Short Lives with Long-Lasting Effects: Filopodia Protrusions in Neuronal Branching Morphogenesis.

The branching behaviors of both dendrites and axons are part of a neuronal maturation process initiated by the generation of small and transient membrane protrusions. These are highly dynamic, actin-enriched structures, collectively called filopodia, which can mature in neurons to form stable branches. Consequently, the generation of filopodia protrusions is crucial during the formation of neuronal circuits and involves the precise control of an interplay between the plasma membrane and actin dynamics. In this issue of PLOS Biology, Hou and colleagues identify a Ca2+/CaM-dependent molecular machinery in dendrites that ensures proper targeting of branch formation by activation of the actin nucleator Cobl

Filopodia formation in five steps.

<p>1. Under resting conditions, actin nucleators (blue and green crosses), filament bundling and crosslinking proteins (grey crosses), and membrane curvature–sensing proteins (purple and orange curved lines) reside in the cytosol and preserve a base-line level of filamentous (F-) actin (red lines). Black dots denote plasma membrane microdomains rich in specific lipids and transmembrane proteins. 2. Upon signaling (e.g., increases in Ca²⁺ in the cytosol or activation of growth factor induced Phosphatidylinositol-4,5-bisphosphate (PIP2)-Phosphatidylinositol-3,4,5-trisphosphate (PIP3) turnover), nucleators are activated and, together with elongation factors, promote rapid actin polymerization and actin patch formation. 3. Filaments extend rapidly towards the membrane, and changes in membrane curvature are sensed and/or induced by curvature-sensing proteins. 4. A growing filopodium has established a mixture of unbundled and bundled or crosslinked actin filaments. It is conceivable that different sets of nucleators and elongators may contribute to increased actin polymerization. Other proteins that uncap or cap barbed ends or proteins that sever filaments at the root of filopodia may regulate filament turnover during this process. 5. In a “mature” filopodium, additional signals may guide microtubule invasion in order to stabilize the nascent filopodium into a branch.</p

Subcellular targeting and dynamic regulation of PTEN:Implications for neuronal cells and neurological disorders

PTEN is a lipid and protein phosphatase that regulates a diverse range of cellular mechanisms. PTEN is mainly present in the cytosol and transiently associates with the plasma membrane to dephosphorylate PI(3,4,5)P3, thereby antagonizing the PI3-Kinase signaling pathway. Recently, PTEN has been shown to associate also with organelles such as the endoplasmic reticulum (ER), the mitochondria, or the nucleus, and to be secreted outside of the cell. In addition, PTEN dynamically localizes to specialized sub-cellular compartments such as the neuronal growth cone or dendritic spines. The diverse localizations of PTEN imply a tight temporal and spatial regulation, orchestrated by mechanisms such as posttranslational modifications, formation of distinct protein–protein interactions, or the activation/recruitment of PTEN downstream of external cues. The regulation of PTEN function is thus not only important at the enzymatic activity level, but is also associated to its spatial distribution. In this review we will summarize (i) recent findings that highlight mechanisms controlling PTEN movement and sub-cellular localization, and (ii) current understanding of how PTEN localization is achieved by mechanisms controlling posttranslational modification, by association with binding partners and by PTEN structural or activity requirements. Finally, we will discuss the possible roles of compartmentalized PTEN in developing and mature neurons in health and disease

Domain structure, phylogeny and conservation of critical catalytic features of ciliate PIPKs.

