1,721,049 research outputs found
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
The role and regulation of Asterless in the centrosome cycle
Get PDFCentrosomes are the main microtubule organizing centres in animal cells and are formed by a pair of centrioles together with surrounding pericentriolar material (PCM). Cycling cells duplicate their centrosomes strictly once per cell cycle. This process is driven by the semi-conservative duplication of the centrioles that are found at the centrosome core. During the exit from mitosis the two centrioles within the single inherited centrosome separate, and upon the start of S-phase each of these inherited mother centrioles assembles an adjacent daughter at its side. This process results in two complete centrosomes that can form the poles of the mitotic spindle, and thus segregate evenly to the next cell generation. The formation of a daughter centriole suppresses the initiation of new duplication events from the same templating mother centriole until this daughter separates - disengages - at the end of the cell cycle. This regulation - that acts to repress centriole amplification - is summarized in the 'licensing model of centriole duplication' (Tsou and Stearns, 2006). This model states that centriole disengagement provides the license for the re-duplication of mother centrioles. Importantly, experiments show that while abolishing centriole engagement is sufficient to allow mother centrioles to re-duplicate within the same cycle, it is insufficient to allow daughter centrioles the assembly of a granddaughter before they mature into mothers towards the end of their first cell cycle. The molecular nature of this daughter-to-mother transition remains mysterious. In this thesis I show that in Drosophila embryos the essential centriole duplication protein Asl is not incorporated into daughter centrioles as they assemble during S-phase, but is only incorporated once mother and daughter separate at the end of mitosis. The initial incorporation of Asterless (Asl) is irreversible, and is dependent on centriolar DSas-4. Crucially, Asl incorporation is essential for daughter centrioles to mature into mothers that can support centriole duplication. I propose that Asl acts as a permanent primary license that allows new centrioles to duplicate for the first time. Once acquired, this primary license is not lost but rather further regulation is taken over by the reduplication licensing mechanism, disengagement. This work extends the previously proposed licensing model to also explain how new centrioles are licensed for their first duplication event
Dissecting Polo kinase recruitment to the centrosome
Get PDFPolo kinase is a major regulator of the cell cycle. Polo is present at many different subcellular structures, including centrosomes. Centrosomes are formed when mother centrioles recruit pericentriolar material (PCM) around themselves. The PCM expands dramatically in preparation for mitosis, in a process that relies on Polo phosphorylation of the PCM protein Cnn. Polo is also known to regulate centriole disengagement, and Polo binding to the centriolar protein Sas-4 is crucial for Asl recruitment to daughter centrioles during their conversion into fully functional mothers.
The primary aim of my thesis was to fully characterise how Polo is recruited to centrosomes to perform its different functions. As a model, I use syncytial fly embryos. In addition to the previously known Polo binding site in Sas-4, I found that the PCM protein Spd-2 directly interacts with the PBD domain of Polo. While Spd-2 is not required for Polo recruitment to the centriole, it is crucial to recruit Polo to the PCM. I propose that this interaction is part of a Spd-2/Polo/Cnn positive feedback loop that drives mitotic PCM assembly. Furthermore, I found that a third protein (Ana1) is required for overall Polo recruitment to the centrosome. Ana1 has been previously linked to Asl recruitment and/or maintenance and centriole conversion. Losing Ana1-mediated Polo recruitment did not affect these processes, but it did severely hinder mitotic PCM expansion. I propose that, independently of its Asl-related role, Ana1 acts as a crucial reservoir of centriolar Polo. Thus, centriolar Polo recruited via Ana1 can phosphorylate Spd-2 to allow it to bind the PBD, in an example of Polo “self-priming”
Studying centrosome formation and the consequences of centrosome loss in Drosophila melanogaster
Get PDFCentrioles are conserved microtubule-based structures that are required for the formation of two important cellular organelles, centrosomes and cilia. Centrosomes form the poles of the mitotic spindle and consist of a pair of centrioles surrounded by a matrix of pericentriolar material (PCM) that has the ability to nucleate and organise microtubules. Centrosome defects are implicated into a variety of human diseases including cancer, microcephaly, and ciliopathies. Therefore it is of great interest to understand the mechanisms that lead to centrosome formation and the consequences that centrosome defects have in cells. I have analysed the roles of several centrosomal proteins in centrosome assembly in Drosophila. My results indicate that Sak/PLK4 is only required for the initial step of centriole duplication, but has no further role in recruitment of PCM. I show that two proteins important for PCM recruitment, Asterless (Asl) and Spd-2, are preferentially phosphorylated when they are integrated into the centrosome and I identified these phosphorylation sites using a phosphoproteomic screen. A phosphorylation site in Asl is specifically phosphorylated in mitosis, and the phosphorylation state of Spd-2 regulates its maintenance at the centrosome, suggesting that phosphorylation of PCM proteins is an important mechanism to ensure PCM assembly specifically at the centrosome and in mitosis. I have performed a global transcriptional analysis of flies lacking centrosomes or having extra centrosomes to investigate the effects of centrosomal defects on a cellular level. Surprisingly, my results indicate that centrosome defects per se do not dramatically alter cellular physiology. Finally, I demonstrate that in the absence of centrioles acentrosomal microtubule-organising centres (aMTOCs) are formed in an Asl- and Cnn-dependent fashion, and I show that these aMTOCs can contribute to spindle focusing in acentrosomal cells
