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Biomedical subjects

C N David

Publications and source records attributed to C N David.

At least 19 recordsLinked to original sources
PubMed1992

The primitive metazoan Hydra expresses antistasin, a serine protease inhibitor of vertebrate blood coagulation: cDNA cloning, cellular localisation and developmental regulation.

We have isolated and characterized cDNAs from Hydra which encode antistasin, a potent inhibitor of factor Xa in the vertebrate blood clotting cascade. Hydra antistasin is expressed in gland cells and represents a major class of transcripts from Hydra's head. Sequence analysis revealed that Hydra antistasin contains 6 internal repeats of a 25-26 amino acid sequence with a highly conserved pattern of 6 cysteine and 2 glycine residues identical to that in leech antistasin. Conservation of antistasin in a lower metazoan provides a potential link between the vertebrate and invertebrate coagulation systems.

Amino Acid Sequence↗
PubMed1991

Interstitial stem cell proliferation in hydra: evidence for strain-specific regulatory signals.

We have examined the growth behavior of small numbers of interstitial stem cells transplanted into tissue of genetically unrelated strains of Hydra magnipapillata. We show that such stem cells, which are at low density following transplantation, proliferate more rapidly than the stem cells of the host, which are at normal density. The rapid proliferation is similar to the proliferation rate of stem cells transplanted into interstitial cell free tissue. The results suggest that stem cells transplanted into heterotypic tissue are unable to "sense" the presence of host stem cells and to adopt their growth rate to that of the surrounding cells. Thus, the feedback signal which negatively regulates stem cell growth as a function of stem cell density must be strain specific.

Animals↗
PubMed1991

Pattern of epithelial cell cycling in hydra.

We have investigated the spatial pattern of epithelial cell cycling in a mutant strain of Hydra magnipapillata (sf-1). This strain has temperature sensitive interstitial stem cells and thus polyps containing only epithelial cells can be obtained by growth at the restrictive temperature. Epithelial animals were pulse labeled with the thymidine analog 5'-bromo-2'-deoxyuridine (Brdu) and stained with anti-Brdu antibody to visualize S phase cells. Our results indicate that Brdu-labeled cells are broadly and fairly evenly distributed along the body column. Feeding stimulates a rapid decrease and then an increase in labeled cells in gastric tissue; labeled cells in the head are not affected. Starvation leads to a twofold decrease in labeled cells in the gastric region; the density of labeled cells in head tissue remains similar to that in well-fed animals. During bud formation the number of labeled epithelial cells increases significantly in the evaginating bud. During head regeneration the number of labeled cells declines sharply during the first 12 hr and then increases to a density typical of head tissue by 24-36 hr of regeneration. The results indicate the release of signals by feeding and regeneration which inhibit mitosis. By contrast head tissue and developing buds express signals stimulating mitosis. Thus changes in epithelial cell cycling in hydra are closely correlated with morphogenetic events as well as with feeding stimuli.

Animals↗
PubMed1991

Mini-collagens in hydra nematocytes.

We have isolated and characterized four collagen-related c-DNA clones (N-COL 1, N-COL 2, N-COL 3, N-COL 4) that are highly expressed in developing nematocytes in hydra. All four c-DNAs as well as their corresponding transcripts are small in size (600-1,000 bp). The deduced amino acid sequences show that they contain a central region consisting of 14 to 16 Gly-X-Y triplets. This region is flanked amino-terminal by a stretch of 14-23 proline residues and carboxy-terminal by a stretch of 6-9 prolines. At the NH2- and COOH-termini are repeated patterns of cysteine residues that are highly conserved between the molecules. A model is proposed which consists of a central stable collagen triple helix of 12-14 nm length from which three 9-22 nm long polyproline II type helices emerge at both ends. Disulfide linkage between cysteine-rich segments in these helices could lead to the formation of oligomeric network structures. Electrophoretic characterization of nematocyst extracts allows resolution of small proline-rich polypeptides that correspond in size to the cloned sequences.

Amino Acid Sequence↗
PubMed1990

Transplantation stimulates interstitial cell migration in hydra.

Migration of interstitial cells and nerve cell precursors was analyzed in Hydra magnipapillata and Hydra vulgaris (formerly Hydra attenuata). Axial grafts were made between [3H]thymidine-labeled donor and unlabeled host tissue. Migration of labeled cells into the unlabeled half was followed for 4 days. The results indicate that the rate of migration was initially high and then slowed on Days 2-4. Regrafting fresh donor tissue on Days 2-4 maintained high levels of migration. Thus, migration appears to be stimulated by the grafting procedure itself.

