Cell sorting during regenerative tissue formation

Regeneration of transected peripheral nerves is a complex process involving the coordinated action of neuronal axons, glial cells, and fibroblasts. Using rodent models of nerve repair, Parrinello et al. (2010) find that ephrin signaling between fibroblasts and Schwann cell progenitors, involving the stemness factor Sox2, is required for nerve regeneration.     

Cell sorting and chondrogenic aggregate formation in micromass culture

A fundamental feature of cartilage differentiation in the developing limb is the formation of a prechondrogenic cell condensation. An apparently similar process of prechondrogenic cell aggregation occurs in micromass cultures of limb bud mesenchyme with the formation of cellular aggregates which often differentiate into cartilage nodules. We have investigated the process of aggregate formation in micromass culture using chimaeric mixtures of potentially chondrogenic and nonchondrogenic cell types. Two systems were studied: mixtures of distal and proximal limb mesenchyme cells and mixtures of distal limb cells with avian tendon fibroblasts. In both cases cultures of varying proportions of each cell type have been prepared. The results demonstrate that aggregate formation in vitro is the consequence of a cell sorting process which can involve prechondrogenic cells of widely different spatial origins within the developing limb. This contrasts with in vivo prechondrogenic condensation in which there is no evidence of cell sorting (Searls, R.L. (1967), J. Exp. Zool. 166, 39-50). However, our findings do indicate that cell surface differences occur in apparently undifferentiated limb mesenchyme. The results also suggest that mesenchymal cell aggregates must achieve a threshold size before chondrogenesis can proceed. In addition, the results show that under some culture conditions nonchondrogenic cells will form aggregates.     

The evolution of signalling pathways in animal development

Despite the bewildering number of cell types and patterns found in the animal kingdom, only a few signalling pathways are required to generate them. Most cell-cell interactions during embryonic development involve the Hedgehog, Wnt, transforming growth factor-beta, receptor tyrosine kinase, Notch, JAK/STAT and nuclear hormone pathways. Looking at how these pathways evolved might provide insights into how a few signalling pathways can generate so much cellular and morphological diversity during the development of individual organisms and the evolution of animal body plans.     

Endosomal sorting and signalling: an emerging role for sorting nexins

The endocytic network comprises a series of interconnected tubulo-vesicular membranous compartments that together regulate various sorting and signalling events. Although it is clear that defects in endocytic function underlie a variety of human diseases, our understanding of the molecular entities that regulate these sorting and signalling events remains limited. Here we discuss the sorting nexins family of proteins and propose that they have a fundamental role in orchestrating the formation of protein complexes that are involved in endosomal sorting and signalling.     

Classification of cell signalling in tissue development

The traditional classification of signalling in biological systems is insufficient and outdated and novel efforts must take into account advances in systems theory, information theory and linguistics. We present some of the classification systems currently used both within and outside of the biological field and discuss some specific aspects of the nature of signalling in tissue development. The analytical methods used in understanding non-biological networks provide a valuable vocabulary, which requires integration and a system of classification to further facilitate development.     

Cell migration: mechanisms of rear detachment and the formation of migration tracks

Cell migration is central to many biological and pathological processes, including embryogenesis, tissue repair and regeneration as well as cancer and the inflammatory response. In general, cell migration can be usefully conceptualized as a cyclic process. The initial response of a cell to a migration-promoting agent is to polarize and extend protrusions in the direction of migration. These protrusions can be large, broad lamellipodia or spike-like filopodia, are usually driven by actin polymerization, and are stabilized by adhering to the extracellular matrix (ECM) via transmembrane receptors of the integrin family linked to the actin cytoskeleton. These adhesions serve as traction sites for migration as the cell moves forward over them, and they must be disassembled at the cell rear, allowing it to detach. The mechanisms of rear detachment and the regulatory processes involved are not well understood. The disassembly of adhesions that is required for detachment depends on a coordinated interaction of actin and actin-binding proteins, signaling molecules and effector enzymes including proteases, kinases and phosphatases. Originally, the biochemically regulated processes leading to rear detachment of migrating cells were thought not to be necessarily accompanied by any loss of cell material. However, it has been shown that during rear detachment long tubular extensions, the retracting fibers, are formed and that "membrane ripping" occurs at the cell rear. By this process, a major fraction of integrin-containing cellular material is left behind forming characteristic migration tracks that exactly mark the way a cell has taken.     

