk and Torso-like were active when co-transfected with Torso and when presented to Torso-expressing cells in conditioned medium. Trunk and Torso-like were also taken up from conditioned medium specifically by cells expressing Torso. At low levels of Torso, similar to those present in the embryo, Trunk and Torso-like alone were ineffective but acted synergistically to stimulate Torso signaling. Our results suggest that Torso interacts with both Trunk and Torso-like, which cooperate to mediate dimerization and activation of Torso at the ends of the Drosophila embryo.

The microRNA pathway regulates the temporal pattern of Notch signaling in Drosophila follicle cells.

Multicellular development requires the correct spatial and temporal regulation of cell division and differentiation. These processes are frequently coordinated by the activities of various signaling pathways such as Notch signaling. From a screen for modifiers of Notch signaling in Drosophila we have identified the RNA helicase Belle, a recently described component of the RNA interference pathway, as an important regulator of the timing of Notch activity in follicle cells. We found that loss of Belle delays activation of Notch signaling, which results in delayed follicle cell differentiation and defects in the cell cycle. Because mutations in well-characterized microRNA components phenocopied the Notch defects observed in belle mutants, Belle might be functioning in the microRNA pathway in follicle cells. The effect of loss of microRNAs on Notch signaling occurs upstream of Notch cleavage, as expression of the constitutively active intracellular domain of Notch in microRNA-defective cells restored proper activation of Notch. Furthermore, we present evidence that the Notch ligand Delta is an important target of microRNA regulation in follicle cells and regulates the timing of Notch activation through cis inhibition of Notch. Here we have uncovered a complex regulatory process in which the microRNA pathway promotes Notch activation by repressing Delta-mediated inhibition of Notch in follicle cells.

Distinct functional specificities are associated with protein isoforms encoded by the Drosophila dorsal-ventral patterning gene pipe.

Spatially regulated transcription of the pipe gene in ventral cells of the Drosophila ovary follicle cell epithelium is a key event that specifies progeny embryo dorsal-ventral (DV) polarity. pipe encodes ten putative protein isoforms, all of which exhibit similarity to vertebrate glycosaminoglycan-modifying enzymes. Expression of one of the isoforms, Pipe-ST2, in follicle cells has previously been shown to be essential for DV patterning. pipe is also expressed in the embryonic salivary gland and its expression there is required for normal viability. Here, we show that in addition to Pipe-ST2, seven of the other Pipe isoforms are expressed in the ovary, whereas all Pipe isoforms are abundantly expressed in the embryo. Of the ten isoforms, only Pipe-ST2 can restore ventral and lateral pattern elements to the progeny of otherwise pipe-null mutant females. By contrast, three Pipe isoforms, but not Pipe-ST2, support the production of a novel pipe-dependent epitope present in the embryonic salivary gland. These data indicate that differences in functional specificity, and presumably enzymatic specificity, are associated with several of the Pipe isoforms. In addition, we show that uniform expression of the Pipe-ST2 isoform in the follicle cell layer of females otherwise lacking pipe expression leads to the formation of embryos with a DV axis that is appropriately oriented with respect to the intrinsic polarity of the eggshell. This suggests the existence of a second mechanism that polarizes the Drosophila embryo, in addition to the ventrally restricted transcription of the pipe gene.

Learning our L.I.M.I.T.S.: less is more in teaching science.

The rapid and accelerating pace of change in physiology and cell biology, along with the easy access to huge amounts of content, have altered the playing field for science students, yet most students are still mainly taught from textbooks. Of necessity, textbooks are usually broad in scope, cover topics much more superficially than do journal articles, and present the scientific process as a linear string of successful experiments, largely ignoring the reality of rejected hypotheses, unanticipated discoveries, or surprising findings that may shift paradigms. We suggest that a more narrow focus on scientific thinking, using a new method for reading a series of journal articles that track the evolution of a single project over a period of years, can more realistically convey the excitement and challenges of research science and perhaps stimulate some students to consider research careers for themselves. Our approach, termed "CREATE" (for Consider, Read, Elucidate hypotheses, Analyze data, and Think of the next Experiment), has proven successful at both demystifying the scientific literature and humanizing science/scientists in undergraduate biology courses (8), and we suggest that it could be profitably expanded to physiology courses.

Isolation of secreted proteins from Drosophila ovaries and embryos through in vivo BirA-mediated biotinylation.

