Cell fate and organogenesis in bacteria

Cell density, alignment, and orientation correlate with C-signal-dependent gene expression during Myxococcus xanthus development

Starving Myxococcus xanthus bacteria use short-range C-signaling to coordinate their movements and construct multicellular mounds, which mature into fruiting bodies as rods differentiate into spherical spores. Differentiation requires efficient C-signaling to drive the expression of developmental genes, but how the arrangement of cells within nascent fruiting bodies (NFBs) affects C-signaling is not fully understood. Here, we used confocal microscopy and cell segmentation to visualize and quantify the arrangement, morphology, and gene expression of cells near the bottom of NFBs at much higher resolution than previously achieved. We discovered that "transitioning cells" (TCs), intermediate in morphology between rods and spores, comprised 10 to 15% of the total population. Spores appeared midway between the center and the edge of NFBs early in their development and near the center as maturation progressed. The developmental pattern, as well as C-signal-dependent gene expression in TCs and spores, were correlated with cell density, the alignment of neighboring rods, and the tangential orientation of rods early in the development of NFBs. These dynamic radial patterns support a model in which the arrangement of cells within the NFBs affects C-signaling efficiency to regulate precisely the expression of developmental genes and cellular differentiation in space and time. Developmental patterns in other bacterial biofilms may likewise rely on short-range signaling to communicate multiple aspects of cellular arrangement, analogous to juxtacrine and paracrine signaling during animal development.

From cell differentiation to cell collectives: Bacillus subtilis uses division of labor to migrate

The organization of cells, emerging from cell-cell interactions, can give rise to collective properties. These properties are adaptive when together cells can face environmental challenges that they separately cannot. One particular challenge that is important for microorganisms is migration. In this study, we show how flagellum-independent migration is driven by the division of labor of two cell types that appear during Bacillus subtilis sliding motility. Cell collectives organize themselves into bundles (called "van Gogh bundles") of tightly aligned cell chains that form filamentous loops at the colony edge. We show, by time-course microscopy, that these loops migrate by pushing themselves away from the colony. The formation of van Gogh bundles depends critically on the synergistic interaction of surfactin-producing and matrix-producing cells. We propose that surfactin-producing cells reduce the friction between cells and their substrate, thereby facilitating matrix-producing cells to form bundles. The folding properties of these bundles determine the rate of colony expansion. Our study illustrates how the simple organization of cells within a community can yield a strong ecological advantage. This is a key factor underlying the diverse origins of multicellularity.

Growth of Myxococcus xanthus in continuous-flow-cell bioreactors as a method for studying development

Nutrient sensors and developmental timers are two classes of genes vital to the establishment of early development in the social soil bacterium Myxococcus xanthus. The products of these genes trigger and regulate the earliest events that drive the colony from a vegetative state to aggregates, which ultimately leads to the formation of fruiting bodies and the cellular differentiation of the individual cells. In order to more accurately identify the genes and pathways involved in the initiation of this multicellular developmental program in M. xanthus, we adapted a method of growing vegetative populations within a constant controllable environment by using flow cell bioreactors, or flow cells. By establishing an M. xanthus community within a flow cell, we are able to test developmental responses to changes in the environment with fewer concerns for effects due to nutrient depletion or bacterial waste production. This approach allows for greater sensitivity in investigating communal environmental responses, such as nutrient sensing. To demonstrate the versatility of our growth environment, we carried out time-lapse confocal laser scanning microscopy to visualize M. xanthus biofilm growth and fruiting body development, as well as fluorescence staining of exopolysaccharides deposited by biofilms. We also employed the flow cells in a nutrient titration to determine the minimum concentration required to sustain vegetative growth. Our data show that by using a flow cell, M. xanthus can be held in a vegetative growth state at low nutrient concentrations for long periods, and then, by slightly decreasing the nutrient concentration, cells can be allowed to initiate the developmental program.

Is caspase-dependent apoptosis only cell differentiation taken to the extreme?

