Letters in 2013

Differentiation of pluripotent cells into renal lineages has had limited success so far. Melissa Little and colleagues have used defined medium conditions that induce posterior primitive streak and intermediate mesoderm using growth factors used during normal embryogenesis. This results in the synchronous induction of both components of the kidney, the ureteric bud and metanephric mesenchyme, which form a self-organizing nephron structure in vitro.

Differentiation of pluripotent cells into renal lineages has so far demonstrated limited success. Juan Carlos Ispizua Belmonte and colleagues have used defined medium conditions to obtain committed renal progenitor cells that are able to integrate into a ureteric bud in a three-dimensional culture system.

During vertebrate embryogenesis, the pharyngeal arch arteries (PAA) connect segments of the primitive circulation. Burns and colleagues show that Nkx2.5⁺ heart precursors from the lateral plate mesoderm surprisingly give rise to the PAA angioblasts, and that Nkx2.5 is required for PAA development.

The spindle assembly checkpoint (SAC) keeps the APC/C ubiquitin ligase inactive until all chromosomes are attached to the spindle. Pines and colleagues tagged endogenous cyclin A with a fluorescent protein by gene targeting and used cyclin A degradation as an assay for SAC activity. They found that the SAC does not show an all-or-nothing response—instead, SAC strength depends on the amount of MAD2 (a checkpoint protein) at kinetochores.

The spindle assembly checkpoint (SAC) arrests cells in metaphase until all chromosomes are attached to the spindle. Dick and Gerlich used laser microsurgery to detach individual chromosomes, revealing that the SAC does not have a switch-like response — instead, SAC strength depends on the number of unattached chromosomes.

Actin is abundant in the nuclei of oocytes but its role has been unclear. Feric and Brangwynne find that actin forms a network that prevents the sedimentation of RNA and protein bodies caused by gravitational forces.

Blau and colleagues show that non-dividing heterokaryons between mouse embryonic stem cells and human fibroblasts, in which human nuclei reprogram to a more embryonic state, can be used to identify signalling pathways involved in reprogramming. They delineate that IL-6 signalling and JAK/STAT target kinase Pim1 signalling are induced during this reprogramming, and that they increase the efficiency of factor-mediated reprogramming to induced pluripotent stem cell status.

The mammalian heart is formed from two distinct groups of mesodermal cells, the first and second heart fields (FHF and SHF). Little is known about the progenitors giving rise to the FHF, but Chien and colleagues have now identified HCN4 (hyperpolarization-activated cyclic nucleotide-gated channel 4) as an FHF marker. They show by lineage tracing that cells expressing HCN4 primarily contribute to cardiomyogenic lineages, and use this marker to isolate FHF progenitors from differentiation cultures of human embryonic stem cells.

Hyman and colleagues modulate the activity of the XMAP215 microtubule polymerase and show that spindle length is proportional to the maximum microtubule growth velocity set by XMAP215.

Oct4 is a core component of the regulatory network of pluripotency. Schöler and colleagues found that establishment of totipotency was not affected by genetic elimination of maternal Oct4a, with live offspring being born and oocytes able to reprogram somatic cell nuclei. However, ablation of both maternal and zygotic Oct4a led to the formation of an inner cell mass that was not pluripotent.

Niethammer and colleagues report that leukocytes are recruited to damaged tissue sites in zebrafish through an osmotic signalling network that responds to changes in osmolarity between the interstitial fluid and the external environment.

It has been shown that planar cell polarity (PCP) signalling and cilium positioning are linked. Montcouquiol and colleagues now show that G-protein signalling, through the GTP-binding G-protein subunit Gα_i3 and the apicobasal polarity protein mPins, modulates cilium positioning cell autonomously. They also show that PCP signalling controls cilium positioning at the tissue level.

Chromosomal fragile sites (CFSs) are prone to breakage and thus can cause genomic instability. Hickson and colleagues demonstrate that the MUS81 endonuclease localizes to CFSs and mediates their processing, revealing that CFS cleavage is an active process.

Breakage and rearrangement of common fragile sites (CFSs) can cause genomic instability. Rosselli and colleagues demonstrate that the endonuclease subunit ERCC1 and the endonuclease MUS18–EME1 are recruited to FANCD2-binding CFSs in mitosis to mediate their processing.

Mabuchi, Balasubramanian and colleagues develop a system to study the contraction of the fission yeast contractile ring in vitro. They identify components required for this process and find that myosin ATPase activity, but not actin disassembly, is needed.

Emergence of primordial germ cells (PGCs) from the syncytial Drosophila embryo is not well understood. Lehmann and colleagues show that a cytokinetic furrow emerges for PGC formation that does not require a canonical anaphase spindle cleavage pathway but involves regulators of the small GTPase Rho and the BTB-domain protein Germ cell-less.

The polycomb protein BMI1 has been linked to maintenance of adult stem cells. Klein and colleagues find that BMI1 is also required for the maintenance of stem cells in the continuously growing mouse incisor, through repression of the Ink4a/Arf locus to modulate the proliferation of stem cells and repression of Hox genes to prevent inappropriate lineage decisions in stem cell progeny.

Howard and colleagues demonstrate that the XMAP215 and EB1 microtubule plus-end binding proteins synergistically promote microtubule growth in vitro, at rates comparable to those measured in cells.

For double-strand breaks to be repaired by homologous recombination, the homologous template must be in the nuclear space. By elegant genetic manipulation of yeast chromosomes, Kupiec and colleagues show that nuclear organization influences repair efficiency.

Frog embryonic cell cycles are driven by oscillations in the activity of cyclin B–Cdk1. This complex activates the E3 ligase APC/C^Cdc20, which in turn causes cyclin B degradation, creating a negative feedback loop. Yang and Ferrell combine experimentation and mathematical modelling to demonstrate that the loop functions as a time-delayed ultrasensitive switch that promotes robust oscillations.