Animals↗
PubMed1990

Cloned interstitial stem cells grow as contiguous patches in hydra.

The migration of interstitial cells was analyzed during the growth of stem cell clones in vivo. The spatial distribution of cloned cells was analyzed at a time by which extensive migration of interstitial cells could have occurred. All interstitial cell clones were found to form large contiguous patches of cells. The results indicate that there is little migration of large interstitial cells in undisturbed tissue during normal growth. This finding is surprising since numerous grafting experiments have shown extensive migration of these cells. The implications of finding nonrandomly distributed stem cells are discussed.

Animals↗
PubMed1990

Cell cycle length, cell size, and proliferation rate in hydra stem cells.

We have analyzed the cell cycle parameters of interstitial cells in Hydra oligactis. Three subpopulations of cells with short, medium, and long cell cycles were identified. Short-cycle cells are stem cells; medium-cycle cells are precursors to nematocyte differentiation; long-cycle cells are precursors to gamete differentiation. We have also determined the effect of different cell densities on the population doubling time, cell cycle length, and cell size of interstitial cells. Our results indicate that decreasing the interstitial cell density from 0.35 to 0.1 interstitial cells/epithelial cell (1) shortens the population doubling time from 4 to 1.8 days, (2) increases the [3H]thymidine labeling index from 0.5 to 0.75 and shifts the nuclear DNA distribution from G2 to S phase cells, and (3) decreases the length of G2 in stem cells from 6 to 3 hr. The shortened cell cycle is correlated with a significant decrease in the size of interstitial stem cells. Coincident with the shortened cell cycle and increased growth rate there is an increase in stem cell self-renewal and a decrease in stem cell differentiation.

Animals↗
PubMed1990

Putative intermediates in the nerve cell differentiation pathway in hydra have properties of multipotent stem cells.

We have investigated the properties of nerve cell precursors in hydra by analyzing the differentiation and proliferation capacity of interstitial cells in the peduncle of Hydra oligactis, which is a region of active nerve cell differentiation. Our results indicate that about 50% of the interstitial cells in the peduncle can grow rapidly and also give rise to nematocyte precursors when transplanted into a gastric environment. If these cells were committed nerve cell precursors, one would not expect them to differentiate into nematocytes nor to proliferate apparently without limit. Therefore we conclude that cycling interstitial cells in peduncles are not intermediates in the nerve cell differentiation pathway but are stem cells. The remaining interstitial cells in the peduncle are in G1 and have the properties of committed nerve cell precursors (Holstein and David, 1986). Thus, the interstitial cell population in the peduncle contains both stem cells and noncycling nerve precursors. The presence of stem cells in this region makes it likely that these cells are the immediate targets of signals which give rise to nerve cells.

Animals↗
PubMed1986

Gland cells in Hydra: cell cycle kinetics and development.

The proliferative capacity of gland cells in Hydra attenuata was investigated. The results indicate that both gland cell proliferation and interstitial cell differentiation to gland cells contribute to the maintenance of the whole population. On the basis of [3H]thymidine incorporation and nuclear DNA measurements, gland cells consist of at least three different populations. One population consists of rapidly proliferating cells with a cell cycle of about 72 h. These cells are distributed throughout the body column. In the lower gastric region there is a population of non-cycling cells in G2 while in the upper gastric region there is a population of non-cycling cells in G1. About half the G1 population becomes a new antigen, SEC 1, which is typical of mucus cells.

Animals↗
PubMed1979

Stem cell growth and differentiation in Hydra attenuata. I. Regulation of the self-renewal probability in multiclone aggregates.

Interstitial stem cells in Hydra are rapidly proliferating multipotent stem cells which continuously give rise to precursors for nerve and nematocyte differentiation. Growth of the stem cell population is controlled by the cell cycle time of the stem cells and the self-renewal probability, Ps (the fraction of stem cells in each generation which divide to yield more stem cells). In normal Hydra the stem cell generation time is 24 h and Ps = 0.6; under these conditions the stem cell population doubles in 3.5 days. In the present experiments we have systematically investigated the dependence of Ps on stem cell density. We culture stem cells in a feeder layer system consisting of aggregates of nitrogen-mustard (NM)-inactivated Hydra cells. In this system stem cell density can be varied over a wide range by changing the number of clone-forming units (CFU) added to the aggregates. We have measured the growth rate of the stem cell population and the cell cycle of stem cells in NM aggregates after 4--7 days of culture. From these data we calculate the value of Ps. The results indicate that the growth rate decreases 4-fold as the number of CFU seeded per aggregate increases from 10 to 400. Under these same conditions the cell cycle remains constant. The values of Ps calculated from these results indicate the Ps decreases from 0.75 in aggregates seeded with 10--30 CFU to 0.55 in aggregates seeded with 200--400 CFU. These results support a model in which Ps is controlled by negative feedback from neighbouring stem cells. In addition, our experiments indicate that Ps decreases during the growth of stem cell clones. When only a few stem cells are seeded in aggregates, they give rise to isolated clones distributed throughout the aggregate. Ps decreases markedly within such clones as they grow in size presumably due to increasing stem cell content of the clones. Since Ps in such isolated clones declines with growth, we infer that the local stem cell concentration is what controls Ps and that the spatial range of the negative feedback signal is short compared to the dimensions of NM aggregates.