The benefits of apoptosis for a multicellular organism are obvious and fit the current dogma that the maintenance and viability of such organisms are dependent on the selective elimination of unneeded or deleterious cell types. However, self destruction at the level of the individual cell defies the most basic precepts of biology (sustaining life). If apoptosis is viewed through this construct then one question becomes paramount, i.e., why would an individual cell and its progeny develop, retain, or evolve a mechanism the sole purpose of which is to eliminate itself? In consideration of such a paradox, it is reasonable to postulate that prospective apoptotic pathways coevolved with and or were co-opted from another basic cell function(s) that did not involve the death of the cell per se. In the following article, we present the hypothesis that the conserved biochemical pathways of apoptosis are integral components of terminal cell differentiation and it is the time of engagement and activity level of these pathways that ultimately determines the choice between cell death or cell maturation.

The phagosomal transporter A couples threonine acquisition to differentiation and replication of Legionella pneumophila in macrophages

Differentiation in response to environmental cues is integral to the success of many intracellular pathogens. By characterizing a Legionella pneumophila mutant defective for differentiation in broth and replication in macrophages, we identified a subfamily of major facilitator superfamily transporters, here named Pht (phagosomal transporter), that also is conserved in two other vacuolar pathogens, Coxiella burnetii and Francisella tularensis. Biolog phenotype microarray analysis indicated that PhtA transports threonine, an essential amino acid. Either excess threonine or threonine peptides bypass phtA function. In minimal medium, phtA mutants do not replicate; in rich broth, the bacteria prematurely differentiate to the transmissive phase, as judged by the kinetics of flaA-gfp expression, heat resistance, and sodium sensitivity. PhtA is dispensable for transmissive L. pneumophila to establish and persist within a replication vacuole but is essential for their differentiation to the replicative phase, based on phenotypic and RT-PCR analysis. Accordingly, we propose that the Pht transporter family equips transmissive L. pneumophila, C. burnetii, and F. tularensis to assess their phagosomal nutrient supply before committing to reenter the cell cycle.

Multicellular microorganisms: laboratory versus nature

Our present in-depth knowledge of the physiology and regulatory mechanisms of microorganisms has arisen from our ability to remove them from their natural, complex ecosystems into pure liquid cultures. These cultures are grown under optimized laboratory conditions and allow us to study microorganisms as individuals. However, microorganisms naturally grow in conditions that are far from optimal, which causes them to become organized into multicellular communities that are better protected against the harmful environment. Moreover, this multicellular existence allows individual cells to differentiate and acquire specific properties, such as forming resistant spores, which benefit the whole population. The relocation of natural microorganisms to the laboratory can result in their adaptation to these favourable conditions, which is accompanied by complex changes that include the repression of some protective mechanisms that are essential in nature. Laboratory microorganisms that have been cultured for long periods under optimized conditions might therefore differ markedly from those that exist in natural ecosystems.

Towards microbial tissue engineering?

Tissue engineering involves the creation of multicellular tissues from individual cells. It was previously perceived that tissues were only formed by higher organisms such as plants and animals. However, it is now known that multicellular systems of microorganisms, such as microbial colonies, biofilms, flocs and aggregates, can also show extensive spatial organization. Here, we discuss methods that can be used to spatially organize microorganisms--bacteria, in particular--into tissue-like materials with defined internal architectures. Some potential uses of such "microbial tissues" are covered.

Ammonia signaling in yeast colony formation

Multicellular structures formed by microorganisms possess various properties, which make them interesting in terms of processes that occur in tissues of higher eukaryotes. These include processes important for morphogenesis and development of multicellular structures as well as those evoked by stress, starvation, and aging. Investigation of colonies created by simple nonmotile yeast cells revealed the existence of various regulators involved in their development. One of the identified signaling compounds, unprotonated volatile ammonia, is produced by colonies in pulses and seems to represent a long-distance signal notifying the colony population of incoming nutrient starvation. This alarm evokes changes in colonies that are important for their long-term survival. Models of the action of ammonia on yeast cells as well as the routes of its production are proposed. Interestingly, ammonia/ammonium also act as a signaling molecule in other organisms. Ammonia regulates several steps of the multicellular development of Dictyostelium discoideum and evidence indicates that ammonia/ammonium plays a role in neural tissues of higher eukaryotes.

