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Neurobiology of the Axon in Health and Disease
No full textThis eBook is a collection of articles from a Frontiers Research Topic. Frontiers Research Topics are very popular trademarks of the Frontiers Journals Series: they are collections of at least ten articles, all centered on a particular subject. With their unique mix of varied contributions from Original Research to Review Articles, Frontiers Research Topics unify the most influential researchers, the latest key findings and historical advances in a hot research area! Find out more on how to host your own Frontiers Research Topic or contribute to one as an author by contacting the Frontiers Editorial Office: frontiersin.org/about/contac
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
Nanospring behaviour of ankyrin repeats
No full textAnkyrin repeats are an amino-acid motif believed to function in protein recognition; they are present in tandem copies in diverse proteins in nearly all phyla(1). Ankyrin repeats contain antiparallel alpha-helices that can stack to form a superhelical spiral(2). Visual inspection of the extrapolated structure of 24 ankyrin-R repeats(2) indicates the possibility of spring-like behaviour of the putative superhelix. Moreover, stacks of 17-29 ankyrin repeats in the cytoplasmic domains of transient receptor potential (TRP) channels have been identified as candidates for a spring that gates mechanoreceptors in hair cells as well as in Drosophila bristles(3-5). Here we report that tandem ankyrin repeats exhibit tertiary-structure-based elasticity and behave as a linear and fully reversible spring in single-molecule measurements by atomic force microscopy. We also observe an unexpected ability of unfolded repeats to generate force during refolding, and report the first direct measurement of the refolding force of a protein domain. Thus, we show that one of the most common amino-acid motifs has spring properties that could be important in mechanotransduction and in the design of nanodevices.
A Novel Function of Giant Ankyrin-G in Promoting the Formation of Somatodendritic GABAA Receptor Synaptogenesis
No full textThe formation and retention of distinct membrane domains in the fluidic membrane bilayer is the key process in establishing spatial organization for mediating physiological functions in metazoans. The spectrin-ankyrin network organizes diverse membrane domains including T-tubule and intercalated disc of cardiomyocytes, basolateral membrane of epithelial cells, costameres of striatal muscle, and axon initial segments and nodes of Ranvier in nervous system. This thesis identifies a novel function of 480 kDa ankyrin-G, an alternatively spliced isoform of the ankyrin family, in promoting somatodendritic GABAA receptor synaptogenesis both in vitro and in vivo. In the nervous system, an insertion of a neuronal specific exon (exon 37) occurs in ankyrin-G polypeptide which results in a 480 kDa isoform. 480 kDa ankyrin-G (giant ankyrin-G) has been shown to coordinate formation and maintenance of the axon initial segment (AIS) and nodes of Ranvier. This thesis research began with the discovery that giant ankyrin-G, previously thought to be confined to the axon initial segment, forms developmentally-regulated and cell-type specific somatodendritic "outposts" on the plasma membrane of pyramidal neurons. This somatodendritic 480 kDa ankyrin-G outpost forms micron-scale membrane domains where it associates with canonical AIS binding partners including voltage-gated sodium channel and neurofascin. This thesis further discovered that the giant insert of 480 kDa ankyrin-G interacts with GABARAP, a GABAA receptor-associated protein. Both the interaction with GABARAP and the membrane association through palmitoylation of giant ankyrin-G are required for the formation of somatodendritic GABAergic synapses. This work further found that ankyrin-G associates with extrasynaptic GABAA receptors and stabilizes receptors on the extrasynaptic membrane through opposing endocytosis. This story demonstrates for the first time the existence of giant ankyrin-G somatodendritic outpost as well as its function in directing the formation of GABAergic synapses that provides a rationale for studies linking ankyrin-G genetic variation with psychiatric disease and neurodevelopmental disorders.Additional work presented in the Appendix characterized novel ankyrin-G full length transcripts in the heart and kidney with unique domain compositions though alternative splicing. The preliminary work further identified biochemical properties and potential role of an insert C in the C-terminus of ankyrin-G in mediating cytokinesis and cellular migration in mouse fibroblasts. Together, this thesis work expands the knowledge of giant ankyrin-G functions in the nervous system and offers insights into the diversified roles of distinct ankyrin-G peptides acquired from alternative splicing in organizing specific membrane domains and interacting with defined intracellular pathways in different tissues.</p
Decoding the Function of Ankyrin-B in Organelle Transport
No full textOrganelle transport in eukaryotic cells is regulated by a precisely coordinated activity of phosphoinositide lipids, small GTPases, and molecular motors. Despite the extensive study of functional activities of individual regulators, how these activities promote precise deliveries of particular membrane proteins to specific cellular locations remained unclear. Ankyrin-B, which is previously well recognized as a plasma membrane adaptor that assembles diverse specialized plasma membrane domains, exhibited an unexpected role in intracellular transport. This thesis establishes ankyrin-B as a master integrator of the polarized long range organelle transport via direct interactions with Rab GTPase Activating Protein 1 Like (RabGAP1L), phosphatidylinositol 3-phosphate (PI3P) and dynactin 4. In Chapter 2, I identified an ankyrin-B death domain binding partner, RabGAP1L, that specifically interacts with ankyrin-B on intracellular organelles and requires ankyrin-B for its proper localization. In Chapter 3, I demonstrated that ankyrin-B is a PI3P-effector in mouse embryonic fibroblasts (MEFs) and