The extraordinarily strong non-covalent interaction between biotin and avidin (kD = 10-14-10-16) has permitted this interaction to be used in a wide variety of experimental contexts. The Biotin Acceptor Peptide (BAP), a 15 amino acid motif that can be biotinylated by the E. coli BirA protein, has been fused to proteins-of-interest, making them substrates for in vivo biotinylation. Here we report on the construction and characterization of a modified BirA bearing signals for secretion and endoplasmic reticulum (ER) retention, for use in experimental contexts requiring biotinylation of secreted proteins. When expressed in the Drosophila female germline or ovarian follicle cells under Gal4-mediated transcriptional control, the modified BirA protein could be detected and shown to be enzymatically active in ovaries and progeny embryos. Surprisingly, however, it was not efficiently retained in the ER, and instead appeared to be secreted. To determine whether this secreted protein, now designated secBirA, could biotinylate secreted proteins, we generated BAP-tagged versions of two secreted Drosophila proteins, Torsolike (Tsl) and Gastrulation Defective (GD), which are normally expressed maternally and participate in embryonic pattern formation. Both Tsl-BAP and GD-BAP were shown to exhibit normal patterning activity. Co-expression of Tsl-BAP together with secBirA in ovarian follicle cells resulted in its biotinylation, which permitted its isolation from both ovaries and progeny embryos using Avidin-coupled affinity matrix. In contrast, co-expression with secBirA in the female germline did not result in detectable biotinylation of GD-BAP, possibly because the C-terminal location of the BAP tag made it inaccessible to BirA in vivo. Our results indicate that secBirA directs biotinylation of proteins bound for secretion in vivo, providing access to powerful experimental approaches for secreted proteins-of-interest. However, efficient biotinylation of target proteins may vary depending upon the location of the BAP tag or other structural features of the protein.

Selective use of the primary literature transforms the classroom into a virtual laboratory.

CREATE (consider, read, elucidate hypotheses, analyze and interpret the data, and think of the next experiment) is a new method for teaching science and the nature of science through primary literature. CREATE uses a unique combination of novel pedagogical tools to guide undergraduates through analysis of journal articles, highlighting the evolution of scientific ideas by focusing on a module of four articles from the same laboratory. Students become fluent in the universal language of data analysis as they decipher the figures, interpret the findings, and propose and defend further experiments to test their own hypotheses about the system under study. At the end of the course students gain insight into the individual experiences of article authors by reading authors' responses to an e-mail questionnaire generated by CREATE students. Assessment data indicate that CREATE students gain in ability to read and critically analyze scientific data, as well as in their understanding of, and interest in, research and researchers. The CREATE approach demystifies the process of reading a scientific article and at the same time humanizes scientists. The positive response of students to this method suggests that it could make a significant contribution to retaining undergraduates as science majors.

The Drosophila embryonic patterning determinant torsolike is a component of the eggshell.

The development of the head and tail regions of the Drosophila embryo is dependent upon the localized polar activation of Torso (Tor), a receptor tyrosine kinase that is uniformly distributed in the membrane of the developing embryo. Trunk (Trk), the proposed ligand for Tor, is secreted as an inactive precursor into the perivitelline fluid that lies between the embryonic membrane and the vitelline membrane (VM), the inner layer of the eggshell. The spatial regulation of Trk processing is thought to be mediated by the secreted product of the torsolike (tsl) gene, which is expressed during oogenesis by a specialized population of follicle cells present at the two ends of the oocyte. We show here that Tsl protein is specifically localized to the polar regions of the VM in laid eggs. We further demonstrate that although Tsl can associate with nonpolar regions of the VM, the activity of polar-localized Tsl is enhanced, suggesting the existence of another spatially restricted factor acting in this pathway. The incorporation of Tsl into the VM provides a mechanism for the transfer of spatial information from the follicle cells to the developing embryo. To our knowledge, Tsl represents the first example of an embryonic patterning determinant that is a component of the eggshell.

The CREATE Strategy for Intensive Analysis of Primary Literature Can Be Used Effectively by Newly Trained Faculty to Produce Multiple Gains in Diverse Students.