Coupling cell movement to multicellular development in myxobacteria

The myxobacteria are Gram-negative organisms that are capable of multicellular, social behaviour. In the presence of nutrients, swarms of myxobacteria feed cooperatively by sharing extracellular digestive enzymes, and can prey on other bacteria. When the food supply runs low, they initiate a complex developmental programme that culminates in the production of a fruiting body. Myxobacteria move by gliding and have two, polarly positioned engines to control their motility. The two engines undergo coordinated reversals, and changes in the reversal frequency and speed are responsible for the different patterns of movement that are seen during development. The myxobacteria communicate with each other and coordinate their movements through a cell-contact-dependent signal. Here, the cell movements that culminate in the development of the multicellular fruiting body are reviewed.

Origin and evolution of eukaryotic apoptosis: the bacterial connection

The availability of numerous complete genome sequences of prokaryotes and several eukaryotic genome sequences provides for new insights into the origin of unique functional systems of the eukaryotes. Several key enzymes of the apoptotic machinery, including the paracaspase and metacaspase families of the caspase-like protease superfamily, apoptotic ATPases and NACHT family NTPases, and mitochondrial HtrA-like proteases, have diverse homologs in bacteria, but not in archaea. Phylogenetic analysis strongly suggests a mitochondrial origin for metacaspases and the HtrA-like proteases, whereas acquisition from Actinomycetes appears to be the most likely scenario for AP-ATPases. The homologs of apoptotic proteins are particularly abundant and diverse in bacteria that undergo complex development, such as Actinomycetes, Cyanobacteria and alpha-proteobacteria, the latter being progenitors of the mitochondria. In these bacteria, the apoptosis-related domains typically form multidomain proteins, which are known or inferred to participate in signal transduction and regulation of gene expression. Some of these bacterial multidomain proteins contain fusions between apoptosis-related domains, such as AP-ATPase fused with a metacaspase or a TIR domain. Thus, bacterial homologs of eukaryotic apoptotic machinery components might functionally and physically interact with each other as parts of signaling pathways that remain to be investigated. An emerging scenario of the origin of the eukaryotic apoptotic system involves acquisition of several central apoptotic effectors as a consequence of mitochondrial endosymbiosis and probably also as a result of subsequent, additional horizontal gene transfer events, which was followed by recruitment of newly emerging eukaryotic domains as adaptors.

Salt causes ion disequilibrium-induced programmed cell death in yeast and plants

Programmed cell death (PCD) is a fundamental cellular process conserved in metazoans, plants and yeast. Evidence is presented that salt induces PCD in yeast and plants because of an ionic, rather than osmotic, etiology. In yeast, NaCl inhibited growth and caused a time-dependent reduction in viability that was preceded by DNA fragmentation. NaCl also induced the cytological hallmarks of lysigenous-type PCD, including nuclear fragmentation, vacuolation and lysis. The human anti-apoptotic protein Bcl-2 increased salt tolerance of wild-type yeast strain and calcineurin-deficient yeast mutant (cnb1Delta) that is defective for ion homeostasis, but had no effect on the NaCl or sorbitol sensitivity of the osmotic hypersensitive hog1Delta mutant -- results that further link PCD in the response to the ion disequilibrium under salt stress. Bcl-2 suppression of cnb1Delta salt sensitivity was ENA1 (P-type ATPase gene)-dependent, due in part to transcriptional activation. Salt-induced PCD (TUNEL staining and DNA laddering) in primary roots of both Arabidopsis thaliana wild type (Col-1 gl1) and sos1 (salt overly sensitive) mutant seedlings correlated positively with treatment lethality. Wild-type plants survived salt stress levels that were lethal to sos1 plants because secondary roots were produced from the shoot/root transition zone. PCD-mediated elimination of the primary root in response to salt shock appears to be an adaptive mechanism that facilitates the production of roots more able to cope with a saline environment. Both salt-sensitive mutants of yeast (cnb1Delta) and Arabidopsis (sos1) exhibit substantially more profound PCD symptoms, indicating that salt-induced PCD is mediated by ion disequilibrium.