<p>A, Domain structure of <i>Tetrahymena</i> PIPKs. The RING domain predicted in PIPK2b and transmembrane and SecY domains in PIPK5 have been removed (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0078848#pone.0078848.s005" target="_blank">Table S1</a> and Methods). Domain boundaries, e-values and further details are given in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0078848#pone.0078848.s005" target="_blank">Table S1</a>. B, Unrooted neighbor-joining tree of catalytic domains from 37 ciliate PIPKs. Bootstrap values from 5000 replicates higher than 60% are indicated near the corresponding branches. Group 1, 2, 3 and 4 PIPKs are color coded (blue, green, red and purple respectively). Circles and triangles represent <i>Tetrahymena</i> and <i>Paramecium</i> PIPKs, respectively. Bar indicates number of amino acid substitutions per site. Phylogenetic relationships of ciliate group 2 PIPK genes were less resolved with less nodes supported by high bootstrap values. In <i>Paramecium</i>, 4 additional group 3 PIPKs that are organized in 2 pairs of paralogs (PtPIPK3c,d and PtPIPK3e,f) and they are most related to TtPIPK3 are not shown. C, Sequence alignment of the catalytic kinase domains from ciliate PIPKs and mammalian PIPKIα and PIPKIIα. The position of prominent catalytic features is indicated by arrows and arrowheads and numbered residues refer to the mouse PIPKIIβ structure described in reference 46. Polygons indicate residues that interact with ATP or the phosphoinositide substrate (PtdIns5P) in the crystal structure of PIPKIIβ and they are conserved in both type I and II PIPKs <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0078848#pone.0078848-Rao1" target="_blank">[46]</a>. The variable inserts between the MDYSL and IID motifs present in all PIPKs have been omitted. The residues K¹⁵⁰, D²⁷⁸ and D³⁶⁹, essential for catalytic activity, are conserved in all but 2 <i>Tetrahymena</i> PIPKs (highlighted in grey; see text and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0078848#pone.0078848.s004" target="_blank">Figure S4</a> for details). The DLKGS motif in TtPIPK2c (highlighted in grey) has been reconstituted from RNA sequencing data (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0078848#pone.0078848.s005" target="_blank">Table S1</a>). The position of the KKxE/AxxxK motif in the specificity loop is indicated by a bar; further K residues that may contribute are highlighted by light blue and most ciliate PIPK1, but not PIPK2, genes confront to the consensus KK motif. Note that in all but 2 ciliate PIPKs the +2 position (E/A residues) in the specificity loop is occupied by a Glu residue as in all PtdInsP 5-kinases.</p

Phylogenetic analysis of PIPKs reveals evolutionary conservation of PIPKIII/FAB1 orthologs in most unicellular organisms.

<p>Unrooted maximum likelihood tree of catalytic domains from 92 eukaryotic PIPKs. Ciliate and apicomplexa group 1 PIPKs (reassigned as type I PIPKs due to the characterization of PfPIPK/NCS <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0078848#pone.0078848-Leber1" target="_blank">[51]</a>), ciliate and <i>Dictyostelium</i>/<i>Phytopthora</i> type IV PIPKs, type III PIPKs, metazoa/<i>Monosiga</i> type I and II PIPKs as well as fungi and plant/<i>Chlamydomonas</i> PIPKs are color-coded as indicated. Circles indicate <i>Tetrahymena</i> PIPKs, triangles <i>Paramecium</i> PIPKs and squares mammalian PIPKs. Bar indicates number of amino acid substitutions per site. Nodes that are supported by bootstrap analysis (1000 replicates, >69%) and neighbor-joining trees are highlighted according to the color code of each group. For PIPK repertoires of selected organisms, accession numbers and further details see Methods and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0078848#pone.0078848.s007" target="_blank">Table S3</a>.</p

Distinct <i>Tetrahymena</i> gene networks are associated with TtPI4K1, TtPI4K4 and TtPI4K6 type III PI4Ks.

<p>Gene networks associated with the indicated TtPI4K genes were extracted from the TetraFGD site (<a href="http://tfgd.ihb.ac.cn/" target="_blank">http://tfgd.ihb.ac.cn/</a>). The actual numbers of genes were 76, 100 and 100 for TtPI4K1, TtPI4K4, and TtPI4K6, respectively. A significant number of genes was annotated and/or characterized in terms of at least one Gene Ontology category annotation in the Tetrahymena Genome Database; percentages were 56%, 55%, and 48% for TtPI4K1, TtPI4K4 and TtPI4K6, respectively. Results are shown as pie charts indicating the number of non-annotated genes and numbers of genes involved in specific cellular processes or metabolic activities or sharing structural/functional homology. The graph in the PI4K6 panel shows the classification of TtPI4K6-associated protein kinases. The table in the PI4K4 panel shows the names, localization and possible function of TtPI4K4-associated Rabs. Two more Rab-like genes that were recovered are not included in the Bright et al. study (reference 16 in the manuscript) and they were omitted from this table.</p