Inhibitory gradients of head and foot regeneration in Hydra viridis

Signaling gradients in surface dynamics as basis for planarian regeneration

Based on experimental data, we introduce and analyze a system of reaction-diffusion equations for the regeneration of planarian flatworms. We model dynamics of head and tail cells expressing positional control genes that translate into localized signals which in turn guide stem cell differentiation. Tissue orientation and positional information are encoded in a long range wnt-related signaling gradient. Our system correctly reproduces typical cut and graft experiments, and improves on previous models by preserving polarity in regeneration over orders of magnitude in body size during growth phases. Key to polarity preservation in our model flatworm is the sensitivity of cell differentiation to gradients of wnt-related signals relative to the tissue surface. This process is particularly relevant in small tissue layers close to cuts during their healing, and modeled in a robust fashion through dynamic boundary conditions.

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.

Isolation and characterization of two new morphogenetically active peptides from Hydra vulgaris

Foot-specific differentiation processes in hydra are controlled by activating and inhibiting potentials. In an attempt to understand the molecular mechanisms underlying these processes, two substances were isolated from Hydra vulgaris that stimulate foot-specific differentiation measured as acceleration of foot regeneration. These substances were shown to be peptides of 13 and 21 amino acids, respectively, with sequences that bear no significant homology to known peptides or proteins. Polyclonal antibodies were raised against both peptides. The data obtained by biological and radioimmunoassays show that the shorter peptide, pedin, is an excellent candidate for a major component of the ‘foot-activating potential’.

Hydra transplantation phenomena and the mechanism of Hydra head regeneration. II. Properties of the head activation

Measurements were made of the "head activation" in transplanted fragments of Hydra tissue. The measurements confirm that head activation is graded in the body and that activation increases during head regeneration. Experiments of two different sorts show that regeneration-specific activation increase is confined to the presumptive head zone. When head formation is initiated and subsequently blocked, the activation decays back to its original level in about 12 hr; this is three times faster than the activation decay observed in body tissue moved to a more basal position. Experiments on foot formation phenomena show a similar lability difference. It thus appears that regeneration-specific activation increase involves a mechanism different from the one responsible for the gradient of activation in the body. Cutting produces a local increase of the head and/or foot activation level; this increase decays in about 12 hr. These results and those of the preceding paper are consistent with a version of the Gierer-Meinhardt model which also accounts for the regulation of the head/body proportion in Hydra.

Hydra transplantation phenomena and the mechanism of hydra head regeneration. I. Properties of the head inhibition

Measurements were made of the "head inhibition" and the "head inhibition gradient" in Hydra. Following decapitation, the head inhibition decays with a half-time of 2-3 hr. The slope of the inhibition gradient increases about twofold when the temperature is raised from 20 to 24 degrees C; the increase (at the higher temperature) occurs with a half-time of about 1.2 hr. These results are consistent with the idea that the head inhibition and the head inhibition gradient are due to a diffusing substance, made in the head and broken down in the body of Hydra, with a half-life of about 2 hr at 20 degrees C and about 1 hr at 24 degrees C. Calculations suggest a diffusion constant of 1-3 X 10(-6) cm2/sec and a gradient range (ratio of maximum to minimum concentration) of about 2. During head regeneration, the head inhibition returns much more slowly than one would expect if its production were switched on at the time of "head determination." The slow return of the inhibition can be explained if one assumes that determination is due to the "activation" of a few cells, while the restoration of inhibition depends on the lateral expansion of the activated region. This behavior is observed in a "proportion-regulating" diffusion-reaction model proposed by Gierer and Meinhardt, 1972, Kybernetik 12, 30-39. Transplants to beheaded animals demonstrate a substantial inhibition gradient even at 6 hr after decapitation, a time at which most of the inhibitor from the original head should have decayed and no substantial amount of inhibitor is expected from the regenerated head. Experiments in which parts of a hydra's body are removed suggest that this inhibition gradient is due to the production of some inhibitor in the body, with larger amounts produced in apical portions.

Isolation of a substance activating foot formation in hydra

We have developed an assay for a substance from hydra that accelerates foot regeneration in the animal. This substance is specific for the foot as evidenced by the following findings: (1) It is present in the animal as a steep gradient descending from foot to head, paralleling the foot-forming potential of the tissue (2) It does not accelerate head regeneration, nor do the head factors of hydra discovered by Schaller (1973) and Berking (1977) accelerate foot regeneration. We propose that the foot-activating substance is a morphogen responsible for foot formation in hydra. The foot activator can be extracted from hydra tissue with methanol and separated from other known morphogens of hydra by gel filtration and ion-exchange chromatography. A substance with similar biological and physicochemical properties can be isolated from sea anemones.

The budding region as source of diffusible inhibitors of head and foot regeneration inHydra viridis

An implication of Crick's (1970, 1971) "source-sink" model of diffusion gradients is that the addition of sources along the length of a gradient would cause it to bulge away from linearity in the direction of the source. The activity of the budding region ofHydra viridis as a source of the diffusible inhibitors of head and of foot regeneration is investigated in this light, by placing multiple series of budding regions along the length of the inhibitory gradients of head and of foot regeneration previously described (Shostak, 1972, 1973).Samples of 30-50 animals were used to determine the frequencies of head and of foot regeneration at each graft border formed by grafting 2 to 5 gastric-plus budding regions in tandem, the distal one having a terminal apical head, and the proximal one part of a host animal having terminal basal peduncle and foot. These frequencies were compared to the corresponding frequencies at appropriate distances along the lengths of the inhibitory gradients, as computed from the linear equations for these gradients based on earlier work. The curves for the regeneration of heads and feet for animals with three or fewer additional budding regions deviate in the direction of greater inhibition, but do not differ significantly from the gradients on animals having only grafted gastric regions. The curves for animals with four additional budding regions, however, bulge out toward greater inhibition, and differ with statistical significance from the linear inhibitory gradients at the fourth graft border.The results show, therefore, that the budding region is a source of both the inhibitors of head and of foot regeneration also produced by the head and foot, respectively. The suggestion arises that the diffusible inhibitors produced by the morphogenetically active regions ofHydra have no effect on the normal homeostatic processes and budding occurring in these regions, but normally prevent regeneration that might otherwise occur during cellular turnover.

Morphogenetic gradients in moltiple-graft Hydra viridis. I. The effects of colcemid and colchicine

The morphogenesis of so-called secondary (2 degree) heads, budding regions and feet, and separations at graft borders, is studied in multiple-graft animals containing three gastric regions (3g animals) treated with Colcemid or colchicine. Control animals consist of three non-treated pieces. Animals consisting of two non-treated and one treated piece are also employed. The main effects of Colcemid are the promotion of graft healing at the distal graft border, and of 2 degree head formation on the proximal gastric region (g-1). Colchicine also promotes graft healing at the distal graft border, but in contrast to the effect of Colcemid, promotes the formation of ectopic feet (tertiary feet) and inhibits head regeneration at the distal end of the middle gastric region (g-2). Colchicine also accelerates an inversion of polarity in 2 degree budding regions on the g-2 pieces of animals with 2 degree heads on their g-1 pieces. Treatment of a distal graft piece with Colcemid promotes 2 degree head formation on the nearest proximal piece which is not treated, but the inhibition of 2 degree head formation by colchicine occurs only on colchicine treated pieces. The morphogenetic effects of the drugs are interpreted as consequences of their actions on nerve differentiation and cell division. The paper argues that homeostatic control of a hydra's cell population depends on dividing cells influencing cell loss, and that similar mechanisms are involved in the rejection of a graft and the separation of a bud.