Both the epithelial cells and the nerve cells are involved in the head inhibition properties in Hydra attenuata

Rethinking the Role of the Nervous System: Lessons From the Hydra Holobiont

Here we evaluate our current understanding of the function of the nervous system in Hydra, a non-bilaterian animal which is among the first metazoans that contain neurons. We highlight growing evidence that the nervous system, with its rich repertoire of neuropeptides, is involved in controlling resident beneficial microbes. We also review observations that indicate that microbes affect the animal's behavior by directly interfering with neuronal receptors. These findings provide new insight into the original role of the nervous system, and suggest that it emerged to orchestrate multiple functions including host-microbiome interactions. The excitement of future research in the Hydra model now relies on uncovering the common rules and principles that govern the interaction between neurons and microbes and the extent to which such laws might apply to other and more complex organisms.

Expression of Cnox-2, a HOM/HOX homeobox gene in hydra, is correlated with axial pattern formation

Cnox-2 is a HOM/HOX homeobox gene that we have identified in the simple metazoan Hydra vulgaris (Cnidaria: Hydrozoa). Cnox-2 is most closely related to anterior members of the Antennapedia gene complex from Drosophila, with the greatest similarity to Deformed. The Cnox-2 protein is expressed in the epithelial cells of adult hydra polyps in a region-specific pattern along the body axis, at a low level in the head and at a high level in the body column and the foot. The expression pattern of Cnox-2 is consistent with a role in axial pattern formation. Alteration of hydra axial patterning by treatment with diacylglycerol (DAG) results in an increase of head activation down the body column and in a coordinate reduction of Cnox-2 expression in epithelial cells in ‘head-like’ regions. These results suggest that Cnox-2 expression is negatively regulated by a signaling pathway acting through protein kinase C (PKC), and that the varying levels of expression of Cnox-2 along the body axis have the potential to result in differential gene expression which is important for hydra pattern formation.

A symbiogenetic theory for the origins of cnidocysts in Cnidaria

Did cnidarian cnidocysts originate from cnidocyst-bearing protoctistans living as symbiotic partners with an epithelial placula? If an increase in the fitness of symbiotic partners was "locked in" by an evolutionary stable strategy, co-evolution and compartmentalization could have led phyletically separate, eukaryotic symbionts to fuse and undergo nuclear merger. Traits originating in the symbiotic partners would have been brought to the "synthetic" organism and reworked through evolution into the development of an integrated organism. Support for the theory of symbiogenetic origins of Cnidaria rests on traces of symbiosis detected in the relationship of cnidarian epithelium to interstitial cells (I-cells), the precursors of cnidocyst-producing cnidoblasts: (1) epithelium and I-cell are autonomous and differ in morphology, cellular dynamics, the relationship of differentiation to proliferation and the variety of cell types formed; (2) hydras and planulas can be "cured" of I-cells and their derivatives, thereby creating "epithelial" animals which lack responsiveness but retain vegetative properties. (3) The reintroduction of I-cells into "epithelial" animals which lack responsiveness but retain vegetative properties. (3) The reintroduction of I-cells into "epithelial" animals restores missing differentiated cell and organismic characteristics. Symbiogenesis as a source of metazoan species has consequences for concepts of development, from the origins of cell lines to the evolution of differentiation.

Nucleotide sequence of an actin-encoding gene from Hydra attenuata: structural characteristics and evolutionary implications

We have determined the complete nucleotide sequence of an actin-encoding gene from Hydra attenuata as well as partial sequences of cDNA clones from two additional actin-encoding genes. The gene from the genomic clone contains a single intron, and has promoter and polyadenylation signals similar to those found in other species. The hydra genome has a very A + T-rich base composition (71%). This is reflected in the codon usage of the actin-encoding genes, which is strongly biased towards codons having A or T in the third position. The hydra actin-encoding gene family consists of three or more transcribed genes, two of which are very closely related to each other and probably arose by a recent gene duplication. Hydra actin, like other invertebrate actins, is more similar to the non-muscle isotypes of vertebrates than to the vertebrate muscle actins. Hydra actin is more similar to animal actins than to those of plants or fungi, which is consistent with the view that all metazoans arose from a single protist ancestor.

Gland cells arise by differentiation from interstitial cells in Hydra attenuata

The origin of the gland cells in asexually reproducing adult hydra is unclear. There is evidence suggesting that the gland cells are a self-renewing population as well as contrary evidence suggesting that they must arise from another cell type. We have reexamined the question and found the latter to be the case. Analysis of ectoderm/endoderm chimeras in which the ectoderm was labeled with [3H]thymidine indicates a precursor for gland cells in the ectoderm which migrates into the endoderm. Analysis of grafts between labeled lower halves and unlabeled upper halves of animals indicates the migratory precursor is either a large or a small interstitial cell. Measurement of the cell cycle times of the gland cells and the epithelial cells provided further support. The cell cycle time of the gland cells appears to be longer than that of the epithelial cells of the endoderm throughout the animal. This means that in the steady-state growth condition of hydra tissue, the gland cells cannot maintain their population size simply by cell division. These results and other data suggest the following dynamics for the gland cell population. Gland cells arise by differentiation from large interstitial cells, undergo a limited number of cell divisions, and then become postmitotic.