promotes the polarized transport of associated organelles in migrating cells in a RabGAP1L-dependent manner. I continued to investigate what membranes/membrane-associated proteins utilize the ankyrin-B/RabGAP1L pathway in Chapter 4 and identified α5β1-integrin as a cargo whose polarized transport and recycling are ankyrin-B-dependent. I further presented that ankyrin-B, through recruiting RabGAP1L to PI3P-positive/Rab22A-associated endosomes containing α5β1-integrin, promotes polarized recycling of α5β1-integrin in migrating mouse embryonic fibroblasts. In collaboration with James Bear (UNC Chapel Hill), we further demonstrated that this ankyrin-B/RabGAP1L-mediated pathway is required for haptotaxis along fibronectin gradients. In Chapter 5, I elucidated the in vivo interaction between ankyrin-B and RabGAP1L. I demonstrated that ankyrin-B specifically interacts with RabGAP1L at long axon tracts in the brain and at costameres in the skeletal muscle. I also show the phenotypic analysis of ankyrin-B floxDD mice as an initial attempt to determine the physiological function of the ankyrin-B death domain in vivo. Together, this thesis dissects an ankyrin-B-mediated molecular mechanism for polarized endosomal trafficking and α5β1-integrin recycling during directional cell migration, and provides new insights into how phosphoinositide lipids, Rab GTPases, and molecular motor activities are coordinated to control the directional transport of specialized membrane cargos.</p
Giant Ankyrins Awake: Select Roles of Giant Ankyrins in Axons of the Central Nervous System
No full textOrganization of membrane domains in axons is key for the normal transmission of information from one neuron to another. Giant ankyrin proteins are two such members of axonal membrane domains, where giant ankyrin-B (440kDa) is localized to the plasma membrane of axons and giant ankyrin-G (480kDa) is localized to excitable membrane domains of axons as well as somatodendritic regions later in development. Mutations in these proteins are found in autistic individuals (giant ankyrin-B) and bipolar and schizophrenia patients (giant ankyrin-G). However, the function of giant ankyrin-B and the regulation of giant ankyrin-G remain elusive. Here we developed two transgenic mouse lines lacking giant ankyrin-B: one that completely eliminates giant ankyrin-B by Cre-dependent removal of the inserted sequence, and another CRISPR induced frameshift inside of the inserted sequence that mimics the variant in one autistic individual, to decipher its role in the central nervous system. Using analyses of cellular, brain-wide connectivity, and behavioral assays of these transgenic mice, we show the L1CAM-giant ankyrin-B complex is essential for normal axon branching and brain connectivity. Perturbations to brain connectivity by giant ankyrin-B mutation leads to selective deficits in pup ultrasonic vocalizations, male territory marking, and increased plasticity for reversal learning in a binary water T-maze. Secondly, I developed a novel solubilization technique for the denatured isolation of previously biochemically inaccessible giant ankyrin proteins. With this technique, we determined that phosphorylation was the major post-translational modification on giant ankyrin-G, and also on beta-4 spectrin sigma six isoforms. The stoichiometry of the phosphorylation was increased in these two excitable membrane proteins in comparison to the other isoforms of these proteins. By comparing developmental stages, there is a trend of decreased phosphorylation of giant ankyrin proteins within the giant inserted region during maturation of excitable membrane domains. Finally, we show a developmental change in ankyrin-G polypeptide levels and pinceau synapse formation on Purkinje cells in a mouse model with a disrupted ankyrin-G/GABARAP binding site. Together, these findings demonstrate that giant ankyrin proteins are critical proteins for the normal connectivity and conductance of neurons and that they are dynamically regulated through post-translational phosphorylation.</p
Decoding Ankyrin-G Targeting and Function
Get PDFThe spectrin-ankyrin network assembles diverse plasma membrane domains including axon initial segments and nodes of Ranvier, cardiomyocyte T-tubules and intercalated discs, epithelial lateral membranes, costameres and photoreceptor inner and outer segments. However the mechanism that targets the spectrin-ankyrin network to those plasma membrane domains is unknown. This thesis identifies two lipid inputs from protein palmitoylation and phosphoinositides that together control the precise localization of the spectrin-ankyrin network. In Chapter 2, we identify a linker peptide encoded by a single divergent exon that distinguishes the subcellular localization of ankyrin-B and -G by selectively suppressing protein binding through autoinhibition. In Chapter 3, we demonstrate that ankyrin-G is S-palmitoylated at a conserved C70 residue which is required to assemble epithelial lateral membranes and neuronal axon initial segments. We continue to interrogate how palmitoylation regulates ankyrin-G activities in Chapter 4, and identify DHHC5 and DHHC8 as the palmitoyltransferases in MDCK cells. We showed that palmitoylated ankyrin-G, in concert with phosphoinositide lipids, determines the polarized localization of beta II spectrin though a coincidence detection mechanism. This palmitoyltransferases/ ankyrin-G/beta II spectrin pathway determines the cell height of columnar epithelial cells. In Chapter 5, we elucidated the molecular mechanism through which the spectrin-ankyrin network assembles epithelial lateral membranes. We demonstrated that ankyrin-G and beta II spectrin function by opposing clathrin-mediated endocytosis to build the lateral membrane in MDCK cells. Together, this thesis dissects the mechanisms of how the spectrin-ankyrin network achieves precise membrane targeting and how it assembles lateral membranes to determine the morphogenesis of columnar epithelial cells, and provides the first molecular insight to understand how cells control the assembly of diverse plasma membrane domains.</p
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
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