Cytology in cnidaria using Exaiptasia as a model

A need exists for additional methods to examine cnidaria at the cellular level to aid our understanding of health, anatomy, and physiology of this important group of organisms. This need is particularly acute given that disease is emerging as a major factor in declines of ecologically important functional groups such as corals. Here we describe a simple method to process cnidarian cells for microscopic examination using the model organism Exaiptasia. We show that this organism has at least 18 cell types or structures that can be readily distinguished based on defined morphological features. Some of these cells can be related back to anatomic features of the animal both at the light microscope and ultrastructural level. The cnidome of Exaiptasia may be more complex than what is currently understood. Moreover, cnidarian cells, including some types of cnidocytes, phagocytize cells other than endosymbionts. Finally, our findings shed light on morphologic complexity of cell-associated microbial aggregates and their intimate intracellular associations. The tools described here could be useful for other cnidaria.

Immunocytochemical localization of a putative strychnine-sensitive glycine receptor in Hydra vulgaris

Previous biochemical studies have identified strychnine-sensitive glycine receptors in membrane preparations of Hydra vulgaris (Cnidaria: Hydrozoa). Electrophysiological and behavioral evidence has shown that these receptors play a role in modulating pacemaker activity and feeding behavior. Here, we present our genomic analysis that revealed hydra proteins having strong homology with the strychnine-binding region of the human receptor protein, GlyRα1. We further present immunocytochemical evidence for the specific labeling of cell and tissue preparations of hydra by a commercially available polyclonal anti-GlyRα1 antibody, selected through our genomic analysis. Tissue pieces and cell macerates from the upper and lower thirds of the body and ablated tentacles were double-labeled with this antibody and with an antibody specific for α-tubulin, to identify the glycine receptors and microtubules, respectively. Extensive receptor labeling was evident on the membranes, cell bodies and myonemes of endodermal and ectodermal epithelial cells, cell bodies and neurites of nerve cells, cnidocytes and interstitial cells. Labeling of the membranes of epithelial cells frequently corresponded to conspicuous varicosities (presumptive presynaptic sites) in the associated nerve net. Our findings support the idea that glycine receptors form an integral part of the nerve and effector systems that control hydra behavior.

The cell specificity of gene expression in the response to heat stress in corals

Previous transcriptional studies in heat-stressed corals have shown that many genes are responsive to generalized heat stress whereas the expression patterns of specific gene networks after heat stress show strong correlations with variation in bleaching outcomes. However, where these specific genes are expressed is unknown. In this study, we employed in situ hybridization to identify patterns of spatial gene expression of genes previously predicted to be involved in general stress response and bleaching. We found that tumor necrosis factor receptors (TNFRs), known to be strong responders to heat stress, were not expressed in gastrodermal symbiont-containing cells but were widely expressed in specific cells of the epidermal layer. The transcription factors AP-1 and FosB, implicated as early signals of heat stress, were widely expressed throughout the oral gastrodermis and epidermis. By contrast, a G protein-coupled receptor gene (GPCR) and a fructose bisphosphate aldolase C gene (aldolase), previously implicated in bleaching, were expressed in symbiont-containing gastrodermal cells and in the epidermal tissue. Finally, chordin-like/kielin (chordin-like), a gene highly correlated to bleaching, was expressed solely in the oral gastrodermis. From this study, we confirm that heat-responsive genes occur widely in coral tissues outside of symbiont-containing cells. Joint information about expression patterns in response to heat and cell specificity will allow greater dissection of the regulatory pathways and specific cell reactions that lead to coral bleaching.

Nervous system immunohistochemistry of the parasitic cnidarian Polypodium hydriforme at its free-living stage

Polypodium hydriforme, the only species in Polypodiozoa, which is currently considered a class of Cnidaria, and likely a sister group to Medusozoa (together with Myxozoa), is a cnidarian adapted to intracellular parasitism inside sturgeon oocytes. Free-living P. hydriforme lives on river bottoms; it walks on supporting tentacles and uses sensory tentacles to capture food and bring it to the mouth. The nervous system of free-living P. hydriforme was studied by confocal microscopy and immunohistochemistry using antibodies to FMRF-amide and α-tubulin combined with phalloidin-staining of F-actin fibres. A sensory FMRF-amide immunoreactive (IR) nerve net and an α-tubulin IR nerve net have been identified. The FMRF-amide IR nerve net underlies the epidermis along the tentacles and around the mouth; it consists of neurites emanating from epidermal sensory cells and basiepidermal ganglion cells, and it connects with cnidocytes. A deeper-lying α-tubulin IR nerve net occurs only in tentacles and looks like chains of different-sized beads crossing the mesoglea and entwining muscles. Anti-α-tubulin staining also reveals microtubules in muscle cells following the longitudinal muscle fibres or the thin circular F-actin fibres of the tentacles. Cnidocytes in the tentacles are embedded in a regular hexagonal non-neural network formed by the tubulin IR cytoskeleton of epidermal cells. Cnidocils of the cnidocytes around the mouth and in walking tentacles are identical, but those in sensory tentacles differ in length and width. The possible homology of the tubulin IR nerve net with motor nerve nets of cnidarians is discussed. The absence of a classic nerve ring around the mouth and the lack of specialised sense organs are considered to be plesiomorphic characters for Cnidaria.