Neuronal cell fate: windows of opportunity

It is becoming increasingly evident that both vertebrate and invertebrate neural progenitor cells exhibit programmed temporal changes in their competence to generate specific neuronal cell types, but how is this competency controlled? Two articles in this issue address this question by studying Drosophila neuroblast lineage progression.

On p. 657, Michael Cleary and co-workers investigate the role of Polycomb repressor complexes (PRCs) in regulating neuroblast competence. In the Drosophila neuroblast 7-1 (NB7-1) lineage, the transcription factor Kruppel (Kr) specifies the third-born U3 motoneuron, but competence to generate U3 cells is limited to early divisions and is gradually lost when the neuroblasts transition to making interneurons. The researchers show that PRC loss of function extends the ability of Kr to induce U3 fate, whereas PRC gain of function causes precocious loss of competence to make motoneurons. The analysis of other neuroblast lineages that undergo a motoneuron-to-interneuron production transition demonstrates that PRCs also act to restrict motoneuron competence in these lineages. The researchers thus propose a model in which PRCs act to set up motoneuron-specific windows of competence in various neuroblasts that transition from motoneuron-to-interneuron production.

On p. 678, Stefan Thor and colleagues focus on the Drosophila embryonic neuroblast NB5-6T lineage to investigate how cell proliferation and cell fate specification are integrated during development. NB5-6T neuroblasts give rise to a lineage of 20 cells, including a differentiated set of neurons that are born at the very end of the lineage and that express Apterous: the Ap neurons. The authors identify two independent factors, Prospero and Notch, that act in concert to control the proliferation of NB5-6T daughter cells as the lineage progresses temporally; Prospero controls daughter cell proliferation in the early lineage and Notch activity then limits daughter cell proliferation in the late lineage, when Ap neurons are generated directly from neuroblasts, resulting in a programmed differentiation switch. Thus, the authors conclude, the control of neuronal daughter cell proliferation is integrated with temporal progression to ensure that the correct numbers of each unique cell type are generated.

Bypassing auxin signalling

Plant development is regulated by a number of mobile factors. The Arabidopsis BYPASS1 (BPS1) gene was previously shown to control shoot and root development by preventing formation of a mobile compound, but how this compound functions and whether it modulates other signalling pathways is unclear. Now, Leslie Sieburth and colleagues show that Arabidopsis BPS1, as well as two related genes, BPS2 and BPS3, control the production of a mobile factor, the bps signal, which regulates patterning and growth in parallel with auxin signalling (p. 805). By analysing single, double and triple mutants, the researchers show that all three BPS genes control bps signal synthesis. Importantly, bps triple mutants display severe embryogenesis defects, including disruptions to vascular, root and shoot stem cell populations. Finally, bps triple mutants exhibit normal auxin-induced gene expression and localisation of the PIN1 auxin transporter, suggesting that the bps signal functions in an auxin-independent manner. Although the nature of the bps signal remains unknown, these studies identify a novel pathway that regulates multiple stages of plant patterning and growth.

ABC of germline development

Plasma membrane ABC transporters serve dual functions in the cell: they export toxins to protect against damage and morphogens to mediate communication. It is thought that the activity of ABC transporters in embryos and stem cells should be high, so that mutagens are efficiently removed. Here, Joseph Campanale and Amro Hamdoun (p. 783) report the surprising finding that ABC transporter activity is reduced in germline precursors, the small micromeres, of the sea urchin embryo. This reduction in efflux pump activity can likely be ascribed to an increase in the rate of endocytosis specifically in the micromeres. What are the functional consequences of manipulating ABC transporter activity? The authors take a first step towards understanding this, showing that ABC transporter inhibition disrupts migration of the small micromeres at later stages of embryogenesis. While there is still much to be understood about the regulation and role of these plasma membrane pumps, this study provides evidence for the developmental importance of controlling their surface expression and activity.

