Transgenic Hydra allow in vivo tracking of individual stem cells during morphogenesis

Fluorescent proteins and their use in marine biosciences, biotechnology, and proteomics

This review explores the field of fluorescent proteins (FPs) from the perspective of their marine origins and their applications in marine biotechnology and proteomics. FPs occur in hydrozoan, anthozoan, and copepodan species, and possibly in other metazoan niches as well. Many FPs exhibit unique photophysical and photochemical properties that are the source of exciting research opportunities and technological development. Wild-type FPs can be enhanced by mutagenetic modifications leading to variants with optimized fluorescence and new functionalities. Paradoxically, the benefits from ocean-derived FPs have been realized, first and foremost, for terrestrial organisms. In recent years, however, FPs have also made inroads into aquatic biosciences, primarily as genetically encoded fluorescent fusion tags for optical marking and tracking of proteins, organelles, and cells. Examples of FPs and applications summarized here testify to growing utilization of FP-based platform technologies in basic and applied biology of aquatic organisms. Hydra, sea squirt, zebrafish, striped bass, rainbow trout, salmonids, and various mussels are only a few of numerous instances where FPs have been used to address questions relevant to evolutionary and developmental research and aquaculture.

Symmetry breaking in stem cells of the basal metazoan Hydra

Among the earliest diverging animal phyla are the Cnidaria. Cnidaria were not only first in evolution having a tissue layer construction and a nervous system but also have cells of remarkable plasticity in their differentiation capacity. How a cell chooses to proliferate or to differentiate is an important issue in stem cell biology and as critical to human stem cells as it is to any other stem cell. Here I revise the key properties of stem cells in the freshwater polyp Hydra with special emphasis on the nature of signals that control the growth and differentiation of these cells.

Hydra molecular network reaches criticality at the symmetry-breaking axis-defining moment

We study biological, multicellular symmetry breaking on a hollow cell sphere as it occurs during hydra regeneration from a random cell aggregate. We show that even a weak temperature gradient directs the axis of the regenerating animal--but only if it is applied during the symmetry-breaking moment. We observe that the spatial distribution of the early expressed, head-specific gene ks1 has become scale-free and fractal at that point. We suggest the self-organized critical state to reflect long range signaling, which is required for axis definition and arises from cell next-neighbor communication.

Programmed cell death in Hydra

Hydra is one of the simplest metazoans and thus an important model organism for studies on the evolution of developmental mechanisms in multi-cellular animals. In Hydra apoptosis is involved in the regulation of cell numbers in response to feeding, in regeneration and in the removal of non-self cells. It also participates in the maintenance of cellular homeostasis in germ cells. During oogenesis a special "arrested" apoptosis of nurse cells is observed. The morphology of apoptotic hydra cells is almost indistinguishable from apoptosis in higher animals and caspases as well as members of the Bcl-2 family participate in the process.

Why polyps regenerate and we don't: towards a cellular and molecular framework for Hydra regeneration

The basis for Hydra's enormous regeneration capacity is the "stem cellness" of its epithelium which continuously undergoes self-renewing mitotic divisions and also has the option to follow differentiation pathways. Now, emerging molecular tools have shed light on the molecular processes controlling these pathways. In this review I discuss how the modular tissue architecture may allow continuous replacement of cells in Hydra. I also describe the discovery and regulation of factors controlling the transition from self-renewing epithelial stem cells to differentiated cells.

The Wnt code: cnidarians signal the way

Cnidarians are the simplest metazoans with a nervous system. They are well known for their regeneration capacity, which is based on the restoration of a signalling centre (organizer). Recent work has identified the canonical Wnt pathway in the freshwater polyp Hydra, where it acts in organizer formation and regeneration. Wnt signalling is also essential for cnidarian embryogenesis. In the sea anemone Nematostella vectensis 11 of the 12 known wnt gene subfamilies were identified. Different wnt genes exhibit serial and overlapping expression domains along the oral-aboral axis of the embryo (the 'wnt code'). This is reminiscent of the hox code (cluster) in bilaterian embryogenesis that is, however, absent in cnidarians. It is proposed that the common ancestor of cnidarians and bilaterians invented a set of wnt genes that patterned the ancient main body axis. Major antagonists of Wnt ligands (e.g. Dkk 1/2/4) that were previously known only from chordates, are also present in cnidarians and exhibit a similar conserved function. The unexpectedly high level of genetic complexity of wnt genes evolved in early multi-cellular animals about 650 Myr ago and suggests a radical expansion of the genetic repertoire, concurrent with the evolution of multi-cellularity and the diversification of eumetazoan body plans.

Bridging the regeneration gap: genetic insights from diverse animal models

Significant progress has recently been made in our understanding of animal regenerative biology, spurred on by the use of a wider range of model organisms and an increasing ability to use genetic tools in traditional models of regeneration. This progress has begun to delineate differences and similarities in the regenerative capabilities and mechanisms among diverse animal species, and to address some of the key questions about the molecular and cell biology of regeneration. Our expanding knowledge in these areas not only provides insights into animal biology in general, but also has important implications for regenerative medicine and stem-cell biology.

Hydra, a niche for cell and developmental plasticity

The silencing of genes whose expression is restricted to specific cell types and/or specific regeneration stages opens avenues to decipher the molecular control of the cellular plasticity underlying head regeneration in hydra. In this review, we highlight recent studies that identified genes involved in the immediate cytoprotective function played by gland cells after amputation; the early dedifferentiation of digestive cells into blastema-like cells during head regeneration, and the early late proliferation of neuronal progenitors required for head patterning. Hence, developmental plasticity in hydra relies on spatially restricted and timely orchestrated cellular modifications, where the functions played by stem cells remain to be characterized.