Regulation of cell-fate determination in Dictyostelium

A key step in the development of all multicellular organisms is the differentiation of specialized cell types. The eukaryotic microorganism Dictyostelium discoideum provides a unique experimental system for studying cell-type determination and spatial patterning in a developing multicellular organism. Unlike metazoans, which become multicellular by undergoing many rounds of cell division after fertilization of an egg, the social amoeba Dictyostelium achieves multicellularity by the aggregation of approximately 10(5) cells in response to nutrient depletion. Following aggregation, cell-type differentiation and morphogenesis result in a multicellular organism with only a few cell types that exhibit a defined patterning along the anterior-posterior axis of the organism. Analysis of the mechanisms that control these processes is facilitated by the relative simplicity of Dictyostelium development and the availability of molecular, genetic, and cell biological tools. Interestingly, analysis has shown that many molecules that play integral roles in the development of higher eukaryotes, such as PKA, STATs, and GSK-3, are also essential for cell-type differentiation and patterning in Dictyostelium. The role of these and other signaling pathways in the induction, maintenance, and patterning of cell types during Dictyostelium development is discussed.     

Selection of initial conditions for recursive production of multicellular organisms

The development of a multicellular organism is a dynamic process. Starting from one or a few cells, the organism becomes a set of cells with different types that form well-determined patterns. It is rather surprising that differentiation in cell types and formation of controlled patterns are compatible, because the former gives morphogenetic diversification whereas the latter implies recursive production of a cell ensemble, reducing individual differences. We studied this problem by taking a simple cell model with intracellular reaction dynamics of chemical concentrations, cell-cell interactions, and increase in cell numbers. We observed successive differentiation from a cell type with diverse chemicals and chaotic concentration dynamics to cell types with oscillatory or fixed-point dynamics, leading to morphogenetic diversity in a spatial pattern. We further show that, by starting from an initial object consisting of both the former cell type with diverse chemicals and the latter differentiated cell type, the recursive production of a multicellular organism with morphogenetic diversity is possible. By relating the former type to a cell in the vegetal pole and the latter to one in the animal pole, classic experimental results with separation of blastomeres in sea urchin eggs are coherently explained, while some predictions are made for in vitro morphogenesis from embryonic stem cells.     

The evolution of robust development and homeostasis in artificial organisms

During embryogenesis, multicellular animals are shaped via cell proliferation, cell rearrangement, and apoptosis. At the end of development, tissue architecture is then maintained through balanced rates of cell proliferation and loss. Here, we take an in silico approach to look for generic systems features of morphogenesis in multicellular animals that arise as a consequence of the evolution of development. Using artificial evolution, we evolved cellular automata-based digital organisms that have distinct embryonic and homeostatic phases of development. Although these evolved organisms use a variety of strategies to maintain their form over time, organisms of different types were all found to rapidly recover from environmental damage in the form of wounds. This regenerative response was most robust in an organism with a stratified tissue-like architecture. An evolutionary analysis revealed that evolution itself contributed to the ability of this organism to maintain its form in the face of genetic and environmental perturbation, confirming the results of previous studies. In addition, the exceptional robustness of this organism to surface injury was found to result from an upward flux of cells, driven in part by cell divisions with a stable niche at the tissue base. Given the general nature of the model, our results lead us to suggest that many of the robust systems properties observed in real organisms, including scar-free wound-healing in well-protected embryos and the layered tissue architecture of regenerating epithelial tissues, may be by-products of the evolution of morphogenesis, rather than the direct result of selection.     

Theoretical prerequisites for an experimental corroboration of the existence of a specialized cellular system in control of multicellular organism tissue proliferation

A theoretical analysis of cell proliferation as a selflimiting process designed to maintain the integrity of an entire multicellular organism and based on the principles of a "hypercycle" suggests the need for the existence, starting at a certain level of multicellular organization, of a specialized system in control of tissue proliferation, a system represented by a body of cells capable of both stimulating and inhibiting the proliferation of a variety of cell types. An analysis of experimental data in different fields of the biological science points to certain T-cell populations as probable candidate for the role of cellular regulators of tissue proliferation. Using as an example the induction of murine liver regeneration by the administration of CCL4, the author demonstrates the dynamics of the formation of cells stimulating and inhibiting regeneration, which conforms well to theoretical considerations.     