Glutamate keeps hair follicles in touch

The hairs of our skin are mechanoreceptive: displacement of the hair is detected via sensory afferents in the hair follicle piloneural collar. In this complex structure, neurons, Schwann cells and keratinocytes are closely apposed, and interactions between these three cell types may influence differentiation and function of the piloneural collar. Here, David Owens and colleagues demonstrate that glutamate, which is known to mediate communication between neurons and Schwann cells in the central nervous system, has analogous activities in the periphery (p. 740). Signalling between VGLUT2-expressing neurons and NMDA receptor-expressing Schwann cells directs both formation and maintenance of the piloneural collar in mice. In conditional VGLUT2 mutants, the Schwann cells are disorganised and overall collar structure is severely disrupted. Moreover, treating the skin of adult mice with an NMDA receptor antagonist impairs touch-evoked responses, demonstrating defects in piloneural collar activity. Thus, continuous glutamate signalling between neurons and Schwann cells in the piloneural collar of the skin is essential for the integrity and function of this elaborate mechanosensory structure.

FGF signalling: keeping migrating cells on track

In Drosophila embryos, the longitudinal muscle cells surrounding the gut – the caudal visceral mesoderm (CVM) – arise in the posterior mesoderm and migrate anteriorly to reach their destination where they differentiate. Although this represents the longest cell migration event of Drosophila embryogenesis, the signals directing it are poorly understood. On p. 699, Angelike Stathopoulos and colleagues identify the FGF ligands Pyramus and Thisbe as crucial guidance cues for the CVM, signalling via the Heartless receptor to promote proper migration. The researchers use detailed live imaging and cell tracking analyses to describe wild-type migration, and analyse the consequences of disrupting FGF signalling, revealing defects in migration speed, directionality and cell survival. Intriguingly, by manipulating both the levels and location of ligand expression, they provide evidence for synergistic effects of Pyramus and Thisbe, although the mechanistic basis of such synergism remains to be investigated. Together, these data establish a new system for studying collective cell migration, and suggest additional complexities in FGF ligand-receptor interactions and signalling.

Plus…

Patterning embryos with oscillations: structure, function and dynamics of the vertebrate segmentation clock

The segmentation clock is an oscillating genetic network thought to govern the rhythmic and sequential subdivision of the elongating body axis of the vertebrate embryo into somites. Recent work, reviewed here by Oates et al, has provided evidence for how the period of the segmentation clock is regulated and how this affects the anatomy of the embryo. See the Review article on p. 625

Myoblast fusion: lessons from flies and mice

The fusion of myoblasts into multinucleate syncytia plays a fundamental role in muscle development and function. Here, Abmayr and Pavlath review the molecular events that drive myoblast fusion in the Drosophila embryo, in developing and regenerating mouse muscle, and in cultured muscle cells. See the Review article on p. 641

CycA takes control of endoreplication

Endocycles – repeated rounds of DNA replication without intervening mitoses – are involved in several terminal differentiation events. In Drosophila, for example, endoreplication occurs during the terminal differentiation of mechanosensory bristles. Endocycles are thought not to involve mitotic cyclins but here (p. 547), Agnès Audibert, Michel Gho and colleagues overturn that view by showing that cyclin A (CycA), which was thought to function exclusively in mitosis in Drosophila, is involved in endoreplication in the bristle lineage. The researchers show that CycA accumulates during the last part of endoreplication. CycA loss- and gain-of-function both induce changes in the dynamics of endoreplication, they report, and reduce the number of endocycles. Finally, CycA is required for relocalisation of ORC2, a member of the pre-replication complex, to the heterochromatin. These and other data reveal that CycA oscillations regulate endocycle dynamics in the fly mechanosensory bristle lineage and suggest that endoreplication involves remodelling of the entire cell-cycle network rather than simply a restriction of the canonical cell cycle as previously suggested.

Plane fact: aPKC orientates mitotic spindles

Mitotic spindle orientation, which is essential for epithelial morphogenesis and tissue maintenance, involves interactions between cortical polarity components and astral microtubules. The molecular machinery that regulates spindle apicobasal orientation during asymmetric cell division is well understood but what orientates the spindle along the epithelial plane in symmetrically dividing epithelial cells? On p. 503, Rui Gonçalo Martinho and colleagues provide the first in vivo evidence that atypical protein kinase C (aPKC) is involved in this process. Using a temperature-sensitive aPKC allele, the researchers show that Drosophila aPKC is required in imaginal discs for spindle planar orientation and for apical exclusion of Pins, a component of the molecular machinery that links the cell cortex to the astral microtubules. Apically localized aPKC is important for spindle planar orientation in mammalian epithelial cells in tissue culture; thus, these new observations suggest that the cortical cues required for spindle planar orientation are conserved between Drosophila and mammalian cells and are similar to those required for spindle orientation during asymmetric cell division.

