Cnidarians: an evolutionarily conserved model system for regeneration?

Cnidarians are among the simplest metazoan animals and are well known for their remarkable regeneration capacity. They can regenerate any amputated head or foot, and when dissociated into single cells, even intact animals will regenerate from reaggregates. This extensive regeneration capacity is mediated by epithelial stem cells, and it is based on the restoration of a signaling center, i.e., an organizer. Organizers secrete growth factors that act as long-range regulators in axis formation and cell differentiation. In Hydra, Wnt and TGF-beta/Bmp signaling pathways are transcriptionally up-regulated early during head regeneration and also define the Hydra head organizer created by de novo pattern formation in aggregates. The signaling molecules identified in Cnidarian regeneration also act in early embryogenesis of higher animals. We suppose that they represent a core network of molecular interactions, which could explain at least some of the mechanisms underlying regeneration in vertebrates.

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.

Identification of a Hydra homologue of the beta-catenin/plakoglobin/armadillo gene family.

The beta-catenin/plakoglobin/armadillo gene family encodes a group of highly conserved proteins which play important roles in cadherin-mediated cell adhesion and in signal transduction mechanisms involved in regulating development. This gene family previously had been isolated only from higher metazoans. Here, we describe the isolation and characterization of a beta-catenin (beta Ctn) homologue from Hydra magnipapillata, a diploblastic lower metazoan. Comparison of the putative amino acid (aa) sequence of Hydra beta Ctn, with its homologues in higher metazoans, shows that a repeating 42-aa motif present in its central domain is highly conserved throughout the metazoa. This suggests that beta Ctn appeared very early in metazoan evolution, possibly when primitive multicellular animals started to form epithelial cell layers.

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.