Overweight and Abdominal Obesity Association with All-Cause and Cardiovascular Mortality in the Elderly Aged 80 and Over: A Cohort Study

OBJECTIVE

To evaluate the association between overweight and abdominal obesity with all-cause and cardiovascular mortality in the elderly aged 80 and over.

DESIGN

A prospective cohort study.

SETTING

A population-based study of community-dwelling very elderly adults in a city in southern Brazil.

PARTICIPANTS

236 very elderly adults, number that represents 85% of the population aged 80 and over living in the city in the period (mean age 83.4 ± 3.2).

MEASUREMENTS

Overweight and abdominal obesity were assessed using recommended cut-off points for body mass index (BMI), waist circumference (WC), waist-hip ratio (WHR) and waist-height ratio (WHtR). The association between these anthropometric measurements and all-cause and cardiovascular mortality were independently estimated by Cox proportional hazards model. Kaplan-Meier was used to assess survival time.

RESULTS

Increased WC (>80cm F and >94cm M) and WHtR (>0.53 F and >0.52 M) were associated with lower all-cause mortality, but only WHtR remained associated even after controlling for residual confounding (HR 0.55 CI95% 0.36-0.84; p<0.001). Additionally increased WC was independently associated with lower mortality from cardiovascular diseases (HR 0.57 CI95% 0.34-0.95; p<0.030). BMI and WHR did not show significant independent association with mortality in the main analysis.

CONCLUSION

Greater abdominal fat accumulation, as estimated by WC and WHtR, presented an association with lower allcause and cardiovascular mortality in the elderly aged 80 and over, but not by BMI and WHR.

Male and female stem cells and sex reversal in Hydra polyps

Single interstitial stem cells of male polyps of Hydra magnipapillata give rise to clones that differentiate either male or female gametes. To test the sexual stability of these clones, stem cells were recloned. The results indicate that stem cells from female clones are stable in their sexual differentiation capacity; male stem cells, by comparison, switch sexual phenotype at the rate of 10(-2) per cell per generation. As a result, female polyps contain only female stem cells; male polyps contain a mixture of male and female stem cells. A model is presented in which the sexual phenotype of Hydra polyps is controlled by (i) the switching rate of male and female stem cells and (ii) the repression of female differentiation by male stem cells.

Oogenesis in Hydra: Nurse cells transfer cytoplasm directly to the growing oocyte

Oogenesis in Hydra occurs in so-called egg patches containing several thousand germ cells. Only one oocyte is formed per egg patch; the remaining germ cells differentiate as nurse cells. Whether and how nurse cells contribute cytoplasm to the developing oocyte has been unclear. We have used tissue maceration to characterize the differentiation of oocytes and nurse cells in developing egg patches. We show that nurse cells decrease in size at the same time that developing oocytes increase dramatically in volume. Nurse cells are also tightly attached to oocytes at this stage and confocal images of egg patches stained with the fluorescent membrane dye FM 4-64 clearly show large gaps (10 microm) in the cell membranes separating nurse cells from the developing oocyte. We conclude that nurse cells directly transfer cytoplasm to the developing oocyte. Following this transfer of cytoplasm, nurse cells undergo apoptosis and are phagocytosed by the oocyte. These results demonstrate that basic mechanisms of alimentary oogenesis typical of Caenorhabditis and Drosophila are already present in the early metazoan Hydra.