pecanex wraps Notch up

During early development of the Drosophila nervous system, Notch signalling limits neuroblast numbers by preventing the cells that neighbour neuroblasts from also choosing a neuroblast fate. Disruption of Notch signalling prevents this ‘lateral inhibition’ and produces hyperplasias of the embryonic nervous system. The absence of pecanex (pcx), which encodes a conserved multi-pass transmembrane protein of unknown function, also causes a similar neurogenic phenotype. Now, Kenji Matsuno and colleagues propose that Pcx is a novel component of the Notch signalling pathway in Drosophila (see p. 558). They show that Pcx resides in the endoplasmic reticulum (ER) and is required upstream of activated Notch. Disruption of pcx function, they report, results in ER enlargement. However, hyper-induction of the unfolded protein response in the absence of pcx suppresses both ER enlargement and the development of a neurogenic phenotype. Together, these results suggest that the ER plays a previously unrecognised role in Notch signalling that involves Notch folding and that this ER function depends on pcx activity.

Wnt signalling regulates ciliogenesis

In zebrafish embryos, motile cilia lining the Kupffer’s vesicle (KV; the fish equivalent of the mouse node) help to establish left-right (LR) asymmetry. Wnt/β-catenin signalling is also involved in this process but precisely how it functions is unclear. Xueying Lin and colleagues now reveal that Wnt/β-catenin signalling directly regulates ciliogenesis in the zebrafish KV (see p. 514). The researchers show that reduced Wnt signalling disrupts LR patterning and ciliogenesis and downregulates Foxj1, a transcription factor that is required for the biosynthesis of motile cilia. KV-specific expression of foxj1a, they report, requires the presence of putative Lef1/Tcf binding sites in the foxj1a enhancer region, which suggests that Wnt signalling activates fox1ja transcription directly. Importantly, reduction of Wnt signalling also impairs foxj1 expression and ciliogenesis in developing zebrafish pronephric ducts and otic vesicles, epithelial structures that require Wnt activity for their development and function. The researchers propose, therefore, that the regulation of Foxj1 expression and ciliogenesis by Wnt/β-catenin signalling is a general developmental mechanism in zebrafish.

Egg cell orchestrates gametophyte development

Plant germ cells develop in specialised haploid structures called gametophytes. The female gametophyte of flowering plants contains an egg cell (which develops into the embryo), a central cell (which generates the endosperm that nurtures the embryo) and two accessory cell types, but what coordinates the development of these different cell types? On p. 498, Rita Groß-Hardt and co-workers report that egg-cell signalling mediated by LACHESIS (LIS), which encodes a homologue of a yeast pre-RNA splicing factor, regulates the development of Arabidopsis female gametophytic cells. lis mutants form supernumerary egg cells at the expense of accessory cells. The researchers now show that reducing LIS transcript levels specifically in the egg cell affects all the gametophyte cell types, which suggests that the egg cell orchestrates gametophyte differentiation. Notably, reduced LIS transcript levels in the egg cell interfere with homotypic nuclei fusion in the central cell and, consequently, endosperm formation. Thus, LIS-mediated egg-cell signalling ensures that endosperm only forms in the presence of a functional egg cell.

DAZL takes heat off testes

Mammalian testes are usually located outside the body cavity to ensure that the male germ cells are maintained at a low enough temperature to develop properly. If the testes get too warm, heat stress can cause germ cell apoptosis, which reduces fertility. Now, on p. 568, Kunsoo Rhee and co-workers reveal a cellular mechanism that protects male germ cells from heat stress. The researchers show that brief exposure of mouse testes to the core body temperature induces the assembly of stress granules (SGs; non-membranous cytoplasmic particles that contain translationally inert messenger ribonucleoproteins) in male germ cells. DAZL, a germ cell-specific translational regulator, translocates to SGs upon heat stress, they report, and is essential for their assembly. Importantly, DAZL-containing SGs sequester specific signalling molecules, such as RACK1 (receptor for activated protein kinase C), thereby blocking the apoptotic MAPK pathway. Together, these results suggest that DAZL is an essential component of SGs and that SGs prevent male germ cells from undergoing apoptosis upon heat stress.

Plus…

New for this year: poster articles!

Cilia in vertebrate development and disease

A taste of TGFβ in Tuscany

Regulation of DNA replication during development