Cell sorting by differential cell motility: a model for pattern formation in Dictyostelium

In the slug stage of the cellular slime mold Dictyostelium discoideum, prespore cells and four types of prestalk cells show a well-defined spatial distribution in a migrating slug. We have developed a continuous mathematical model for the distribution pattern of these cell types based on the balance of force in individual cells. In the model, cell types are assumed to have different properties in cell motility, i.e. different motive force, the rate of resistance against cell movement, and diffusion coefficient. Analysis of the stationary solution of the model shows that combination of these parameters and slug speed determines the three-dimensional shape of a slug and cell distribution pattern within it. Based on experimental data of slug motive force and velocity measurements, appropriate sets of parameters were chosen so that the cell-type distribution at stationary state matches the distribution in real slugs. With these parameters, we performed numerical calculation of the model in two-dimensional space using a moving particle method. The results reproduced many of the basic features of slug morphogenesis, i.e. cell sorting, translocation of the prestalk region, elongation of the slug, and its steady migration.     

From fruitflies to mammals: mechanisms of signalling via the Sonic hedgehog pathway in lung development

Chorioamnionitis is frequently associated with preterm deliveries before 30 weeks gestation. Chorioamnionitis correlates both with an increased risk of bronchopulmonary dysplasia and with a decreased risk of respiratory distress syndrome. Both interleukin-1α and endotoxin can induce inflammation in the fetal lungs and lung maturation after preterm birth when given by intra-amniotic injection. Inflammation can also result in an arrest of alveolarization, and this lung developmental abnormality is prominent in the lungs of preterm infants that die of bronchopulmonary dysplasia. The mechanisms by which infection/inflammation can have both beneficial and injurious effects on the preterm lung remain to be characterized.     

Cell movement in somite formation and development in the chick: Inhibition of segmentation

The contribution of active cell movement to somite formation (segmentation) and the later dispersal of the somite sclerotome was examined using cytochalasin D (CD). Stage 14–16 chick embryos were grown over liquid medium. After 8 hr in culture, control embryos had an average of six additional pairs of somites while CD (1–2 μg/ml dissolved in DMSO)-treated embryos had no new somites. DMSO alone had no effect on somitogenesis. CD-treated embryos transferred to drug-free medium recovered and segmentation resumed. Normal and CD-treated segmental plates were examined by SEM. Drug-treated segmental plate cells rounded up, consistent with the interaction of CD on contractile microfilaments. Embryos cultured 8 hr with or without CD were fractured through somite pair 20 and examined by SEM. In untreated embryos the sclerotome had dispersed and was migrating toward the notochord. CD stopped sclerotome dispersal. To test whether CD interfered with elaboration of extracellular matrix material associated with somite development, incorporation of [³H]glucosamine and Na₂³⁵SO₄ by somites and segmental plate was determined. There was no difference in total label incorporation. Molecular-weight profiles of proteoglycan obtained using controlled-pore glass-bead columns showed only small proteoglycans for both treated and control tissues. Therefore, the alteration of segmentation and somite morphogenesis by CD was not due to detectable changes in proteoglycan synthesis.     

Kinetochore fiber formation in animal somatic cells: dueling mechanisms come to a draw