The CREATE (Consider Read, Elucidate the hypotheses, Analyze and interpret the data, and Think of the next Experiment) strategy aims to demystify scientific research and scientists while building critical thinking, reading/analytical skills, and improved science attitudes through intensive analysis of primary literature. CREATE was developed and piloted at the City College of New York (CCNY), a 4-yr, minority-serving institution, with both upper-level biology majors and first-year students interested in science, technology, engineering, and mathematics. To test the extent to which CREATE strategies are broadly applicable to students at private, public, research-intensive, and/or primarily undergraduate colleges/universities, we trained a cohort of faculty from the New York/New Jersey/Pennsylvania area in CREATE pedagogies, then followed a subset, the CREATE implementers (CIs), as they taught all or part of an existing course on their home campuses using CREATE approaches. Evaluation of the workshops, the CIs, and their students was carried out both by the principal investigators and by an outside evaluator working independently. Our data indicate that: intensive workshops change aspects of faculty attitudes about teaching/learning; workshop-trained faculty can effectively design and teach CREATE courses; and students taught by such faculty on multiple campuses make significant cognitive and affective gains that parallel the changes documented previously at CCNY.

Maternal control of the Drosophila dorsal-ventral body axis.

UNLABELLED: The pathway that generates the dorsal-ventral (DV) axis of the Drosophila embryo has been the subject of intense investigation over the previous three decades. The initial asymmetric signal originates during oogenesis by the movement of the oocyte nucleus to an anterior corner of the oocyte, which establishes DV polarity within the follicle through signaling between Gurken, the Drosophila Transforming Growth Factor (TGF)-α homologue secreted from the oocyte, and the Drosophila Epidermal Growth Factor Receptor (EGFR) that is expressed by the follicular epithelium cells that envelop the oocyte. Follicle cells that are not exposed to Gurken follow a ventral fate and express Pipe, a sulfotransferase that enzymatically modifies components of the inner vitelline membrane layer of the eggshell, thereby transferring DV spatial information from the follicle to the egg. These ventrally sulfated eggshell proteins comprise a localized cue that directs the ventrally restricted formation of the active Spätzle ligand within the perivitelline space between the eggshell and the embryonic membrane. Spätzle activates Toll, a transmembrane receptor in the embryonic membrane. Transmission of the Toll signal into the embryo leads to the formation of a ventral-to-dorsal gradient of the transcription factor Dorsal within the nuclei of the syncytial blastoderm stage embryo. Dorsal controls the spatially specific expression of a large constellation of zygotic target genes, the Dorsal gene regulatory network, along the embryonic DV circumference. This article reviews classic studies and integrates them with the details of more recent work that has advanced our understanding of the complex pathway that establishes Drosophila embryo DV polarity. For further resources related to this article, please visit the WIREs website. CONFLICT OF INTEREST: The authors have declared no conflicts of interest for this article.

Localized serine protease activity and the establishment of Drosophila embryonic dorsoventral polarity.

Drosophila embryo dorsoventral polarity is established by a maternally encoded signal transduction pathway in which three sequentially acting serine proteases, Gastrulation Defective, Snake and Easter, generate the ligand that activates the Toll receptor on the ventral side of the embryo. The spatial regulation of this pathway depends upon ventrally restricted expression of the Pipe sulfotransferase in the ovarian follicle during egg formation. Several recent observations have advanced our understanding of the mechanism regulating the spatially restricted activation of Toll. First, several protein components of the vitelline membrane layer of the eggshell have been determined to be targets of Pipe-mediated sulfation. Second, the processing of Easter by Snake has been identified as the first Pipe-dependent, ventrally-restricted processing event in the pathway. Finally, Gastrulation Defective has been shown to undergo Pipe-dependent, ventral localization within the perivitelline space and to facilitate Snake-mediated processing of Easter. Together, these observations suggest that Gastrulation Defective, localized on the interior ventral surface of the eggshell in association with Pipe-sulfated eggshell proteins, recruits and mediates an interaction between Snake and Easter. This event leads to ventrally-restricted processing and activation of Easter and consequently, localized formation of the Toll ligand, and Toll activation.

A ventrally localized protease in the Drosophila egg controls embryo dorsoventral polarity.