Selective disruption of gap junctional communication interferes with a patterning process in hydra

The cells that make up the body column of hydra are extensively joined by gap junctions, capable of mediating the rapid exchange of small hydrophilic molecules between the cytoplasms of neighboring cells. Both the rate of transfer of small molecules through the gap junctions and the rate of return of gap junction coupling after grafting experiments are sufficiently rapid to mediate events in the patterning of hydra tissue. Antibodies to the major rat liver gap junction protein (27,000 daltons) recognize a gap junction antigen in hydra and are effective in eliminating junctional communication between hydra cells. The antibodies perturb the head inhibition gradient in grafting operations, suggesting that cell-cell communication via gap junctions is important in this defined tissue patterning process.

Numerical simulations of the effects of retinoids on pattern formation in a hydroid

Retinoids have been shown to influence pattern formation in hydroid polyps (Hydractinia echinata) at various levels. These effects are counteracted by an inhibitor isolated from Hydra. The present study provides a theoretical attempt to elucidate the role of retinoids by computer simulations based on a simple model of pattern formation in Hydractinia. The elements of this model are morphogens of the Gierer-Meinhardt type, namely a long-range inhibitor and a short-range activator. From the calcualtions, a reducing effect of retinoids on the propagation of the inhibitor seems most probable.

Growth regulation of the interstitial cell population in hydra. I. Evidence for global control by nerve cells in the head

The interstitial cells of hydra form a multipotent stem cell system, producing terminally differentiated nerve cells and nematocytes during asexual growth. Under well-fed conditions the interstitial cell population doubles in size every 4 days. We have investigated the possible role of nerve cells in regulating this behavior. Nerve cells are normally found in highest concentrations in the head region of hydra, while interstitial cells are primarily located in the body column. Our experimental approach was to construct, by grafting, animals in which the density of nerve cells varied in (1) the head region, or (2) the body column. The growth of the interstitial cell population was then measured in these hydra. The results indicate that differences in head nerve cell density are closely correlated with how fast the interstitial cell population increases in size. Variations in the level of either nerve cells or interstitial cells in the body column showed no such correlation. These findings suggest the existence of a signaling mechanism in the head region. This signal, which is a function of the density of head nerve cells, emanates from the head tissue and exerts global control on the growth of the interstitial cell population in the body column.

A subset of cells in the nerve net of Hydra oligactis defined by a monoclonal antibody: its arrangement and development

A monoclonal antibody, termed JD1, was generated that bound to a subset of the nerve cells in the hypostome and tentacles of Hydra oligactis. Using a whole-mount technique the spatial pattern of the subset of nerve cells and their processes could be clearly visualized using indirect immunofluorescence. The subset largely corresponds to the epidermal sensory cells. Using the same technique the development of the pattern during head regeneration and budding was examined. The appearance of the nerve cells coincides with the formation of both the tentacles and hypostome. When head regeneration does not occur, JD1+ cells do not appear suggesting the differentiation of JD1+ cells is an integral event in head formation dependent on antecedent patterning processes.

Genetic analysis of developmental mechanisms in hydra. IX. Effect of food on development of a slow-budding strain (L4)

A mutant hydra strain L4 produces buds at a much lower rate than the standard wild-type strain (105) when fed with brine shrimp nauplii (T. Sugiyama and T. Fujisawa, 1979b, Dev. Growth Differ. 21, 361-375). It was found that addition of a small amount of tubifex worm tissue to the normal brine shrimp diet significantly improved the budding rate of L4 but not of 105. Detailed examination and comparison of food effect on various developmental processes in L4 and 105 have provided the following observations. (1) L4 development is strongly affected by food and has significantly lower rates than 105 in all the developmental parameters examined which involve the bud initiation process. In contrast, such effects and differences are not observed in parameters not involving bud initiation. These observations suggest that L4 has a defect(s) in its bud initiating mechanisms and that the expression of this defect is somehow strongly affected by food. (2) Epithelial cells proliferate in L4 nearly as rapidly as in 105 and without food effect. Epithelial cells produced by cell division are mostly utilized to form buds in mature 105 polyps. These cells, however, are not fully utilized for this purpose in L4 polyps which bud very slowly. Instead, they appear to be somehow lost from tissue in these animals.