The act operon controls the level and time of C-signal production for Myxococcus xanthus development

The C-signal is a morphogen that controls the assembly of fruiting bodies and the differentiation of myxospores. Production of this signal, which is encoded by the csgA gene, is regulated by the act operon of four genes that are co-transcribed from the same start site. The act A and act B genes regulate the maximum level of the C-signal, which never rises above one-quarter of the maximum wild-type level of CsgA protein in null mutants of either gene. The act A and act B mutants have the same developmental phenotype: both aggregate, neither sporulates, both prolong rippling. By sequence homology, act A encodes a response regulator, and act B encodes a sigma-54 activator protein of the NTRC class. The similar phenotypes of act A and act B deletion mutants suggest that the two gene products are part of the same signal transduction pathway. That pathway responds to C-signal and also regulates the production of CsgA protein, thus creating a positive feedback loop. The act C and act D genes regulate the time pattern of CsgA production, while achieving the same maximum level. An act C null mutant raises CsgA production 15 h earlier than the wild type, whereas an act D null mutant does so 6 h later than wild type. The loop explains how the C-signal rises continuously from early development to a peak at the time of sporulation, and the act genes govern the time course of that rise.

Spatial control of cell differentiation in Myxococcus xanthus

Myxococcus xanthus develops species-specific multicellular fruiting bodies. Starting from a uniform mat of cells, some cells enter into nascent fruiting body aggregates, whereas other cells remain outside. The cells within the fruiting body differentiate from rods into spherical, heat-resistant spores, whereas the cells outside the aggregates, called peripheral cells, remain rod-shaped. Early developmentally regulated genes are expressed in peripheral cells as well as by cells in the fruiting bodies. By contrast, late developmental genes are only expressed by cells within the nascent fruiting bodies. The data show that peripheral cells begin to develop, but are unable to express genes that are switched on later than about 6 h after the start of development. All of the genes whose expression is limited to the fruiting body are dependent on C-signaling either directly or indirectly, whereas the genes that are equally expressed in peripheral rods and in fruiting body cells are not. One of the C-signal-dependent and spatially patterned operons is called dev, and the dev operon has been implicated in the process of sporulation. It is proposed that expression of certain genes, including those of the dev operon, is limited to the nascent fruiting body because fruiting body cells engage in a high level of C-signaling. Peripheral cells do less C-signaling than fruiting body cells, because they have a different spatial arrangement and are at lower density. As a consequence, peripheral cells fail to express the late genes necessary for spore differentiation.

Yeast colonies synchronise their growth and development

The ability to emit and receive signals over long distances is one of the characteristic attributes of multicellular organisms. Such communication can be mediated in different manners (by chemical compounds, light waves, acoustic waves etc.) and usually is reflected in the behaviour of the communicating organisms. Recently, we reported that individual yeast colonies, organised multicellular structures, can also communicate at long distance by means of volatile ammonia, which is produced by colonies in pulses separated by acidification of the medium. Here, we demonstrate that the colony that first reached the stage of intense ammonia production induces ammonia production response in surrounding colonies regardless of their age, causing the synchronisation of their NH(3) pulses and, consequently, the mutual affection of their growth. Also an artificial source of ammonia (but neither NH(4)(+) nor NaOH gradients) can immediately induce the ammonia production even in the colony starting its acidic stage of the development. The repeated transition of Candida mogii colonies from the acidic phase to the phase of intensive ammonia production is accompanied by dramatic changes in colony morphology and also in cell morphology and growth. Relatively smooth colonies in the acidic phase are formed by growing pseudohyphae. After ammonia induction, pseudohyphae decompose into non-dividing yeast-like cells, which rearrange themselves into ruffled spaghetti-like structures. The synchronisation of colony growth, that also exists between yeast colonies of different genera, could be important in establishing their optimal distribution in a natural habitat.

PII signal transduction proteins

PII proteins, found in Bacteria, Archaea and plants, help coordinate carbon and nitrogen assimilation by regulating the activity of signal transduction enzymes in response to diverse signals. Recent studies of bacterial PII proteins have revealed a solution to the signal transduction problem of how to coordinate multiple receptors in response to diverse stimuli yet permit selective control of these receptors under various conditions and allow adaptation of the system as a whole to long-term stimulation.

Quorum sensing and starvation: signals for entry into stationary phase

Quorum sensing occurs at high cell density in many microorganisms. It regulates specialized processes such as genetic competence, bioluminescence, virulence, and sporulation. However, recent evidence suggests that quorum-sensing may play a more central role in the physiology of bacteria, where quorum-sensing pathways converge with starvation-sensing pathways to regulate cell entry into stationary phase.