Statistical evidence for a random commitment of pluripotent cephalic neural crest cells

Isolation of neural crest-derived stem cells from rat embryonic mandibular processes

BACKGROUND INFORMATION

Substantial evidence indicates the existence of NCSCs (neural crest-derived stem cells) in embryonic mandibular processes; however, they have not been fully investigated or isolated. The aim of the present study was to isolate stem cells from mandibular process during embryonic development by MACS (magnetic-activated cell sorting). The findings show that the cells are multipotent and self-renewing.

RESULTS

LNGFR (low-affinity nerve-growth-factor receptor)+ cells were isolated from rat embryonic mandibular processes by MACS. The cells were grown in clonal culture by limiting dilution to assess their developmental potential. Clone analysis indicated that, first, LNGFR+ cells are multipotent, being able to generate at least neurons and Schwann cells, similar to peripheral neural crest stem cells. Secondly, multipotent LNGFR+ cells generate multipotent progenies, indicating that they are capable of self-renewal and therefore are stem cells. Thirdly, manipulation of the medium supplementation alters the fate of the isolated LNGFR+ cells.

CONCLUSIONS

These results indicate that LNGFR antibodies label NCSCs with high specificity and purity, and suggest that positive selection using these antibodies may become the method of choice for obtaining multipotent cells from rat embryonic mandibular processes for tissue engineering or regenerative therapeutic use.

Human neural crest cells display molecular and phenotypic hallmarks of stem cells

The fields of both developmental and stem cell biology explore how functionally distinct cell types arise from a self-renewing founder population. Multipotent, proliferative human neural crest cells (hNCC) develop toward the end of the first month of pregnancy. It is assumed that most differentiate after migrating throughout the organism, although in animal models neural crest stem cells reportedly persist in postnatal tissues. Molecular pathways leading over time from an invasive mesenchyme to differentiated progeny such as the dorsal root ganglion, the maxillary bone or the adrenal medulla are altered in many congenital diseases. To identify additional components of such pathways, we derived and maintained self-renewing hNCC lines from pharyngulas. We show that, unlike their animal counterparts, hNCC are able to self-renew ex vivo under feeder-free conditions. While cross species comparisons showed extensive overlap between human, mouse and avian NCC transcriptomes, some molecular cascades are only active in the human cells, correlating with phenotypic differences. Furthermore, we found that the global hNCC molecular profile is highly similar to that of pluripotent embryonic stem cells when compared with other stem cell populations or hNCC derivatives. The pluripotency markers NANOG, POU5F1 and SOX2 are also expressed by hNCC, and a small subset of transcripts can unambiguously identify hNCC among other cell types. The hNCC molecular profile is thus both unique and globally characteristic of uncommitted stem cells.

Successful elimination of non-neural cells and unachievable elimination of glial cells by means of commonly used cell culture manipulations during differentiation of GFAP and SOX2 positive neural progenitors (NHA) to neuronal cells

BACKGROUND

Although extensive research has been performed to control differentiation of neural stem cells - still, the response of those cells to diverse cell culture conditions often appears to be random and difficult to predict. To this end, we strived to obtain stabilized protocol of NHA cells differentiation - allowing for an increase in percentage yield of neuronal cells.

RESULTS

Uncommitted GFAP and SOX2 positive neural progenitors - so-called, Normal Human Astrocytes (NHA) were differentiated in different environmental conditions to: only neural cells consisted of neuronal [MAP2+, GFAP-] and glial [GFAP+, MAP2-] population, non-neural cells [CD44+, VIMENTIN+, FIBRONECTIN+, MAP2-, GFAP-, S100beta-, SOX2-], or mixture of neural and non-neural cells.In spite of successfully increasing the percentage yield of glial and neuronal vs. non-neural cells by means of environmental changes, we were not able to increase significantly the percentage of neuronal (GABA-ergic and catecholaminergic) over glial cells under several different cell culture testing conditions. Supplementing serum-free medium with several growth factors (SHH, bFGF, GDNF) did not radically change the ratio between neuronal and glial cells--i.e., 1,1:1 in medium without growth factors and 1,4:1 in medium with GDNF, respectively.

CONCLUSION

We suggest that biotechnologists attempting to enrich in vitro neural cell cultures in one type of cells - such as that required for transplantology purposes, should consider the strong limiting influence of intrinsic factors upon extracellular factors commonly tested in cell culture conditions.