Protein Thermodynamics Can Be Predicted Directly from Biological Growth Rates

Life on Earth is capable of growing from temperatures well below freezing to above the boiling point of water, with some organisms preferring cooler and others hotter conditions. The growth rate of each organism ultimately depends on its intracellular chemical reactions. Here we show that a thermodynamic model based on a single, rate-limiting, enzyme-catalysed reaction accurately describes population growth rates in 230 diverse strains of unicellular and multicellular organisms. Collectively these represent all three domains of life, ranging from psychrophilic to hyperthermophilic, and including the highest temperature so far observed for growth (122°C). The results provide credible estimates of thermodynamic properties of proteins and obtain, purely from organism intrinsic growth rate data, relationships between parameters previously identified experimentally, thus bridging a gap between biochemistry and whole organism biology. We find that growth rates of both unicellular and multicellular life forms can be described by the same temperature dependence model. The model results provide strong support for a single highly-conserved reaction present in the last universal common ancestor (LUCA). This is remarkable in that it means that the growth rate dependence on temperature of unicellular and multicellular life forms that evolved over geological time spans can be explained by the same model.     

At the nexus between pattern formation and cell-type specification: the generation of individual neuroblast fates in the Drosophila embryonic central nervous system.

The specification of specific and often unique fates to individual cells as a function of their position within a developing organism is a fundamental process during the development of multicellular organisms. The development of the Drosophila embryonic central nervous system serves as an excellent model system in which to clarify the developmental mechanisms that link pattern formation to cell-type specification. The Drosophila embryonic central nervous system develops from a set of neural stem cells termed neuroblasts. Neuroblasts arise from the ectoderm in an invariant pattern, and each neuroblast acquires a unique fate based on its position within this pattern. Two groups of genes recently have been demonstrated to govern the individual fate specification of neuroblasts. One group, the segment polarity genes, enables neuroblasts that develop in different anteroposterior positions to acquire different fates. The second group, referred to as the columnar genes, ensures that neuroblasts that develop in different dorsoventral domains assume different fates. When integrated, the activities of the segment polarity and columnar genes create a Cartesian coordinate system that bestows unique fates to individual neuroblasts as a function of their position of formation within the ectoderm. BioEssays 1999;21:922-931.     

Cell-type control of membrane biogenesis induced by HMG-CoA reductase

Quantitative increases in HMG-CoA reductase, the rate-limiting enzyme in sterol biosynthesis, induce membrane biogenesis in both yeast and mammalian cells. The subcellular organization of the resulting membrane differs in the two cell types: mammalian cells generate crystalloid endoplasmic reticulum whereas yeast cells assemble karmellae. We examined the consequences of heterologous expression of HMG-CoA reductase to distinguish features of this response that were cell-type specific from those that were isozyme-specific. This analysis demonstrated that membrane proliferation was induced in both mammalian and yeast cells by HMG-CoA reductase from either organism. However, the morphology of the induced membranes was determined by the cell type rather than the particular isozyme. Thus, both yeast and mammalian HMG-CoA reductase contained functional signals for membrane proliferation that were operational in either cell type, but the qualitative response to those signals was cell-type specific.     

Pathways regulating apoptosis during patterning and development

The patterning and development of multicellular organisms require a precisely controlled balance between cell proliferation, differentiation and death. The regulation of apoptosis is an important aspect to achieve this balance, by eliminating unnecessary or mis-specified cells which, otherwise, may have harmful effects on the whole organism. Apoptosis is also important for the morphogenetic processes that occur during development and that lead to the sculpting of organs and other body structures. Here, we review recent progress in understanding how apoptosis is regulated during development, focusing on studies using Drosophila or Caenorhabditis elegans as model organisms.     

The mitochondrial prohibitin complex is essential for embryonic viability and germline function in Caenorhabditis elegans

Prohibitins in eukaryotes consist of two subunits (PHB1 and PHB2) that together form a high molecular weight complex in the mitochondrial inner membrane. The evolutionary conservation and the ubiquitous expression in mammalian tissues of the prohibitin complex suggest an important function among eukaryotes. The PHB complex has been shown to play a role in the stabilization of newly synthesized subunits of mitochondrial respiratory enzymes in the yeast Saccharomyces cerevisiae. We have used Caenorhabditis elegans as model system to study the role of the PHB complex during development of a multicellular organism. We demonstrate that prohibitins in C. elegans form a high molecular weight complex in the mitochondrial inner membrane similar to that of yeast and humans. By using RNA-mediated gene inactivation, we show that PHB proteins are essential during embryonic development and are required for somatic and germline differentiation in the larval gonad. We further demonstrate that a deficiency in PHB proteins results in altered mitochondrial biogenesis in body wall muscle cells. This paper reports a strong loss of function phenotype for prohibitin gene inactivation in a multicellular organism and shows for the first time that prohibitins serve an essential role in mitochondrial function during organismal development.     