Identification of caspases and apoptosis in the simple metazoan Hydra

Apoptosis is a normal process by which cells die and are eliminated from tissue by phagocytosis [1]. It is involved in regulating cell numbers in adult tissues and in eliminating 'excess' cells during embryogenesis and development. Apoptosis is mediated by activation of caspases, which then cleave a variety of cellular substrates and thereby cause the characteristic morphology of apoptotic cells (rounded cells, condensed chromatin, susceptibility to phagocytosis) [2]. Although apoptosis has been well documented in nematodes, insects and mammals, it is not yet clear how early in evolution apoptosis or its component enzymes arose. In the simple metazoan Hydra vulgaris, cell death regulates cell numbers [3] [4] [5]. In starved animals, for example, epithelial cell proliferation continues at a nearly normal rate although the tissue does not increase in size; the excess cells produced are eliminated by phagocytosis. Cell death can also be induced in wild-type hydra by treatment with colchicine [6] or in a mutant strain (sf-1) by temperature shock [7]. Here, we show that cell death in hydra is morphologically indistinguishable from apoptosis in higher animals, that hydra polyps express two genes with strong homology to members of the caspase 3 family, and that caspase-3-specific enzyme activity accompanies apoptosis in hydra. The occurrence of apoptosis and caspases in a member of the ancient metazoan phylum Cnidaria supports the idea that the invention of apoptosis was an essential feature of the evolution of multicellular animals.

Spinalin, a new glycine- and histidine-rich protein in spines of Hydra nematocysts

Here we present the cloning, expression and immunocytochemical localization of a novel 24 kDa protein, designated spinalin, which is present in the spines and operculum of Hydra nematocysts. Spinalin cDNA clones were identified by in situ hybridization to differentiating nematocytes. Sequencing of a full-length clone revealed the presence of an N-terminal signal peptide, suggesting that the mature protein is sorted via the endoplasmic reticulum to the post-Golgi vacuole in which the nematocyst is formed. The N-terminal region of spinalin (154 residues) is very rich in glycines (48 residues) and histidines (33 residues). A central region of 35 residues contains 19 glycines, occurring mainly as pairs. For both regions a polyglycine-like structure is likely and this may be stabilized by hydrogen bond-mediated chain association. Similar sequences found in loricrins, cytokeratins and avian keratins are postulated to participate in formation of supramolecular structures. Spinalin is terminated by a basic region (6 lysines out of 15 residues) and an acidic region (9 glutamates and 9 aspartates out of 32 residues). Western blot analysis with a polyclonal antibody generated against a recombinant 19 kDa fragment of spinalin showed that spinalin is localized in nematocysts. Following dissociation of the nematocyst's capsule wall with DTT, spinalin was found in the insoluble fraction containing spines and the operculum. Immunocytochemical analysis of developing nematocysts revealed that spinalin first appears in the matrix but then is transferred through the capsule wall at the end of morphogenesis to form spines on the external surface of the inverted tubule and the operculum.

Stimulation of tentacle and bud formation by the neuropeptide head activator in Hydra magnipapillata

Stimulation of epithelial cell cycling by the neuropeptide head activator was analyzed in Hydra magnipapillata and compared with the action of head activator on bud formation and tentacle formation during head regeneration. The results obtained indicate that head activator treatment stimulates epithelial cell division and induces the formation of more tentacle-specific epithelial cells. The number of additional epithelial cells which undergo mitosis during head activator treatment accounts for the increased number of epithelial cells present in the regenerated tentacles. Therefore, the head activator stimulation of tentacle formation can be explained by the mitogenic action of head activator on tentacle cell precursors. To analyze stimulation of bud formation by head activator, polyps of different developmental age were tested under conditions of long-term treatment, and effects on bud formation were compared with effects on epithelial cell proliferation. Head activator treatment strongly stimulated bud formation, but had no detectable effect on epithelial cell numbers. Bud formation occurs at smaller polyp size as a result of head activator treatment, indicating that head activator significantly interferes with the patterning system in hydra.

Systematic isolation of peptide signal molecules regulating development in hydra: LWamide and PW families

To isolate new peptide signal molecules involved in regulating developmental processes in hydra, a novel screening project was developed. Peptides extracted from the tissue of Hydra magnipapillata were systematically purified to homogeneity using HPLC. A fraction of each purified peptide was examined by differential display-PCR for its ability to affect gene expression in hydra. Another fraction was used to determine the tentative structure using an amino acid sequence analyzer and/or a mass spectrometer. Based on the results, peptides of potential interest were selected for chemical synthesis, followed by confirmation of the identity of the synthetic with the native peptides using HPLC. Using this approach, 286 peptides have been isolated, tentative amino acid sequences have been determined for 95 of them, and 19 synthetic peptides identical to native ones were produced. The 19 synthetic peptides were active in a variety of biological tests. For example, Hym-54 stimulated muscle contraction in adult polyps of hydra and sea anemone, Anthopleura fuscoviridis, and induced metamorphosis of planula, the larval stage, into polyps in a marine hydrozoan species, Hydractinia serrata. Another peptide, Hym-33H, inhibited nerve cell differentiation in hydra and induced tissue contraction in planula of Hydractinia serrata. The evidence obtained so far suggests that hydra contains a large number (>350) of peptide signal molecules involved in regulating developmental or other processes in cnidaria. These peptides can be isolated and their functions examined systematically with the new approach developed in this study.