Modeling molecular interactions to understand spatial crowding effects on heterodimer formations

Molecular crowding occurs when the density of interacting molecules in some reaction system is sufficient to create deviations from traditional mass-action models of chemistry in diffusive systems. While there is a great deal of theory on the influence of molecular crowding on biochemistry in vivo, the effects are highly dependent on specific assumptions about the shapes, volumes, and diffusion properties of the components of an individual system and are thus difficult to predict from first principles. In this study, we use lattice Monte Carlo simulations to examine the effects on a reaction system for two limiting cases of the diffusion behavior of inert crowding agents. In cells, inert molecules might diffuse throughout a solute along with the reactant species by passive diffusion or may be anchored at fixed positions within the solute. We investigate the relative contributions of the two models to crowding effects by examining moving inert particles versus stationery inert particles on the kinetics of a heterodimer assembly system. The two models of inert crowding agents resulted in highly divergent effects on the reactant system. Stationary particles exhibited a bimodal response in the reaction rate curve that was a function of copy number and spatial arrangement and which accelerated the process at conditions not unlike those found in cellular environments. On the other hand, moving inert particles created a well mixed background that had no effect on the reaction process even under extremely compacted conditions. These results may have applications in developing more realistic simulations of reaction chemistry in crowded environments such as living cells.

A phylogenetic approach to mapping cell fate

Recent, surprising, and controversial discoveries have challenged conventional concepts regarding the origins and plasticity of stem cells, and their contributions to tissue regeneration, and highlight just how little is known about mammalian development in comparison to simpler model organisms. In the case of the transparent worm, Caenorhabditis elegans, Sulston and colleagues used a microscope to record the birth and death of every cell during its life, and the compilation of this "fate map" represents a milestone achievement of developmental biology. Determining a fate map for mammals or other higher organisms is more complicated because they are opaque, take a long time to mature, and have a tremendous number of cells. Consequently, fate mapping experiments have relied on tagging a progenitor cell with a dye or genetic marker in order to later identify its descendants. This approach, however, extracts little information because it demonstrates that a population of cells, all having inherited the same label, shares a common ancestor, but it does not reveal how cells in that population are related to one another. To avoid that problem, as well as technical limitations of current methods for mapping cell fate, we, and others, have developed a new strategy for retrospectively deriving cell fate maps by using phylogenetics to infer the order in which somatic mutations have arisen in the genomes of individual cells during development in multicellular organisms. DNA replication inevitably introduces mutations, particularly at repetitive sequences, every time a cell divides. It is thus possible to deduce the history of cell divisions by cataloging somatic mutations and phylogenetically reconstructing cell lineage. This approach has the potential to produce a complete mammalian cell fate map that, in principle, could describe the developmental lineage of any cell and help resolve outstanding questions of stem cell biology, tissue repair and maintenance, and aging.

Modeling embryogenesis and cancer: an approach based on an equilibrium between the autostabilization of stochastic gene expression and the interdependence of cells for proliferation

A large amount of data demonstrating the stochastic nature of gene expression and cell differentiation has accumulated during the last 40 years. These data suggest that a gene in a cell always has a certain probability of being activated at any time and that instead of leading to on and off switches in an all-or-nothing fashion, the concentration of transcriptional regulators increases or decreases this probability. In order to integrate these data in an appropriate theoretical frame, we have tested the relevance of the selective model of cell differentiation by computer simulation experiments. This model is based on stochastic gene expression controlled by cellular interactions. Our results show that it is readily able to produce tissue organization. A model involving only two cells generated a bi-layer cellular structure of finite growth. Cell death was not a drawback but an advantage because it improved the viability of this bi-layer structure. However, our results also show that cellular interactions cannot be simply based on raw selection between cells. Instead, tissue coordination includes at least two basic components: phenotypic autostabilization (differentiated cells stabilize their own phenotype) and interdependence for proliferation (differentiated cells stimulate the proliferation of alien phenotypes). In this modified autostabilization-selection model, cellular organization and growth arrest result from a quantitative equilibrium between the parameters controlling these two processes. An imbalance leads to tissue disorganization and invasive cancer-like growth. These findings suggest that cancer does not result solely from mutations in the cancerous cell but from the progressive addition of several small alterations of the equilibrium between autostabilization and interdependence for proliferation. In this frame, it is not solely the cancerous cell that is abnormal. The whole organism is involved. Tumor growth is a local effect of an imbalance between all the factors involved in tissue organization.

Self-renewal capacity is a widespread property of various types of neural crest precursor cells

In vertebrates, trunk neural crest (NC) generates glia, neurons, and melanocytes. In addition, it yields mesectodermal derivatives (connective tissues, chondrocytes, and myofibroblasts lining the blood vessels) in the head. Previous in vitro clonal analyses of avian NC cells unraveled a hierarchical succession of highly pluripotent, followed by various intermediate, progenitors, suggesting a model of progressive restrictions in the multiple potentialities of a totipotent stem cell, as prevails in the hematopoietic system. However, which progenitors are able to self-renew within the hierarchy of the NC lineages is still undetermined. Here, we explored further the stem cell properties of quail NC cells by means of in vitro serial subcloning. We identified types of multipotent and oligopotent NC progenitors that differ in their developmental repertoire, ability to self-maintain, and response to exogenous endothelin 3 according to their truncal or cephalic origin. The most striking result is that bipotent progenitors are endowed with self-renewal properties. Thus glia-melanocyte and glia-myofibroblast progenitors behave like stem cells in that they are able both to self-renew and generate a restricted progeny. In our culture conditions, glia-myofibroblast precursors display a modest capacity to self-renew, whereas glia-melanocyte precursors respond to endothelin 3 by extensive self-renewal. These findings may explain the etiology of certain multiphenotypic NC-derived tumors in humans. Moreover, the presence of multiple stem cell phenotypes along the NC-derived lineages may account for the rarity of the "totipotent NC stem cell" and may be related to the large variety and widespread dispersion of NC derivatives throughout the body.

