Reviews & Analysis

The mitochondrial network fragments during mitosis to allow proper segregation of the organelles between daughter cells. Two mitotic kinases, the cyclin B–CDK1 complex and Aurora A, are now shown to cooperate with the small G protein RALA and its effector RALBP1 to promote DRP1 phosphorylation and mitochondrial fission.

How the unique chromatin configuration of embryonic stem cells (ESCs) integrates inputs from exogenous stimuli to maintain pluripotency remains largely unknown. The ESC-specific ATP-dependent chromatin-remodelling (esBAF) complex maintains the accessibility of the target sites of Stat3, a leukaemia inhibitory factor (LIF) signalling effector, by preventing repressive localized polycomb-mediated trimethylation of Lys 27 of histone 3 (H3K27me3).

Regulatory mechanisms to prevent centriole overduplication during the cell cycle are not completely understood. In this issue, FBXW5 is shown to control the degradation of the centriole assembly factor HsSAS-6. Moreover, the study proposes that FBXW5 is a substrate of both PLK4 and APC/C, two established regulators of centriole duplication.

The retromer complex coordinates retrograde transport of cargo proteins between endosomes and the trans-Golgi network. The sorting nexin SNX3 is required for the retrograde trafficking of Wntless, but not of other retrograde cargo proteins, revealing that the cargo specificity of retromer is provided by the sorting nexins.

Both symmetric and asymmetric divisions rely on alignment of the mitotic spindle with cues from the environment. A study now shows that mitotic spindles find their position by reading the map of forces that load-bearing retraction fibres exert on the cell body.

Microtubule-depolymerizing motor proteins regulate microtubule dynamics during chromosome segregation, but whether they can independently grip the ends of shrinking kinetochore microtubules has remained unresolved. MCAK, a member of the kinesin-13 motor protein family, is now shown to grip microtubules on its own and harness the forces of microtubule disassembly.

The primary cilium is proposed to restrain the level of canonical Wnt signalling, but it was unknown how the cilium achieves this. β-catenin, a component of the canonical Wnt signalling pathway, is now shown to be sequestered to the cilium by the Wnt signalling modulator Jouberin (Jbn) to restrain Wnt responses.

Aurora A kinase is a key regulator of cell division, whose functions were attributed to its ability to phosphorylate diverse substrates. Aurora A is now shown to have a kinase-independent role in the regulation of chromatin-mediated microtubule assembly.

Starvation of animals or cells triggers autophagic degradation of cell contents to retrieve nutrients, but, paradoxically, mitochondria enlarge. This is now shown to result from inhibition of mitochondrial fission through PKA-mediated phosphorylation of the GTPase DRP1. Elongation of mitochondria optimizes ATP production and spares them from autophagy-mediated destruction.

Induced pluripotent stem (iPS) cells offer the possibility to generate patient-specific cell types for use in regenerative medicine. However, a long-lasting question remains: are iPS and embryonic stem cells equivalent? iPS cells retain a transcriptional memory of their origin, which is now shown to endure with passages and to correlate with defects in the re-establishment of DNA methylation. Both selective pressure and genomic environment may account for these defects.

COPII-coated vesicles drive protein export from the endoplasmic reticulum (ER), although the regulation of this event, both spatially and kinetically, remains unclear. TFG is now defined as a factor that modulates recruitment of the coat and links ER sequestration of kinases to oncogenesis.

Two papers in this issue show that dynein-binding proteins may regulate the G1–S transition through an effect on cilia. Nde1, a known partner of dynein light chain LC8, controls ciliary length in vitro and in zebrafish, and influences the G1–S progression. The phosphorylation of Tctex1, a dynein light chain, modulates cilia length and accelerates G1–S, thereby regulating proliferation–differentiation decisions in the developing mouse neocortex.

Under nutrient-rich conditions, the nutrient-sensitive kinase mTOR (mammalian target of rapamycin) is recruited to the surface of lysosomes where it becomes activated and can promote cell growth and inhibit autophagy. In contrast, mTOR is inhibited in nutrient-poor conditions, leading to the induction of autophagy. The intracellular positioning of lysosomes in response to nutrient availability is now shown to orchestrate mTOR activation and regulate autophagy.

Plants reach for the sun by avoiding the shade and by directly growing towards the light. Two studies now suggest that the polar relocation of PIN3, a transporter directing the flow of the plant hormone auxin, drives both growth processes. PIN3 repolarization occurs downstream of shade perception through phytochrome photoreceptors, whereas blue light perceived by phototropin initiates polar recycling of PIN3 and growth towards the light.