Animals↗
PubMed1979

Stem cell growth and differentiation in Hydra attenuata. II. Regulation of nerve and nematocyte differentiation in multiclone aggregates.

The differentiation of nerve cells and nematocytes from interstitial stem cells in Hydra has been investigated under conditions of changing stem cell density. Interstitial stem cells were cultured in a feeder layer system consisting of aggregates of nitrogen mustard-inactivated tissue. The aggregates were seeded with varying numbers of stem cells from 10 to 400 per aggregate; between 4 and 7 days later the rates of nerve and nematocyte differentiation were measured. Nerve differentiation was scored by labelling the stem cell population with [3H]-thymidine and counting nests of 4 proliferating nematoblasts. In both cases the numbers of differentiating cells were normalized to the size of the stem cell population. The results indicate that the rate of nematocyte differentiation increases as the concentration of stem cells increases in aggregates; under the same conditions the rate of nerve differentiation remains essentially constant. To calculate the numbers of stem cells entering each pathway per generation, a computer was programmed to simulate the growth and differentiation of interstitial stem cells. Standard curves were prepared from the simulations relating the rates of nerve and nematocyte differentiation to the fraction of stem cells committed to each pathway per generation. The rates of nerve and nematocyte commitment were then estimated from the experimentally observed rates of differentiation using the standard curves. The results indicate that nerve commitment remains constant at about 0.13 stem cells per generation over a wide range of stem cell concentration. Nematocyte commitment, by comparison, increases from 0.15 to 0.21 stem cells per generation as stem cell concentration increases in aggregates. The fact that the ratio of nerve to nematocyte commitment changes under our conditions suggests that stem cell commitment is not a stochastic process but subject to control by environmental stimuli.

Animals↗
PubMed1979

Ammonia plus another factor are necessary for differentiation in submerged clumps of Dictyostelium.

Differentiation of Dictyostelium amoebae can occur in submerged clumps of cells; under an oxygen atmosphere mature stalk cells and spores form, as has been shown in previous work. This report shows that at least 2 factors are released by the cells under these conditions, and that both, together, are required for differentiation of stalk cells and spores. One of the factors is ammonia (NH3 + NH4+). The other factor(s) is heat stable and dialysable but has not yet been further characterized. The factors can be collected in conditioned medium and, when added to cells, stimulate differentiation. Conditioned medium loses its biological activity upon the removal of the NH3 + NH4+. When NH3 + NH4+ is added back, activity is restored. Because NH3 + NH4+, alone, has no activity, a second factor(s) in the conditioned medium must be required for differentiation. It is also shown that calcium inhibits differentiation in submerged clumps and that in calcium-free medium the timing of differentiation is essentially the same as under aerial conditions.

Ammonia↗
PubMed1978

Regulation of the self-renewal probability in Hydra stem cell clones.

Hydra interstitial stem cells continuously give rise to daughter stem cells as well as precursors for nerve and nematocyte differentiation. Growth of the stem cell population is controlled by the self-renewal probability (Ps): Ps is the fraction of stem cell daughters that remain stem cells in each generation. We have determined Ps for Hydra interstitial stem cells by using a novel technique based on the cell conposition of clones. Stem cell clones were grown in aggregates of nitrogen mustard-inactivated Hydra tissue. They contain several hundred cells after 14 days of growth, including stem cells, differentiating nematocytes, and differentiating nerve cells. Clone size, size variability, and the ratio of differentiating cells to stem cells are sensitive measures of Ps. We have prepared standard curves relating these parameters to Ps, using computer simulations of clone growth. Comparisoon of the experimentally observed parameter of clones to these curves indicates that Ps decreases from 0.8 in 5- to 6-day clones to 0.6 in 10- to 12-day clones. The decrease in Ps coincides with the increase in clone size and suggest that Ps may be regulated by the density of stem cells in clones. Such a mechanism could be responsible for the observed homeostasis of stem cell populations in vivo.

Cell Differentiation↗