Interactions between the foot and bud patterning systems in Hydra vulgaris

Morphogenesis by coupled regulatory networks: reliable control of positional information and proportion regulation

Based on a non-equilibrium mechanism for spatial pattern formation we study how position information can be controlled by locally coupled discrete dynamical networks, similar to gene regulation networks of cells in a developing multicellular organism. As an example we study the developmental problems of domain formation and proportion regulation in the presence of noise, as well as in the presence of cell flow. We find that networks that solve this task exhibit a hierarchical structure of information processing and are of similar complexity as developmental circuits of living cells. Proportion regulation is scalable with system size and leads to sharp, precisely localized boundaries of gene expression domains, even for large numbers of cells. A detailed analysis of noise-induced dynamics, using a mean-field approximation, shows that noise in gene expression states stabilizes (rather than disrupts) the spatial pattern in the presence of cell movements, both for stationary as well as growing systems. Finally, we discuss how this mechanism could be realized in the highly dynamic environment of growing tissues in multicellular organisms.

Thypedin, the multi copy precursor for the hydra peptide pedin, is a β-thymosin repeat-like domain containing protein

Pedin, a peptide of 13 amino acids, stimulates foot formation in hydra, one of the simplest metazoan animals. Here, we show that the corresponding transcripts are 3.8 kb in size encoding a precursor protein with a size of about 110 kDa, which contains 13 copies of the peptide. Interestingly, the deduced amino acid sequence of the precursor comprises 27 copies of a beta-thymosin-like repeat domain. Hence, we named the precursor protein thypedin. Pedin transcripts are present along the body axis of the animal with slightly higher abundance in the foot to bud region and in the head. Pedin is expressed mainly in epithelial cells of the ectoderm and endoderm. During budding it is present in the evaginating bud. The early appearance of transcripts during phases of cell-fate specification like budding indicates that pedin may be involved in differentiation processes in hydra. This is confirmed by the fact that pedin stimulates bud outgrowth. Thymosin-repeat containing proteins are well known for their regulatory influence on actin polymerisation. Here we show the first indirect evidence that thypedin may be able to interact with actin as well. Since actin polymerisation and depolymerisation processes are known to take place during morphogenetic processes, these findings may hint at new aspects of the function of pedin and its precursor in pattern formation in hydra.

Control of foot differentiation in Hydra: Phylogenetic footprinting indicates interaction of head, bud and foot patterning systems

Homeodomain transcription factor CnNK-2 seems to play a major role in foot formation in Hydra. Recently, we reported in vitro evidence indicating that CnNK-2 has autoregulatory features and regulates expression of the morphogenetic peptide pedibin. We proposed that CnNK-2 and pedibin synergistically orchestrate foot differentiation processes. Here, we further analyzed the regulatory network controlling foot formation in Hydra. By phylogenetic footprinting we compared the CnNK-2 5'-flanking sequence from two closely related species, Hydra vulgaris and Hydra oligactis. Unexpectedly, we detected a highly conserved binding site for HNF-3beta, a vertebrate Forkhead transcription factor, in the CnNK-2 5'-flanking region. The Hydra HNF-3beta homolog budhead is predominantly expressed in the apical region of the body column and early during budding. Budhead is absent from tissue expressing CnNK-2 and thought to be involved in determining tissue for head differentiation. By electrophoretic mobility shift assays we demonstrate an in vitro interaction between recombinant budhead protein and the interspecific conserved HNF-3beta binding motif in the CnNK-2 5'-flanking region. Our results strengthen the view of CnNK-2 as an important regulator during foot patterning processes. Furtheron, they point to budhead as a candidate for a transcriptional regulator of CnNK-2 and to an interaction of foot and head patterning processes in Hydra on the molecular level.

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.

Selective inhibition of protein kinases blocks the formation of a new axis, the beginning of budding, in Hydra

In Hydra, head regeneration and bud formation appear to be very similar processes. The fact that there are genes whose expression is specific for one of the two processes suggests that they do not have identical molecular bases. We analyzed the signal transduction pathways regulating bud development using inhibitors of protein kinase C, Src, PI3K and ERK. The four inhibitors reversibly blocked bud formation in Hydra when applied before stage 1. Once the bud reached stage 3, three of them had no effect and the bud developed normally. The inhibitors blocked the expression of Budhead, an early head marker, and of CnOtx which are specific for bud formation. The results are in agreement with the central role of a signaling pathway mediated by Src on bud development.

Interactions between the foot and the head patterning systems in Hydra vulgaris

The Cnidarian, hydra, is an appealing model system for studying the basic processes underlying pattern formation. Classical studies have elucidated much basic information regarding the role of development gradients, and theoretical models have been quite successful at describing experimental results. However, most experiments and computer simulations have dealt with isolated patterning events such as the dynamics of head regeneration. More global events such as interactions among the head, bud, and foot patterning systems have not been extensively addressed. The characterization of monoclonal antibodies with position-specific labeling patterns and the recent cloning and characterization of genes expressed in position-specific manners now provide the tools for investigating global interactions between patterning systems. In particular, changes in the axial positional value gradient may be monitored in response to experimental perturbation. Rather than studying isolated patterning events, this approach allows us to study patterning over the entire animal. The studies reported here focus on interactions between the foot and the head patterning systems in Hydra vulgaris following induction of a foot in close proximity to a head, axial grafting of a foot closer to the head, or doubling the amount of basal tissue by lateral grafting of an additional peduncle-foot onto host animals. Resulting positional value changes as monitored by antigen (TS19) and gene (ks1 and CnNK-2) expression were assessed in the foot, head, and intervening tissue. The results of the experiments indicate that positional values changed rapidly, in a matter of hours, and that there were reciprocal interactions between the foot and the head patterning systems. Theoretical interpretations of the results in the form of computer simulations based on the reaction-diffusion model are presented and predict many, but not all, of the experimental observations. Since the lateral grafting experiment cannot, at present, be simulated, it is discussed in light of what has been learned from the axial grafting experiments and their simulations.