Actin on disease--studying the pathobiology of cell motility using Dictyostelium discoideum

The actin cytoskeleton in eukaryotic cells provides cell structure and organisation, and allows cells to generate forces against membranes. As such it is a central component of a variety of cellular structures involved in cell motility, cytokinesis and vesicle trafficking. In multicellular organisms these processes contribute towards embryonic development and effective functioning of cells of all types, most obviously rapidly moving cells like lymphocytes. Actin also defines and maintains the architecture of complex structures such as neuronal synapses and stereocillia, and is required for basic housekeeping tasks within the cell. It is therefore not surprising that misregulation of the actin cytoskeleton can cause a variety of disease pathologies, including compromised immunity, neurodegeneration, and cancer spread. Dictyostelium discoideum has long been used as a tool for dissecting the mechanisms by which eukaryotic cells migrate and chemotax, and recently it has gained precedence as a model organism for studying the roles of conserved pathways in disease processes. Dictyostelium's unusual lifestyle, positioned between unicellular and multicellular organisms, combined with ease of handling and strong conservation of actin regulatory machinery with higher animals, make it ideally suited for studying actin-related diseases. Here we address how research in Dictyostelium has contributed to our understanding of immune deficiencies and neurological defects in humans, and briefly discuss its future prospects for furthering our understanding of neurodegenerative disorders.     

Getting an embryo into shape

Formation of a multicellular organism is a complex process involving differentiation and morphogenesis. During early vertebrate development, the radial symmetric organization of the egg is transferred into a bilateral symmetric organism with three distinct body axes: anteroposterior (AP), dorsoventral, and left–right. Due to cellular movements and proliferation, the body elongates along the AP axis. How are these processes coupled? Two recent publications now indicate that cell migration as well as orientated cell divisions contribute to axis elongation. The processes are coupled through the planar cell polarity pathway. 1 At the same time, the AP axis is patterned independently of convergent extension. This process, however, is required for cell migration and represents a cue for polarized cell motility during gastrulation. Thus, it is AP polarity that instructs individual cells how to orientate with respect to the embryonic axis and provides positional information for the process of convergent extension. 2 BioEssays 26:1272–1275, 2004. © 2004 Wiley Periodicals, Inc.     

Aggregative multicellularity evolved independently in the eukaryotic supergroup Rhizaria

Multicellular forms of life have evolved many times, independently giving rise to a diversity of organisms such as animals, plants, and fungi that together comprise the visible biosphere. Yet multicellular life is far more widespread among eukaryotes than just these three lineages. A particularly common form of multicellularity is a social aggregative fruiting lifestyle whereby individual cells associate to form a "fungus-like" sorocarp. This complex developmental process that requires the interaction of thousands of cells working in concert was made famous by the "cellular slime mold"Dictyostelium discoideum, which became an important model organism. Although sorocarpic protistan lineages have been identified in five of the major eukaryote groups, the ubiquitous and globally distributed species Guttulinopsis vulgaris has eluded proper classification. Here we demonstrate, by phylogenomic analyses of a 159-protein data set, that G. vulgaris is a member of Rhizaria and is thus the first member of this eukaryote supergroup known to be capable of aggregative multicellularity.     

Transitions in individuality.

The evolution of multicellular organisms is the premier example of the integration of lower levels into a single, higher-level individual. Explaining the evolutionary transition from single cells to multicellular organisms is a major challenge for evolutionary theory. We provide an explicit two locus genetic framework for understanding this transition in terms of the increase of cooperation among cells and the regulation of conflict within the emerging organism. Heritability of fitness and individuality at the new level emerge as a result of the evolution of organismal functions that restrict the opportunity for conflict within and ensure cooperation among cells. Conflict leads, through the evolution of adaptations that reduce it, to greater individuality and harmony for the organism.     