The attachment to and movement of a chromosome on the mitotic spindle are mediated by the formation of a bundle of microtubules (MTs) that tethers the kinetochore on the chromosome to a spindle pole. The origin of these “kinetochore fibers” (K fibers) has been investigated for over 125 years. As noted in 1944 by Schrader [Mitosis, Columbia University Press, New York, 110 pp.], there are three possible ways to form a K fiber: (a) it grows from the pole until it contacts the kinetochore, (b) it grows directly from the kinetochore, or (c) it forms as a result of an interaction between the pole and the chromosome. Since Schrader's time, it has been firmly established that K fibers in centrosome-containing animal somatic cells form as kinetochores capture MTs growing from the spindle pole (route a). It is now similarly clear that in cells lacking centrosomes, including higher plants and many animal oocytes, K fibers “self-assemble” from MTs generated by the chromosomes (route b). Can animal somatic cells form K fibers in the absence of centrosomes by the “self-assembly” pathway? In 2000, the answer to this question was shown to be a resounding “yes.” With this result, the next question became whether the presence of a centrosome normally suppresses K fiber self-assembly or if this route works concurrently with centrosome-mediated K-fiber formation. This question, too, has recently been answered: observations on untreated live animal cells expressing green fluorescent protein-tagged tubulin clearly show that kinetochores can nucleate the formation of their associated MTs in a unique manner in the presence of functional centrosomes. The concurrent operation of these two “dueling” routes for forming K fibers in animal cells helps explain why the attachment of kinetochores and the maturation of K fibers occur as quickly as they do on all chromosomes within a cell.     

Chewing the fat: beta-oxidation in signalling and development

Peroxisomal beta-oxidation is involved not only in fatty acid catabolism and lipid housekeeping but also in metabolism of hormones and amino acids in plants. Recent research in model species has led to new insights into the roles of this pathway in signalling and development, in particular regarding the involvement of beta-oxidation in jasmonic acid biosynthesis. Analysis of associated processes, such as the glyoxylate cycle and redox metabolism has also highlighted the importance of integration of beta-oxidation with cytosolic and mitochondrial metabolism. Mutations that disrupt beta-oxidation can have extremely pleiotropic effects, indicating important and varied roles for this pathway throughout the plant life cycle and making this an exciting topic for future research.     