Environmental signals and cell fate specification in premigratory neural crest

Neural crest cells are multipotent progenitors, capable of producing diverse cell types upon differentiation. Recent studies have identified significant heterogeneity in both the fates produced and genes expressed by different premigratory crest cells. While these cells may be specified toward particular fates prior to migration, transplant studies show that some may still be capable of respecification at this time. Here we summarize evidence that extracellular signals in the local environment may act to specify premigratory crest and thus generate diversity in the population. Three main classes of signals-Wnts, BMP2/BMP4 and TGFbeta1,2,3-have been shown to directly influence the production of particular neural crest cell fates, and all are expressed near the premigratory crest. This system may therefore provide a good model for integration of multiple signaling pathways during embryonic cell fate specification.

Quantitative modeling of stochastic systems in molecular biology by using stochastic Petri nets

An integrated understanding of molecular and developmental biology must consider the large number of molecular species involved and the low concentrations of many species in vivo. Quantitative stochastic models of molecular interaction networks can be expressed as stochastic Petri nets (SPNs), a mathematical formalism developed in computer science. Existing software can be used to define molecular interaction networks as SPNs and solve such models for the probability distributions of molecular species. This approach allows biologists to focus on the content of models and their interpretation, rather than their implementation. The standardized format of SPNs also facilitates the replication, extension, and transfer of models between researchers. A simple chemical system is presented to demonstrate the link between stochastic models of molecular interactions and SPNs. The approach is illustrated with examples of models of genetic and biochemical phenomena where the ULTRASAN package is used to present results from numerical analysis and the outcome of simulations.

The avian embryo as a model in developmental studies: chimeras and in vitro clonal analysis

The avian embryo is a model in which techniques of experimental embryology and cellular and molecular biology can converge to address fundamental questions of development biology. The first part of the chapter describes two examples of transplantation and cell labeling experiments performed in ovo. Thanks to the distinctive histologic and immunocytochemical characteristics of quail and chick cells, the migration and development of definite cells are followed in suitably constructed chimeric quail-chick embryos. Isotopic transplantations of neural tube portions between quail and chick, combined with in situ hybridization with a nucleic probe specific for a quail oligodendrocyte marker, allowed study of the origin and migration of oligodendroblasts in the spinal cord. Heterotopic transplantations of rhombomeres were performed to establish the degree of plasticity of these segments of the hindbrain regarding Hox gene expression, which was revealed by labeling with chick-specific nucleic probes. The second part describes in vitro cell cloning experiments devised to investigate cell lineage segregation and diversification during development of the NC. An original cloning procedure and optimal culture conditions permitted analysis of the developmental potentials of individual NC cells taken at definite migration stages. The results revealed a striking heterogeneity of the crest cell population, which appeared to be composed of precursors at different states of determination. Clonal cultures also provide a means to identify subsets of cells that are the target of environmental factors and to understand how extrinsic signals influence the development of responsive cells.

Lateral specification of cell fate during vertebrate development

Cells within equivalence groups interact via lateral specification to determine cell fates during development in Caenorhabditis elegans and other invertebrates. Populations of cells within the developing zebrafish have features similar to those of invertebrate equivalence groups. In a simple example, two identified zebrafish motoneurons behave as an equivalence pair in which one cell adopts a primary fate and interactions between the cells assign the other cell to a secondary fate. A more complicated situation exists for two initially equivalent populations of zebrafish neural crest cells. We consider whether mechanisms similar to those involved in fate specification within invertebrate equivalence groups also function furing fate specification in vertebrates.

Neural development. Fate diverted

Signals that alter cell fate are probably crucial in metazoan development. Glial growth factor may play such a role in the mammalian neural crest, by regulating the generation of neurons and Schwann cells.

Plasticity in neural crest cell differentiation

The neural crest is a pluripotent population of cells that are endowed with migratory capacities. It has long been known that the differentiation pathway taken by cells derived from the neural crest is largely controlled by the microenvironment to which they home after their migration phase, indicating a high degree of plasticity in their developmental fate. Recent progress has been made concerning the factors which influence survival, growth and differentiation of selected sets of precursors in each embryonic site colonised by derivatives of the neural crest.