Pattern of differentiated nerve cells in hydra is determined by precursor migration

The nervous system of the fresh water polyp hydra is built up as a nerve net spread over the whole body, with higher densities in the head and the foot. In adult hydra, as a result of continuous growth, new nerve cell differentiation takes place continuously. The pattern of nerve cell differentiation and the role of nerve cell precursor migration in establishing the pattern have been observed in vivo by vitally labelling precursor cells with DiI. The results indicate that nerve cell precursors arise directly from stem cells, complete a final cell cycle and divide, giving rise to two daughter cells, which differentiate into nerve cells. A subpopulation of the nerve cell precursors are migratory for a brief interval at the onset of the terminal cell cycle, then complete the cell cycle and divide at the site of differentiation. Labelling small patches of tissue in the head, body column and peduncle/foot with DiI indicated that formation of nerve cell precursors was nearly constant at all three positions. However, at least half of the labelled precursors in the body column migrated to the head or foot before differentiating; by contrast, precursors in head and foot differentiated in situ without significant migration. This redistribution leads to a net increase of nerve cell precursors in head and foot compared to body column and thus to the higher density of nerve cells in these regions.

Ks1, an epithelial cell-specific gene, responds to early signals of head formation in Hydra

As a molecular marker for head specification in Hydra, we have cloned an epithelial cell-specific gene which responds to early signals of head formation. The gene, designated ks1, encodes a 217-amino acid protein lacking significant sequence similarity to any known protein. KS1 contains a N-terminal signal sequence and is rich in charged residues which are clustered in several domains. ks1 is expressed in tentacle-specific epithelial cells (battery cells) as well as in a small fraction of ectodermal epithelial cells in the gastric region subjacent to the tentacles. Treatment with the protein kinase C activator 12-O-tetradecanoylphorbol-13-acetate (TPA) causes a rapid increase in the level of ks1 mRNA in head-specific epithelial cells and also induces ectopic ks1 expression in cells of the gastric region. Sequence elements in the 5′-flanking region of ks1 that are related to TPA-responsive elements may mediate the TPA inducibility of ks1 expression. The pattern of expression of ks1 suggests that a ligand-activated diacyglycerol second messenger system is involved in head-specific differentiation.

Fibrous Mini-Collagens in Hydra Nematocysts

Nematocysts (cnidocysts) are exocytotic organelles found in all cnidarians. Here, atomic force microscopy and field emission scanning electron microscopy reveal the structure of the nematocyst capsule wall. The outer wall consists of globular proteins of unknown function. The inner wall consists of bundles of collagen-like fibrils having a spacing of 50 to 100 nanometers and cross-striations at intervals of 32 nanometers. The fibrils consist of polymers of "mini-collagens," which are abundant in the nematocysts of Hydra. The distinct pattern of mini-collagen fibers in the inner wall can provide the tensile strength necessary to withstand the high osmotic pressure (15 megapascals) in the capsules.

Hydra tropomyosin TROP1 is expressed in head-specific epithelial cells and is a major component of the cytoskeletal structure that anchors nematocytes

A cDNA clone encoding a 253 amino acid tropomyosin was isolated from Hydra in a differential screen for head-specific genes. The Hydra tropomyosin gene, designated trop1, is a single copy gene, lacks introns and is strongly expressed in tentacle-specific epithelial cells. Analysis of protein synthesis in head and gastric tissue indicated a high rate of tropomyosin synthesis in head tissue. Immunolocalization of tropomyosin in tentacle tissue revealed a cushion-like tropomyosin-containing structure within battery cells at the base of nematocytes. The structure appears to form part of the cytoskeletal anchor for nematocytes. Tropomyosin cushions were also observed in epithelial cells along the body column, which contain mounted stenotele nematocytes.

