Cdk1-dependent phosphorylation of Iqg1 governs actomyosin ring assembly prior to cytokinesis.

Contraction of the actomyosin ring (AMR) provides the centripetal force that drives cytokinesis. In budding yeast (Saccharomyces cerevisiae), assembly and contraction of the AMR is coordinated with membrane deposition and septum formation at the bud neck. A central player in this process is Iqg1, which promotes recruitment of actin to the myosin ring and links AMR assembly with that of septum-forming components. We observed early actin recruitment in response to inhibition of cyclin-dependent kinase 1 (Cdk1) activity, and we find that the Cdk1-dependent phosphorylation state of Iqg1 is a determining factor in the timing of bud neck localization of both Iqg1 and actin, with both proteins accumulating prematurely in cells expressing nonphosphorylatable Iqg1 mutants. We also identified the primary septum regulator Hof1 as a binding partner of Iqg1, providing a regulatory link between the septation and contractile pathways that cooperate to complete cytokinesis.

A G protein-coupled receptor is essential for Schwann cells to initiate myelination.

The myelin sheath allows axons to conduct action potentials rapidly in the vertebrate nervous system. Axonal signals activate expression of specific transcription factors, including Oct6 and Krox20, that initiate myelination in Schwann cells. Elevation of cyclic adenosine monophosphate (cAMP) can mimic axonal contact in vitro, but the mechanisms that regulate cAMP levels in vivo are unknown. Using mutational analysis in zebrafish, we found that the G protein-coupled receptor Gpr126 is required autonomously in Schwann cells for myelination. In gpr126 mutants, Schwann cells failed to express oct6 and krox20 and were arrested at the promyelinating stage. Elevation of cAMP in gpr126 mutants, but not krox20 mutants, could restore myelination. We propose that Gpr126 drives the differentiation of promyelinating Schwann cells by elevating cAMP levels, thereby triggering Oct6 expression and myelination.

Kif1b is essential for mRNA localization in oligodendrocytes and development of myelinated axons.

The kinesin motor protein Kif1b has previously been implicated in the axonal transport of mitochondria and synaptic vesicles. More recently, KIF1B has been associated with susceptibility to multiple sclerosis (MS). Here we show that Kif1b is required for the localization of mbp (myelin basic protein) mRNA to processes of myelinating oligodendrocytes in zebrafish. We observe the ectopic appearance of myelin-like membrane in kif1b mutants, coincident with the ectopic localization of myelin proteins in kif1b mutant oligodendrocyte cell bodies. These observations suggest that oligodendrocytes localize certain mRNA molecules, namely those encoding small basic proteins such as MBP, to prevent aberrant effects of these proteins elsewhere in the cell. We also find that Kif1b is required for outgrowth of some of the longest axons in the peripheral and central nervous systems. Our data demonstrate previously unknown functions of kif1b in vivo and provide insights into its possible roles in MS.

KBP is essential for axonal structure, outgrowth and maintenance in zebrafish, providing insight into the cellular basis of Goldberg-Shprintzen syndrome.

Mutations in Kif1-binding protein/KIAA1279 (KBP) cause the devastating neurological disorder Goldberg-Shprintzen syndrome (GSS) in humans. The cellular function of KBP and the basis of the symptoms of GSS, however, remain unclear. Here, we report the identification and characterization of a zebrafish kbp mutant. We show that kbp is required for axonal outgrowth and maintenance. In vivo time-lapse analysis of neuronal development shows that the speed of early axonal outgrowth is reduced in both the peripheral and central nervous systems in kbp mutants. Ultrastructural studies reveal that kbp mutants have disruption to axonal microtubules during outgrowth. These results together suggest that kbp is an important regulator of the microtubule dynamics that drive the forward propulsion of axons. At later stages, we observe that many affected axons degenerate. Ultrastructural analyses at these stages demonstrate mislocalization of axonal mitochondria and a reduction in axonal number in the peripheral, central and enteric nervous systems. We propose that kbp is an important regulator of axonal development and that axonal cytoskeletal defects underlie the nervous system defects in GSS.

alphaII-spectrin is essential for assembly of the nodes of Ranvier in myelinated axons.

Saltatory conduction in myelinated axons requires organization of the nodes of Ranvier, where voltage-gated sodium channels are prominently localized [1]. Previous results indicate that alphaII-spectrin, a component of the cortical cytoskeleton [2], is enriched at the paranodes [3, 4], which flank the node of Ranvier, but alphaII-spectrin's function has not been investigated. Starting with a genetic screen in zebrafish, we discovered in alphaII-spectrin (alphaII-spn) a mutation that disrupts nodal sodium-channel clusters in myelinated axons of the PNS and CNS. In alphaII-spn mutants, the nodal sodium-channel clusters are reduced in number and disrupted at early stages. Analysis of chimeric animals indicated that alphaII-spn functions autonomously in neurons. Ultrastructural studies show that myelin forms in the posterior lateral line nerve and in the ventral spinal cord in alphaII-spn mutants and that the node is abnormally long; these findings indicate that alphaII-spn is required for the assembly of a mature node of the correct length. We find that alphaII-spectrin is enriched in nodes and paranodes at early stages and that the nodal expression diminishes as nodes mature. Our results provide functional evidence that alphaII-spectrin in the axonal cytoskeleton is essential for stabilizing nascent sodium-channel clusters and assembling the mature node of Ranvier.