Aurora kinases, which have been implicated in several vital events in mitosis, represent a protein kinase family highly conserved during evolution. The activity of Aurora kinases is delicately regulated, mainly by phosphorylation and degradation. Deregulation of Aurora kinase activity can result in mitotic abnormality and genetic instability, leading to defects in centrosome function, spindle assembly, chromosome alignment, and cytokinesis. Both the expression level and the kinase activity of Aurora kinases are found to be up-regulated in many human cancers, indicating that these kinases might serve as useful targets for the development of anticancer drugs. This review focuses on recent progress on the roles of Aurora kinases in mitosis and tumorigenesis. (

The small nuclear GTPase Ran controls the directionality of macromolecular transport between the nucleus and the cytoplasm. Ran also has important roles during mitosis, when the nucleus is dramatically reorganized to allow chromosome segregation. Ran directs the assembly of the mitotic spindle, nuclear-envelope dynamics and the timing of cell-cycle transitions. The mechanisms that underlie these functions provide insights into the spatial and temporal coordination of the changes that occur in intracellular organization during the cell-division cycle.

Autophagy is a lysosome-based degradation pathway. During autophagy, lysosomes fuse with autophagosomes to form autolysosomes. Following starvation-induced autophagy, nascent lysosomes are formed from autolysosomal membranes through an evolutionarily conserved cellular process, autophagic lysosome reformation (ALR), which is critical for maintaining lysosome homeostasis. Here we report that clathrin and phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P(2)) regulate ALR. Combining a screen of candidates identified through proteomic analysis of purified ALR tubules, and large-scale RNAi knockdown, we unveiled a tightly regulated molecular pathway that controls lysosome homeostasis, in which clathrin and PtdIns(4,5)P(2) are the central components. Our functional study demonstrates the central role of clathrin and its associated proteins in cargo sorting, phospholipid conversion, initiation of autolysosome tubulation, and proto-lysosome budding during ALR. Our data not only uncover a molecular pathway by which lysosome homeostasis is maintained through the ALR process, but also reveal unexpected functions of clathrin and PtdIns(4,5)P(2) in lysosome homeostasis.

Mitochondrial dysfunction underlies many prevalent diseases including heart disease arising from acute ischemia/reperfusion (I/R) injury. Here, we demonstrate that mitophagy, which selectively removes damaged or unwanted mitochondria, regulated mitochondrial quality and quantity in vivo. Hypoxia induced extensive mitochondrial degradation in a FUNDC1-dependent manner in platelets, and this was blocked by in vivo administration of a cell-penetrating peptide encompassing the LIR motif of FUNDC1 only in wild-type mice. Genetic ablation of Fundc1 impaired mitochondrial quality and increased mitochondrial mass in platelets and rendered the platelets insensitive to hypoxia and the peptide. Moreover, hypoxic mitophagy in platelets protected the heart from worsening of I/R injury. This represents a new mechanism of the hypoxic preconditioning effect which reduces I/R injury. Our results demonstrate a critical role of mitophagy in mitochondrial quality control and platelet activation, and suggest that manipulation of mitophagy by hypoxia or pharmacological approaches may be a novel strategy for cardioprotection.DOI: http://dx.doi.org/10.7554/eLife.21407.001

SummaryTransforming acidic coiled-coil-containing protein 3 (TACC3) has been implicated in mitotic spindle assembly, although the mechanisms involved are largely unknown. Here we identify that clathrin heavy chain (CHC) binds specifically to phosphorylated TACC3 and recruits it to spindle poles for proper spindle assembly and chromosome alignment. Phosphorylation of Xenopus TACC3 at serine 620 (S620) and S626, but not S33, is required for its binding with CHC. Knockdown of CHC by RNA interference (RNAi) abolishes the targeting of TACC3 to spindle poles and results in abnormal spindle assembly and chromosome misalignment, similar to the defects caused by TACC3 knockdown. Furthermore, the binding of CHC with phosphorylated TACC3 is inhibited by importin  and this inhibition is reversed by the presence of the GTP-binding nuclear protein Ran in the GTP-bound state. Together, these results indicate that the recruitment of phosphorylated TACC3 to spindle poles by CHC ensures proper spindle assembly and chromosome alignment, and is regulated by Ran.

The nuclear envelope (NE) forms a controlled boundary between the cytoplasm and the nucleus of eukaryotic cells. To facilitate investigation of mechanisms controlling NE assembly, we developed a cell-free system made from Xenopus laevis eggs to study the process in the absence of chromatin. NEs incorporating nuclear pores were assembled around beads coated with the guanosine triphosphatase Ran, forming pseudo-nuclei that actively imported nuclear proteins. NE assembly required the cycling of guanine nucleotides on Ran and was promoted by RCC1, a nucleotide exchange factor recruited to beads by Ran-guanosine diphosphate (Ran-GDP). Thus, concentration of Ran-GDP followed by generation of Ran-GTP is sufficient to induce NE assembly.

While the adult human heart has very limited regenerative potential, the adult zebrafish heart can fully regenerate after 20% ventricular resection. Although previous reports suggest that developmental signaling pathways such as FGF and PDGF are reused in adult heart regeneration, the underlying intracellular mechanisms remain largely unknown. Here we show that H 2 O 2 acts as a novel epicardial and myocardial signal to prime the heart for regeneration in adult zebrafish. Live imaging of intact hearts revealed highly localized H 2 O 2 (~30 µM) production in the epicardium and adjacent compact myocardium at the resection site. Decreasing H 2 O 2 formation with the Duox inhibitors diphenyleneiodonium (DPI) or apocynin, or scavenging H 2 O 2 by catalase overexpression markedly impaired cardiac regeneration while exogenous H 2 O 2 rescued the inhibitory effects of DPI on cardiac regeneration, indicating that H 2 O 2 is an essential and sufficient signal in this process. Mechanistically, elevated H 2 O 2 destabilized the redox-sensitive phosphatase Dusp6 and hence increased the phosphorylation of Erk1/2. The Dusp6 inhibitor BCI achieved similar pro-regenerative effects while transgenic overexpression of dusp6 impaired cardiac regeneration. H 2 O 2 plays a dual role in recruiting immune cells and promoting heart regeneration through two relatively independent pathways. We conclude that H 2 O 2 potentially generated from Duox/Nox2 promotes heart regeneration in zebrafish by unleashing MAP kinase signaling through a derepression mechanism involving Dusp6.

Autophagic lysosome reformation (ALR) plays an important role in maintaining lysosome homeostasis. During ALR, lysosomes are reformed by recycling lysosomal components from autolysosomes. The most noticeable step of ALR is autolysosome tubulation, but it is currently unknown how the process is regulated. Here, using an approach combining in vivo studies and in vitro reconstitution, we found that the kinesin motor protein KIF5B is required for autolysosome tubulation and that KIF5B drives autolysosome tubulation by pulling on the autolysosomal membrane. Furthermore, we show that KIF5B directly interacts with PtdIns(4,5)P2. Kinesin motors are recruited and clustered on autolysosomes via interaction with PtdIns(4,5)P2 in a clathrin-dependent manner. Finally, we demonstrate that clathrin promotes formation of PtdIns(4,5)P2-enriched microdomains, which are required for clustering of KIF5B. Our study reveals a mechanism by which autolysosome tubulation was generated.