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.

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.

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.

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.

Decision making in interstitial stem cells of Hydra

Interstitial stem cells in Hydra are a continuously proliferating and differentiating cell population. They represent a useful model system for studying mechanisms controlling stem cell differentiation. Here we review our current knowledge of the differentiation potential of these cells. Interstitial stem cells are multipotent and able to differentiate into several different cell types. The differentiation decisions appear to be controlled by positional signals and by the composition of the cellular environment. Since interstitial stem cells can be cultured in an in vivo environment and appear to be accessible to experimental manipulation by a range of new molecular techniques, an in vivo analysis of the molecular mechanisms underlying stem cell decision making can now be approached.

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.

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.

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.

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.

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.

Dependence of cell-type proportioning and sorting on cell cycle phase in Dictyostelium discoideum

The relationship between the cell cycle phase of vegetative amoebae and prestalk and prespore differentiation in the slug stage were investigated in the slime mould Dictyostelium discoideum. Cells were synchronized by release from the stationary phase. Samples were taken at various times during the course of a synchronous cell doubling, fluorescently labelled and mixed with cells of random cell cycle phase from exponentially growing cultures. The fate of the fluorescently labelled cells was recorded at the slug stage. Cells early in the cycle exhibit strong prestalk sorting; cells taken later in the cycle exhibit strong prespore sorting. The period of prestalk sorting occurs immediately following mitosis and lasts about 1 h in a cell cycle of about 7 h duration. Accompanying the altered sorting behaviour is a marked changed in the prestalk-prespore proportions in slugs formed from synchronized populations of cells. Cells synchronized early in the cycle form slugs with 55% prespore cells; cells synchronized late in the cycle form slugs with 90% prespore. The results are discussed in terms of models for the formation of the prestalk-prespore pattern in slugs.

A revision of the Dictyostelium discoideum cell cycle

We have investigated the Dictyostelium discoideum cell cycle using fluorometric determinations of cellular and nuclear DNA contents in exponentially growing cultures and in synchronized cultures. Almost all cells are in G2 during both growth and development. There is no G1 period, S phase is less than 0.5 h, and G2 has an average length of 6.5 h in axenically grown cells. Mitochondrial DNA, which constitutes about half of the total DNA, is replicated throughout the cell cycle. There is no difference in the nuclear DNA contents of axenically grown and bacterially grown cells. Thus the long cell cycle in axenically grown cells is due to a lengthening of the G2 phase.

Growth regulation in Hydra: relationship between epithelial cell cycle length and growth rate

The relationship between epithelial cell production and growth rate was investigated in Hydra attenuata under different feeding regimes. The increase of epithelial cell number was compared to the duration of the epithelial cell cycle using standard methods of cell cycle analysis. The results indicate that cell cycle changes accompanying changes in feeding regime are not sufficient to explain the altered growth rate. Under heavy feeding regimes, epithelial cell production equals tissue growth rate. At low feeding level or under starvation conditions the epithelial cell cycle lengthens and growth rate of epithelial cell population is slowed. However, the cell cycle changes are insufficient to account for the reduction in tissue growth and thus there is an effective overproduction of epithelial cells amounting to 10% per day. Evidence suggests that these excess cells are phagocytized by neighboring cells in the tissue. Thus phagocytosis is directly or indirectly involved in regulating the growth of hydra tissue.

Loss of differentiating nematocytes induced by regeneration and wound healing in Hydra

Cell death was observed in the nematocyte differentiation pathway in Hydra during head and foot regeneration. This death occurs throughout the regenerating piece, is transient in nature and is selective for committed stenotele and desmoneme precursors. Proliferating nematoblasts are unaffected. Cell death appears to be caused by release of a toxic factor rather than loss of a hormone required for differentiation, since regenerating pieces released a factor that inactivated differentiating nematocytes, and injured animals that had intact head and foot tissue also lost differentiating nematocytes. The inactivated nematocytes are removed by phagocytosis by epitheliomuscular cells.

