Boundary cells of endodermal origin define the mouth of Hydra vulgaris (Cnidaria)

From injury to patterning—MAPKs and Wnt signaling in Hydra

Hydra has a regenerative capacity that is not limited to individual organs but encompasses the entire body. Various global and integrative genome, transcriptome and proteome approaches have shown that many of the signaling pathways and transcription factors present in vertebrates are already present in Cnidaria, the sister group of Bilateria, and are also activated in regeneration. It is now possible to investigate one of the central questions of regeneration biology, i.e., how does the patterning system become activated by the injury signals that initiate regeneration. This review will present the current data obtained in Hydra and draw parallels with regeneration in Bilateria. Important findings of this global analysis are that the Wnt signaling pathway has a dual function in the regeneration process. In the early phase Wnt is activated generically and in a second phase of pattern formation it is activated in a position specific manner. Thus, Wnt signaling is part of the generic injury response, in which mitogen-activated protein kinases (MAPKs) are initially activated via calcium and reactive oxygen species (ROS). The MAPKs, p38, c-Jun N-terminal kinases (JNKs) and extracellular signal-regulated kinases (ERK) are essential for Wnt activation in Hydra head and foot regenerates. Furthermore, the antagonism between the ERK signaling pathway and stress-induced MAPKs results in a balanced induction of apoptosis and mitosis. However, the early Wnt genes are activated by MAPK signaling rather than apoptosis. Early Wnt gene activity is differentially integrated with a stable, β-Catenin-based gradient along the primary body axis maintaining axial polarity and activating further Wnts in the regenerating head. Because MAPKs and Wnts are highly evolutionarily conserved, we hypothesize that this mechanism is also present in vertebrates but may be activated to different degrees at the level of early Wnt gene integration.

Hydra's feeding response: Effect of GABAB ligands on GSH-induced electrical activity in the hypostome of H. vulgaris

The feeding response in the Cnidarian, hydra, consists of mouth opening, tentacle writhing, and the cessation of pacemaker-controlled tentacle and body contractions. The behavior can be induced by reduced glutathione (GSH), contained in body fluids that leak from prey impaled by hydra's cnidocysts. Mouth, tentacle, and body-contraction behavior is carried out by hydra's ectodermal and endodermal epitheliomuscular cells. Here, we present the first evidence of GSH-induced electrical activity in the hypostome and its modification by GABA and GABA_B ligands. The 'heads' of hydra were ablated and the tentacles removed. Suction electrodes, positioned on the mouth, recorded electrical activity produced by GSH, contained either within the electrode, or in the surrounding bath, the mouth being shielded. Recorded impulses were characterized, according to size and temporal pattern, as small, medium and (large) pacemaker impulses. GSH applied in the bath caused a frequency increase of small and medium impulses and a decrease in pacemaker bursts. The changes in frequencies of medium and pacemaker bursts, though not obviously affected by GABA, were counteracted by blocking GABA_B inhibition with phaclofen. Only the highest concentration of GSH applied at the mouth potentially decreased pacemaker frequency and potentially increased medium impulses, without affecting small impulses. GABA caused a significant increase in small and medium impulses relative to GSH which was counteracted by baclofen and/or baclofen plus phaclofen. The results indicate that considerable GSH-receptor circuitry is located in hypostomal tissue proximal to hydra's mouth, and substantiate GABA and GABA_B inhibition within the neuroeffector network of the feeding response.

The two nerve rings of the hypostomal nervous system of Hydra vulgaris-an immunohistochemical analysis

In Hydra vulgaris, physiological and pharmacological evidence exists for a hypostomal circumferential neuro-effector pathway that initiates ectodermal pacemaker activity at tentacular-hypostomal loci coordinating body and tentacle contractions. Here, we describe an ectodermal nerve ring that runs below and between the tentacles, and an anti-GABA_B receptor antibody-labeled ring coincident with it. The location of this ring is consistent with the physiology of the hypostomal pacemaker systems of hydra. We also describe a distally located, ectodermal ring of nerve fibers that is not associated with anti-GABA_B receptor antibody labeling. The neurites and cell bodies of sensory cells contribute to both rings. The location of the distal ring and its sensory cell neurites suggests an involvement in the behavior of the mouth. Between the two rings is a network of anastomosing sensory and ganglion cell bodies and their neurites. Phase contrast, darkfield, and antibody-labeled images reveal that the mouth of hydra comprises five or six epithelial folds whose endoderm extensively labels with anti-GABA_B receptor antibody, suggesting that endodermal metabotrobic GABA receptors are also involved in regulating mouth behavior.

Regional modulation of the response to glutathione in Hydra vulgaris

In the presence of prey, or upon exposure to reduced glutathione (GSH), Hydra polyps open a mouth to ingest the captured prey and close it after feeding; at rest the mouth is not evident. In previous papers we have shown that GABA, glycine and NMDA modulate the mechanisms of mouth closure through ligand-gated-ion-channel receptors that are similar to their mammalian analogues in terms of biochemical and pharmacological properties. In order to study the regional distribution of these receptors, we have applied the GSH assay to polyps amputated at different levels of the body column. The response to 1-10 µmol l(-1) GSH of polyps lacking either peduncle and foot or the entire body columns (heads) was not different from control, whole animals. In the presence of GABA or muscimol, duration of the response was significantly decreased in heads; the decrease was suppressed by the GABA antagonists gabazine and bicuculline. By contrast, in animals lacking peduncle and foot, duration of the response did not vary upon GABA administration. Conversely, in the presence of glycine, duration of the response in heads preparations was similar to control, whereas in footless polyps, it was significantly reduced. The decrease was mimicked by the glycine agonists taurine and β-alanine, and counteracted by strychnine. These results suggest a regional distribution of receptors to GABA and glycine in the neuromuscular circuitry modulating the feeding behaviour.