Drosophila embryo dorsoventral (DV) polarity is defined by serine protease activity in the perivitelline space (PVS) between the embryonic membrane and the inner layer of the eggshell. Gastrulation Defective (GD) cleaves and activates Snake (Snk). Activated Snk cleaves and activates Easter (Ea), exclusively on the ventral side of the embryo. Activated Ea then processes Spätzle (Spz) into the activating ligand for Toll, a transmembrane receptor that is distributed throughout the embryonic plasma membrane. Ventral activation of Toll depends upon the activity of the Pipe sulfotransferase in the ventral region of the follicular epithelium that surrounds the developing oocyte. Pipe transfers sulfate residues to several protein components of the inner vitelline membrane layer of the eggshell. Here we show that GD protein becomes localized in the ventral PVS in a Pipe-dependent process. Moreover, ventrally concentrated GD acts to promote the cleavage of Ea by Snk through an extracatalytic mechanism that is distinct from GD's proteolytic activation of Snk. Together, these observations illuminate the mechanism through which spatially restricted sulfotransferase activity in the developing egg chamber leads to localization of serine protease activity and ultimately to spatially specific activation of the Toll receptor in the Drosophila embryo.

The C.R.E.A.T.E. approach to primary literature shifts undergraduates' self-assessed ability to read and analyze journal articles, attitudes about science, and epistemological beliefs.

The C.R.E.A.T.E. (Consider, Read, Elucidate hypotheses, Analyze and interpret data, Think of the next Experiment) method uses intensive analysis of primary literature in the undergraduate classroom to demystify and humanize science. We have reported previously that the method improves students' critical thinking and content integration abilities, while at the same time enhancing their self-reported understanding of "who does science, and why." We report here the results of an assessment that addressed C.R.E.A.T.E. students' attitudes about the nature of science, beliefs about learning, and confidence in their ability to read, analyze, and explain research articles. Using a Likert-style survey administered pre- and postcourse, we found significant changes in students' confidence in their ability to read and analyze primary literature, self-assessed understanding of the nature of science, and epistemological beliefs (e.g., their sense of whether knowledge is certain and scientific talent innate). Thus, within a single semester, the inexpensive C.R.E.A.T.E. method can shift not just students' analytical abilities and understanding of scientists as people, but can also positively affect students' confidence with analysis of primary literature, their insight into the processes of science, and their beliefs about learning.

Pipe-dependent ventral processing of Easter by Snake is the defining step in Drosophila embryo DV axis formation.

The establishment of Drosophila embryonic dorsal-ventral (DV) polarity relies on serine proteolytic activity in the perivitelline space between the embryonic membrane and the eggshell. Gastrulation Defective cleaves and activates Snake, which processes and activates Easter, which cleaves Spätzle to form the activating ligand for the Toll receptor. Ventral restriction of ligand formation depends on the Pipe sulfotransferase, which is expressed in ventral cells of the follicular epithelium surrounding the developing oocyte. Pipe modifies components of the developing eggshell to produce a ventral cue embedded in the vitelline membrane. This ventral cue is believed to promote one or more of the proteolysis steps in the perivitelline space. By examining the processing of transgenic, tagged versions of the perivitelline proteins during DV patterning, we find that the proteolysis of Easter by Snake is the first Pipe-dependent step and therefore the key ventrally restricted event in the protease cascade. We also find that Snake and Easter associate together in a complex in both wild-type and pipe mutant-derived embryos. This observation suggests a mechanism in which the sulfated target of Pipe promotes a productive interaction between Snake and Easter, perhaps by facilitating conformational changes in a complex containing the two proteins.

Sulfation of eggshell components by Pipe defines dorsal-ventral polarity in the Drosophila embryo.

Drosophila embryonic dorsal-ventral (DV) polarity is controlled by a group of sequentially acting serine proteases located in the fluid-filled perivitelline space between the embryonic membrane and the eggshell, which generate the ligand for the Toll receptor on the ventral side of the embryo. Spatial control of the protease cascade relies on the Pipe sulfotransferase, a fly homolog of vertebrate glycosaminoglycan-modifying enzymes, which is expressed in ventral cells of the follicular epithelium surrounding the developing oocyte. Here we show that the vitelline membrane-like (VML) protein undergoes Pipe-dependent sulfation and, consistent with a role in conveying positional information from the egg chamber to the embryo, becomes incorporated into the eggshell at a position corresponding to the location of the follicle cells from which it was secreted. Although VML influences embryonic DV pattern in a sensitized genetic background, VML is not essential for DV axis formation, suggesting that there is redundancy in the composition of the Pipe enzymatic target. Correspondingly, we find that additional structural components of the vitelline membrane undergo Pipe-dependent sulfation. In identifying the elusive targets of Pipe, this work points to the vitelline membrane as the source of signals that generate the Drosophila DV axis.

No requirement for localized Nudel protein expression in Drosophila embryonic axis determination.