Oxygen gradients cause pattern orientation in Dictyostelium cell clumps

We have investigated the formation of the prestalk-prespore pattern in Dictyostelium discoideum. Pattern formation occurs in clumps of Dictyostelium cells embedded in agar under a 100% oxygen atmosphere. Agar embedding allows us to control spatially the environment surrounding the cell clumps. Our results suggest that the ambient oxygen concentration plays a role in controlling the size of the multicellular mass. Further, oxygen gradients established across clumps embedded in agar or held in holes in a plastic barrier cause orientation of the prestalk-prespore pattern such that the anterior prestalk region forms at the highest end of the gradient. The results also indicate that developing cells have the ability to migrate up a gradient of oxygen.

Commitment during nematocyte differentiation in Hydra

Nematocytes in Hydra differentiate from interstitial stem cells. Desmonemes differentiate mainly in the distal half of the body column while stenoteles differentiate predominantly in the proximal half. This difference was used to determine the timing of nematocyte-type commitment in the differentiation pathway. Cells were transferred from distal or proximal regions to all positions in the body column to test when the proportion of stenotele and desmoneme differentiation changed to reflect the new environment. In the first experiment, the distal region of the body column was isolated and permitted to regenerate a whole Hydra. In the second experiment, dissociated cells from distal or proximal regions were transplanted into regenerating aggregates of Hydra tissue. Both experiments effectively transferred cells from distal or proximal positions to positions throughout the body column. By comparing the kinetics of stenotele and differentiation with the time required for distal or proximal cells to differentiate stenoteles and desmonemes in accord with their new environment, it was possible to conclude that stenotele and desmoneme commitment occurs during the terminal cell cycle prior to nematocyte differentiation and not at the stem cell. Additional experiments indicated that the number of rounds of cell division preceding differentiation is fixed at the time stem cells enter the nematocyte pathway.

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.

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.

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.

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 Cycle Kinetics and Development of Hydra Attenuata

The differentiation of nerve cells and nematocytes in Hydra attenuata has been investigated by labelling interstitial cell precursors with [3H]thymidine and following by autoradiography the appearance of labelled, newly differentiated cells. Nematocyte differentiation occurs only in the gastric region where labelled nematoblasts appear 12 h and labelled nematocytes 72-96 h after addition of [3H]thymidine. Labelled nerves appear in hypostome, gastric region, and basal disk about 18 h after addition of [3H]thymidine. The lag in the appearance of labelled cells includes cell division of the precursor as well as differentiation since nerves and nematocytes have 2n postmitotic nuclear DNA content.

A cell flow model is proposed for interstitial cells and their differentiated products. Stem cells occur as single interstitial cells or in pairs. Per cell generation about 60 % of the daughter cells of stem cell divisions remain stem cells and about 40 % differentiate nerves and nematocytes. Nerves differentiate directly from stem cells in about 1 day. Nematocyte differentiation requires 5-7 days including proliferation of a cluster of 4, 8, 16 or 32 interstitial cells and differentiation of a nematocyst capsule in each cell. The numbers of interstitial cells and nematoblasts predicted by the cell flow model from the rates of nerve differentiation (900 nerves/day/ hydra), nematocyte differentiation (1760 nematocyte nests/day/hydra) and stem cell proliferation (stem cell cycle = 24 h), agree with the numbers of these cells observed in hydra. The number of stem cells per hydra is 3000-6000 depending on assumptions about the time of determination. The ratio of nematocyte to nerve differentiation averaged over the whole hydra is 3:1. In the hypostome and basal disk interstitial cell differentiation occurs exclusively to nerve cells while in the gastric region the ratio of nematocyte to nerve differentiation is about 7:1.