The dynamics of centrosome assembly in the early Drosophila embryo
Get PDFCentrosomes are important for many cellular processes. They comprise of a pair of
centrioles surrounded by an amorphous pericentriolar material (PCM). In Drosophila,
although more than hundreds of proteins localise to PCM, it is believed that the PCM
assembly is governed by a small subset of proteins—Polo, Spd-2 and Cnn forming a
network-like underlying scaffold. This scaffold appears to be plastic and adopts different
behaviours in many cell types. Therefore, Drosophila centrosomes allow us to understand
how a simple set of proteins are wired to satisfy biochemical and biophysical needs. In
this thesis, I describe my discovery that Cnn scaffold assembly is initiated and timed by
a Polo oscillation. Centriolar Ana1 helps recruit Polo, which inhibits Ana1 activity, resulting
in a time-delayed negative feedback oscillation. Polo then triggers a Spd-2 oscillation
which together build the Cnn scaffold. In the process, I created an automated analysis
pipeline to look into the scaffold assembly in different systems efficiently. Moreover, I
discovered that in addition to a Cnn solid scaffold centrosomes also possess a liquid
TACC scaffold that is organised by Spd-2 and Aurora A. The liquid TACC scaffold can
enrich many centrosomal proteins
Drosophila melanogaster as a model to study ciliogenesis
Get PDFCilia are microtubule-based extensions of the cell membrane that extend from a mature centriole. Primary (non-motile) cilia are present on most human cells and have important roles in signalling pathways. At the base of the cilium is a selective barrier known as the transition zone (TZ). Defects in the TZ are associated with human congenital diseases such as Meckel-Gruber syndrome (MKS) and Nephronophthisis (NPHP). The TZ is formed by three protein complexes, the MKS, NPHP and Cep290 modules, although the fruit fly Drosophila melanogaster appears to lack the core components of the NPHP module. Results presented in this thesis shows that MKS proteins are spatially separated from Cep290 at the TZ in Drosophila spermatocyte cilia. The TZ of the spermatocyte cilia is perturbed in flies mutant for MKS1 (MKS1Î1) and fails to recruit key TZ proteins, although Cep290 and Chibby are recruited normally. Male fertility, however, is unaffected. Similarly, while there are substantial abnormalities in microtubule and membrane organisation in developing MKS1Î1 mutant cilia, defects in mature MKS1Î1 mutant cilia are limited to subtle changes in IFT and a membrane surrounded volume within the cilium. The function of sensory neurons is not negatively affected by the MKS1Î1 mutation. Evidently, given enough developmental time, ciliary defects can be largely rescued in flies and the localisation of MKS module proteins to the cilia or flagella is not essential for viability or fertility in Drosophila
Investigating centriole orientation in the Drosophila embryo
Get PDFCentrioles are barrel shaped organelles that are known to nucleate microtubules (MT) contributing to the mitotic spindle and act as basal bodies during ciliogenesis. Mammalian centrioles possess distal appendage proteins (DAPs) which allow them to anchor to the cell cortex via their distal end. This distal ended orientation often plays a functional role within the cell. In many other cell types, centrioles are specifically positioned close to the cell cortex. However, we do not yet understand how centrioles may be oriented in these systems. Our lab has long observed that in Drosophila embryos, which do not possess DAPs or make cilia, that most centrioles appear upright. Using a fixed assay and differential fluorescence labelling, I show that most centrioles have a preference to be upright with their distal end pointing towards the cell cortex, much like in mammalian cell types where centriole orientation is known. In collaboration with a statistician, we develop an assay to image embryos in real-time using super-resolution microscopy and analyse spatiotemporal aspects of their orientation using a neural network. Initial analysis further showed that centrioles have a preferential upright, “Ring” oriented state that is mostly stable over S-phase. Using a combination of drug- and candidate-based approaches, my data suggests that centrioles may indirectly be pushed and pulled into different orientations gradually by a balance of surrounding forces from microtubules and the centriole’s surrounding pericentriolar material. However, I also uncover a possible role for scaffold protein Centrosomin (Cnn) and distal cap proteins, most strikingly distal cap protein Cep97, which may be instrumental in maintaining the distal ended orientation of centrioles. My data strongly suggests that distally oriented centrioles are the default state in Drosophila embryos and there is an active mechanism maintaining this orientation
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