Mapping landscape beta diversity of plants across KwaZulu-Natal,South Africa,for aiding conservation planning

Collective properties of biodiversity, such as beta diversity, are suggested as complementary measures of species richness to guide the prioritisation and selection of important biodiversity areas in regional conservation planning. We assessed variation in the rate of plant species turnover along and between environmental gradients in KwaZulu-Natal, South Africa using generalised dissimilarity modelling, in order to map landscape levels of floristic beta diversity. Our dataset consisted of 434 plots (1000 m²) containing 997 grassland and savanna matrix species. Our model explained 79 % of the null deviance observed in floristic dissimilarities. Variable rates of turnover existed along the major environmental gradients of mean annual temperature, median rainfall in February, and soil cation exchange capacity, as well as along gradients of geographical distance. Beta diversity was highest in relatively warm, drier summer regions and on dystrophic soils. Areas of high beta diversity identify areas that should be included in conservation plans to maximise representation of diversity and highlight areas best suited to protected area expansion. Biome transition areas in high beta diversity areas may be susceptible to climate variability. Including beta diversity turnover rates in regional conservation plans will help to preserve evolutionary and ecological processes that create and maintain diversity.     

Cytogerontology at the beginning of the third millenium: from "correlative" to "gist" models

For the most part, research in the area of cytogerontology, i.e., investigation of the mechanisms of aging in the experiments on cultured cells, is carried out using the "Hayflick's model". More than forty years have passed since the appearance of that model, and during this period of time, very much data were obtained on its basis. These data contributed significantly to our knowledge of the behavior of both animal and human cultured cells. Specifically, we already know of the mechanisms underlying the aging in vitro. On the other hand, in my opinion, little has changed in our knowledge of the aging of the whole organism. In all likelihood, this can be explained by that the Hayflick's model is, like many others used in the experimental gerontology, correlative, i.e. based on a number of detected correlations. In the case of Hayflick's model, these are correlations between the mitotic potential of cells (cell population doubling potential) and some "gerontological" parameters and indices: species life-span, donor age, evidence of progeroid syndromes, etc., as well as various changes of normal (diploid) cells during long-term cultivation and during aging of the organism. It is, however, well known that very frequently a good correlation has nothing to do with the essence (gist) of the phenomenon. For example, we do know that the amount of gray hair correlates quite well with the age of an individual but is in no way related to the mechanisms of his/her aging and probability of death. In this case, the absence of cause-effect relationships is evident, which are, at the same time, indispensable for the development of gist models. These models, as distinct from the correlative ones, are based on a certain concept of aging. In the case of Hayflick's model, such a concept is absent: we cannot explain, using the "Hayflick's limit", why our organism ages. This conclusion was convincingly confirmed by the discovery of telomere mechanism which determines the aging of cells in vitro. That discovery initiated the appearance of theories attempting to explain the process of aging in vivo also on its basis. However, it has become clear that the mechanisms of aging of the entire organism, located, apparently, in its postmitotic cells, such as neurons or cardiomyocytes, cannot be explained in the framework of this approach. Hence, we believe that it is essential to develop "gist" models of aging using cultured cells. The mechanisms of cell aging in such models should be similar to the mechanisms of cell aging in the entire organism. Our "stationary phase aging" model could be one of such models, which is based on the assumption of the leading role of cell proliferation restriction in the processes of aging. We assume that the accumulation of "senile" damage is caused by the restriction of cell proliferation either due to the formation of differentiated cell populations during development (in vivo) or to the existence of saturation density phenomenon (in vitro). Cell proliferation changes themselves do not induce aging, they only lead to the accumulation of macromolecular defects, which, in turn, lead to the deterioration of tissues, organs, and, eventually, of the entire organism, increasing the probability of its death. Within the framework of our model, we define cell aging as the accumulation in a cell population of various types of damage identical to the damage arising in senescing multicellular organism. And, finally, it is essential to determine how the cell is dying and what the death of the cell is. These definitions will help to draw real parallels between the "genuine" aging of cells (i.e., increasing probability of their death with "age") and the aging of multicellular organisms.     