Cell Cycle Kinetics and Development of Hydra Attenuata

The cell cycle parameters of interstitial cells in Hydra attenuata have been determined. Interstitial cells were classified according to cluster size in which they occur (1, 2, 4, 8 or 16 cells) and morphology using maceration preparations and histological sections. The lengths of G1, S, G2 and M were determined by standard methods of cell cycle analysis using pulse-chase and continuous labelling with [3H]- and [14C]thymidine. Nuclear DNA contents were measured microfluorimetrically. All classes of interstitial cells proliferate but the cell cycle of large interstitial cells occurring singly or in pairs is longer than that of interstitial cells occurring in clusters of 4, 8 and 16 cells. The S-phase is 11-12 h long and G1 is less than 1 h for all classes of interstitial cells. G2 is 3-4 h long for interstitial cells in clusters of 4, 8 and 16 cells giving these cells a total cell cycle duration of 16-17 h. In contrast, large interstitial cells occurring as singles and in clusters of 2 have G2 durations ranging from 4 to 22 h. Two subpopulations can be discerned among these cells, one having a G1 of about 6 h and a total cell cycle of about 19 h, the other having an average G2 of 14 h and a total cell cycle of about 27 h. The differences in cell cycle duration appear to be associated with interstitial cell function. Cells having a short cell cycle are probably committed to nematocyte differentiation, while large interstitial cells having long and variable cell cycles appear to be undetermined stem cells responsible for proliferating further interstitial cells. The variable length of G2 in these cells suggest it as a possible control point.

Quantitative analysis of cell types during growth and morphogenesis in Hydra

Tissue maceration was used to determine the absolute number and the distribution of cell types in Hydra. It was shown that the total number of cells per animal as well as the distribution of cells vary depending on temperature, feeding conditions, and state of growth. During head and foot regeneration and during budding the first detectable change in the cell distribution is an increase in the number of nerve cells at the site of morphogenesis. These results and the finding that nerve cells are most concentrated in the head region, diminishing in density down the body column, are discussed in relation to tissue polarity.

Cell Cycle Kinetics and Development of Hydra Attenuata

The cell cycle parameters of epithelial cells of Hydra attenuata are described. Specifically the rate of proliferation and the fraction of proliferating cells have been determined under conditions of defined growth rate. Techniques involved standard methods of cell cycle analysis using histological and tissue maceration preparations; pulse-chase and continuous labelling with [3H]thymidine followed by autoradiographic analysis, and microspectrophotometric determination of nuclear DNA content in single cells. The results indicate that more than 90% of hydra epithelial cells are actively proliferating with a cell cycle duration about equal to the tissue doubling time. In well fed hydra the average cell cycle is about 3 days long. S period is 12-15 h, G1 0-1 h, and mitosis 1.5 h. Most of the cell cycle consists of a long G2 period of variable duration (24-72 h). The results provide no evidence for a subpopulation of rapidly proliferating cells as predicted by ‘growth zone’ models of hydra morphogenesis. The results also indicate that the population of epithelial cells is self-sustaining requiring no input by differentiation from other cell types. The long and variable G2 period means that DNA synthesis and the following cell division are effectively uncoupled such that inhibitors of DNA synthesis may not stop epithelial cell division. The variable nature of the G2 period suggests it as a possible point of control of hydra growth.

Ferritin in the fungus Phycomyces

The iron-protein ferritin has been purified from mycelium, sporangiophores, and spores of the fungus Phycomyces blakesleeanus. It has a protein-to-iron ratio of 5, a sedimentation coefficient of 55S, a buoyant density in CsCl of 1.82 g/cm(3), and the characteristic morphology of ferritin in the electron microscope. Apoferritin prepared from Phycomyces ferritin has a sedimentation coefficient of 18S and consists of subunits of molecular weight 25,000. In the cytoplasm of Phycomyces, ferritin is located on the surface of lipid droplets (0.5-2.0 micro in diameter) where it forms crystalline monolayers which are conspicuous in electron micrographs of sporangiophore thin-sections. Ferritin is found in all developmental stages of Phycomyces but is concentrated in spores. The level of ferritin iron is regulated by the iron level in the growth medium, a 50-fold increase occurring on iron-supplemented medium.

Phycomyces

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