Drosophila embryonic dorsal-ventral polarity is defined by a maternally encoded signal transduction pathway. Gastrulation Defective, Snake, and Easter comprise a serine protease cascade that operates in the perivitelline space to generate active ligand for the Toll receptor, which resides in the embryonic membrane. Toll is activated only on the ventral side of the embryo. Spatial regulation of this pathway is initiated by the ventrally restricted expression of the sulfotransferase Pipe in the follicular epithelium that surrounds the developing oocyte. Pipe is thought to modify a target molecule that is secreted and localized within the ventral region of the egg and future embryo, where it influences the activity of the pathway such that active Toll ligand is produced only ventrally. A potential substrate for Pipe is encoded by nudel, which is expressed throughout the follicle cell layer and encodes a large, multi-functional secreted protein that contains a serine protease domain as well as other structural features characteristic of extracellular matrix proteins. A previous mosaic analysis suggested that the protease domain of Nudel is not a target for Pipe activity as its expression is not required in pipe-expressing cells, but failed to rule out such a role for other functional domains of the protein. To investigate this possibility, we carried out a mosaic analysis of additional nudel alleles, including some that affect the entire protein. Our analysis demonstrated that proteolytically processed segments of Nudel are secreted into the perivitelline space and stably localized, as would be expected for the target of Pipe, However, we found no requirement for nudel to be expressed in ventral, pipe-expressing follicle cells, thereby eliminating Nudel as an essential substrate of Pipe sulfotransferase activity.

Synthesis of the sulfate donor PAPS in either the Drosophila germline or somatic follicle cells can support embryonic dorsal-ventral axis formation.

The establishment of dorsal-ventral (DV) polarity in the Drosophila embryo depends upon a localized signal that is generated in the perivitelline space of the egg through the action of a serine proteolytic cascade. Spatial regulation of this pathway is determined by the expression of the pipe gene in a subpopulation of ventral follicle cells in the developing egg chamber. The Pipe protein exhibits homology to vertebrate glycosaminoglycan sulfotransferases. In a previous study, we demonstrated that embryonic DV polarity depends upon the sulfotransferase activity of Pipe. Surprisingly, however, our results also indicated that formation of the embryonic DV axis does not require the synthesis of the high-energy sulfate donor, 3'-phosphoadenosine 5'-phosphosulfate (PAPS) in the follicle cells in which Pipe is presumed to function. Here, we resolve this apparent paradox by demonstrating that dorsalized embryos are only produced by egg chambers in which both germline and follicle cells lack PAPS synthetase activity. Thus, PAPS produced either in the germline or in the follicular epithelium can support the requirement for Pipe sulfotransferase activity in embryonic DV patterning. This finding indicates the existence of a conduit for the movement of PAPS between the germline and the follicle cells, which highlights a previously unappreciated mechanism of soma/germline cooperation affecting pattern formation.

Drosophila dMyc is required for ovary cell growth and endoreplication.

Although the Myc oncogene has long been known to play a role in many human cancers, the mechanisms that mediate its effects in both normal cells and cancer cells are not fully understood. We have initiated a genetic analysis of the Drosophila homolog of the Myc oncoprotein (dMyc), which is encoded by the dm locus. We carried out mosaic analysis to elucidate the functions of dMyc in the germline and somatic cells of the ovary during oogenesis, a process that involves cell proliferation, differentiation and growth. Germline and somatic follicle cells mutant for dm exhibit a profound decrease in their ability to grow and to carry out endoreplication, a modified cell cycle in which DNA replication occurs in the absence of cell division. In contrast to its dramatic effects on growth and endoreplication, dMyc is dispensable for the mitotic division cycles of both germline and somatic components of the ovary. Surprisingly, despite their impaired ability to endoreplicate, dm mutant follicle cells appeared to carry out chorion gene amplification normally. Furthermore, in germline cysts in which the dm mutant cells comprised only a subset of the 16-cell cluster, we observed strictly cell-autonomous growth defects. However, in cases in which the entire germline cyst or the whole follicular epithelium was mutant for dm, the growth of the entire follicle, including the wild-type cells, was delayed. This observation indicates the existence of a signaling mechanism that acts to coordinate the growth rates of the germline and somatic components of the follicle. In summary, dMyc plays an essential role in promoting the rapid growth that must occur in both the germline and the surrounding follicle cells for oogenesis to proceed.