Changing the light environment: chloroplast signalling and response mechanisms

Light is an essential environmental factor required for photosynthesis, but it also mediates signals to control plant development and growth and induces stress tolerance. The photosynthetic organelle (chloroplast) is a key component in the signalling and response network in plants. This theme issue of Philosophical Transactions of the Royal Society of London B: Biology provides updates, highlights and summaries of the most recent findings on chloroplast-initiated signalling cascades and responses to environmental changes, including light and biotic stress. Besides plant molecular cell biology and physiology, the theme issue includes aspects from the cross-disciplinary fields of environmental adaptation, ecology and agronomy.Oxygenic photosynthetic organisms carry out the most intriguing reaction on Earth, namely the conversion of light energy from the sun into chemical energy, which also results in oxygen as a by-product. The photosynthetic end products (sugars) drive most processes in living cells on Earth. As photosynthetic organisms represent the basis of our daily life (food, energy, materials), effects on their primary productivity have an impact on the society in various aspects, for instance economy, ecological sustainability and even our lifestyle. Photosynthetic organisms, particularly plants which are essentially sessile, have to constantly deal with changes in a wide range of abiotic and biotic factors in their immediate environment on a seasonal as well as daily basis. The chloroplast is a light-driven energy factory, but besides this primary mission it perceives signals from surroundings to adjust plant development and induce adaptation to ever-changing environmental cues.The signalling cascades start from various chloroplast processes but merge later or crosstalk with each other and with other signalling cascades (figure 1). For example, acclimation of plants to excess light conditions may also simultaneously increase the tolerance to other abiotic stress factors [1]. Recently, chloroplasts were also recognized to perceive and mediate signals that promote tolerance against plant pathogens (immune defence) or that are involved in hormone perception [2]. Resolving the crosstalk between the cascades is most important for understanding physiological responses in plants under ever-changing environments, and for predicting how plants survive under natural growth conditions. Open in a separate windowFigure 1.Overview of light-induced chloroplast signalling and response mechanisms, covered by papers in this theme issue. Chl, chlorophyll; NPQ, non-photochemical quenching; ROS, reactive oxygen species; ST, state transition; Trx, thioredoxin. (Online version in colour.)This theme issue of Philosophical Transactions of the Royal Society of London B: Biology covers the most recent findings and updates on the molecular short-term mechanisms used by the chloroplast to adjust its function to changes in light conditions, and on the signalling pathways that induce long-term adaptive responses, such as stress tolerance and immune defence in plants (figure 1). It focuses on the current understanding of the crosstalk between signalling networks activated by chloroplasts and mitochondria, light receptors and those induced by biotic stress. It also focuses on the variation of the adaptive mechanisms in natural population and on their agricultural and ecological impacts. Thus, besides plant molecular cell biology and physiology, the theme issue includes aspects from the cross-disciplinary fields of environmental adaptation, ecology and agronomy. It consists of 10 research articles and nine reviews covering the following four topics: (i) short-term adaptive responses in chloroplasts, (ii) chloroplast-to-nucleus signalling and crosstalk with other signalling pathways, (iii) natural variation of regulatory mechanisms to allow for adaptation and (iv) agricultural and ecological perspective of light responses in chloroplasts.Light signals perceived by chlorophyll (Chl) in the thylakoid membrane and by photoreceptors in the cytosol activate various short-term adaptive responses including enzyme regulation, photoprotection and repair (figure 1). Ebenhöh et al. [3] propose a mathematical model for relative contributions of non-photochemical quenching (NPQ) and state transition (ST) in light acclimation. The paper by Cazzaniga et al. [4] identifies photoreceptor-dependent chloroplast movement as an additional pathway used to dissipate the excess absorbed energy, whereas Ruban & Belgio [5] investigate NPQ in relation to maximum light intensity tolerated by plants. Bertrand et al. [6] investigate the different mechanisms involved in NPQ relaxation in diatoms. Nikkanen & Rintamäki [7] and Kirchhoff [8] review the current knowledge on chloroplast processes regulated by thioredoxins under changing light environment and processes in the thylakoid membrane associated with the photosystem II repair cycle in high light stress, respectively.Together with adjustments of metabolic processes and induction of photoprotective mechanisms, light initiates signalling to the nucleus for gene expression, resulting in various long-term adaptive responses, including development and growth, stress and programmed cell death (figure 1). Larkin [9] provides an updated insight into the impact of the GENOMES UNCOUPLED genes on plastid-to-nucleus signalling and reviews the influence of plastids on light receptor signalling and development, whereas the contribution by Blanco et al. [10] searches for new components integrating mitochondrial and plastid retrograde signals that regulate plant energy metabolism. Alsharafa et al. [11] investigate the kinetics of events involved in initiation of high light acclimation, and Tikkanen et al. [12] show that chloroplast signalling interacts with both reactive oxygen species (ROS) and hormonal signalling. ROS signalling is also highlighted in the papers by Heyno et al. (hydrogen peroxide) [13] and by Zhang et al. (singlet oxygen) [14]. Foyer et al. [15] introduce a chloroplast protein belonging to the WHIRLY family and propose that the redox state of the photosynthetic electron transport chain triggers the movement of this protein from the chloroplast to the nucleus where it regulates the gene expression leading to cross tolerance, including light acclimation and immune defence. Gorecka et al. [16] identify novel components for crosstalk of immune reaction-induced signalling networks with two short-term photoprotective mechanisms, ST and NPQ. Trotta et al. [17] further review the increasing evidence for crosstalk between light-induced chloroplast signalling and immune reactions in plants.To allow for adaptation to a changing environment, natural selection of existing genetic variation takes place. Flood, Yin et al. [18] report natural variation in photosystem II protein phosphorylation in the model plant Arabidopsis thaliana and propose a possible role in the adaptation to diverse environments. In addition, Serõdio et al. [19] review the current knowledge of adaptation of macroalgal chloroplasts to life in sea slug following ingestion. Finally, the review by Darko et al. [20] uses selected examples to show how artificial lighting can be used to improve plant growth in agriculture and for production of functional food and materials, whereas Demmig-Adams et al. [21] provide an ecophysiological perspective of light responses in the chloroplast to optimize its function and of the whole plant in a changing environment.This research on light-induced signalling and response is developing in many directions, as reflected by the broad field coverage of the papers of this theme issue. It highlights and summarizes the present knowledge from the individual chloroplast reactions to the variation of the adaptive mechanisms in natural populations and on their agricultural and ecological impacts.