Results and perspectives of cytogerontologic studies in modern time

The overwhelming majority of research in the field of cytogerontology (i.e. investigating mechanisms of aging in experiments with cultured cells) has been done using the widely applied Hayflick's model. More than 40 years have passed since the appearance of the model, and during this time numerous data were obtained on its basis. The data significantly contributed to our knowledge of the behavior of cultured animal and human cells. In particular, we know enough about the in vitro aging phenomenon. But in my opinion, little has changed in our knowledge of aging in the whole organism. This may be, presumably, because Hayflich's model, like many other models used in experimental gerontology, is correlative, i.e. based on a great variety of detected correlations. In Hayflick's model these are correlations between the cell mitotic potential (cell population doubling potential) and the number of "gerontological" parameters and indices, such as the species life span, donor's age, evidence of progeroid syndromes, etc, and also correlations between various changes of normal (diploid) cells during a long-term cultivation and in the course of organismal aging. However, it is well known that a good correlation does not frequently have anything in common with the essence (gist) of the phenomenon under investigation. For example, the amount of grey hair in the individual is known to excellently correlate with his or her age, being, however, in no way associated with mechanisms of aging or probability of death. In this case, the absence of cause-effect relationships is evident. But it is these particular relationships that are totally indispensable for gist models developing. Such models, different from the correlative ones, are based on a definite concept of aging phenomenon. With the Hayflick's model, such a concept is absent, since using "Hayflick's limit" one cannot explain why the human organism is aging eventually. This can be exemplified by a discovery of a telomere mechanism, which is claimed to determine cell aging in vitro. This discovery triggered an outburst of theories aimed to explain on its basis as well the process of aging in vivo. However, now it is clear that mechanisms of the whole organism aging, hidden, presumably, in its postmitotic cells (neurons or cardiomyocytes) cannot be accounted for by this approach. In view of all stated above, we consider as indispensable the elaboration of "gist" models of aging using cultured cells. Mechanism of cell aging in these models must be similar to those in the whole organism. We believe that one of such models may be our "stationary phase aging" model, based on an assumption of the leading role of cell proliferation restriction in aging. We assume that accumulation of "senile" damage may by caused by the restriction of cell proliferation due to both the formation of differentiated cell populations in the course of development, and the existence of saturation density phenomenon (in vitro). Cell proliferation changes by themselves do not induce any aging processes, but lead only to accumulating macromolecular defects, which in their turn generate deterioration of tissues, organs, and eventually of the whole organism, thus increasing the probability of its death. Within the framework of our model, we define cell aging as the accumulation in a cell population of different types of damage identical to the damage arising in senscencing multicellular organism. And finally, we consider as very important the future studies aimed to determine the process of cell dying and cell death in general. Availability of such definitions would help to draw real parallels between the "genuine" cell aging (i.e. the increased probability of cell destruction with "age") and aging of the multicellular organism.     

Cell signaling during development of Dictyostelium

Continuous communication between cells is necessary for development of any multicellular organism and depends on the recognition of secreted signals. A wide range of molecules including proteins, peptides, amino acids, nucleic acids, steroids and polylketides are used as intercellular signals in plants and animals. They are also used for communication in the social ameba Dictyostelium discoideum when the solitary cells aggregate to form multicellular structures. Many of the signals are recognized by surface receptors that are seven-transmembrane proteins coupled to trimeric G proteins, which pass the signal on to components within the cytoplasm. Dictyostelium cells have to judge when sufficient cell density has been reached to warrant transition from growth to differentiation. They have to recognize when exogenous nutrients become limiting, and then synchronously initiate development. A few hours later they signal each other with pulses of cAMP that regulate gene expression as well as direct chemotactic aggregation. They then have to recognize kinship and only continue developing when they are surrounded by close kin. Thereafter, the cells diverge into two specialized cell types, prespore and prestalk cells, that continue to signal each other in complex ways to form well proportioned fruiting bodies. In this way they can proceed through the stages of a dependent sequence in an orderly manner without cells being left out or directed down the wrong path.     

Restricting Apoptosis for Postmitotic Cell Survival and its Relevance to Cancer

The importance of apoptosis in multicellular organisms is signified by the high degree of genetic conservation in the core components of this pathway from C. elegans through mammals. However, as the cells which comprise these organisms have diversified and become more specialized, so have the mechanisms which regulate the apoptotic pathway. The complex regulatory mechanisms by which the apoptotic pathway is refined are perhaps most apparent in differentiated postmitotic cells such as neurons, cardiomyocytes, and skeletal myotubes. The lack of significant regenerative potential in postmitotic cells demands that they must persist long-term, often for the full lifespan of an organism. Recent studies have identified several diverse mechanisms by which postmitotic cells restrict their apoptotic potential. Importantly, these mechanisms may also be co-opted by cancer cells in order to evade apoptosis.