Seasonal variations of digestive enzymes in sea cucumbers (Holothuria tubulosa, G. 1788) under culture conditions

Sea cucumber, Holothuria tubulosa (Gmelin, 1788), is an economically valuable species due to its rich nutrients content that being exported in Turkey. However, culture of this species is difficult due to a lack of knowledge. The main objective of this study is to investigate seasonal activities of main digestive enzymes (protease, lipase, and amylase) for nutritional requirements. In this sense, sea cucumbers were stocked in the glass aquarium and sediment was 10 cm. It was replaced monthly and enriched (1% of the living individual weight) by dried Sargassum sp. Total protease and amylase enzymes have shown higher activity compared to lipase. All three enzyme activities were maximum in the summer and minimum in the winter. In summer, the highest measured total protease, amylase, and lipase activity values were 6.45 ± 0.66, 6.77 ± 0.72, and 2.78 ± 0.32 U/mg per protein, respectively. In winter, the lowest total protease, amylase, and lipase activity values were measured as 2.03 ± 0.16, 1.14 ± 0.14, and 0.12 ± 0.01 U/mg per protein, respectively. As a conclusion of the study, seasonal expression of the main digestive enzymes was strictly dependent on water temperatures and food abundance and also it was ideal to feed this species with food containing high protein and carbohydrate under appropriate temperatures for commercial culture.

Temperature-induced aerobic scope and Hsp70 expression in the sea cucumber Holothuria scabra

The Aerobic Scope (AS), which reflects the functional capacity for biological fitness, is a highly relevant proxy to determine thermal tolerance in various taxa. Despite the importance of this method, its implementation is often hindered, due to lacking techniques to accurately measure standard- (SMR) and maximal- (MMR) metabolic rates, especially in sluggish marine invertebrates with low oxygen consumption rates, such as sea cucumbers. In this study the AS concept was modified to define a Temperature-induced Aerobic Scope (TAS), based on metabolic rate changes due to temperature adjustments rather than traditionally used physical activity patterns. Consequentially, temperature dependent peak and bottom O₂ consumption rates, defined as Temperature-induced Maximal- (TMMR) and Standard Metabolic Rates (TSMR), respectively, served as MMR and SMR alternatives for the sea cucumber Holothuria scabra. TMMR and TSMR were induced through acute temperature change (2°C per hour; 17–41°C) until critical warm (WT_crit) and cold (CT_crit) temperatures were reached, respectively. In addition, Hsp70 gene expression linked to respiration rates served as synergistic markers to confirm critical threshold temperatures. O₂ consumption of H. scabra peaked distinctly at WT_crit of 38°C (TMMR = 33.2 ± 4.7 μgO₂ g^-1 h^-1). A clear metabolic bottom line was reached at CT_crit of 22°C (TSMR = 2.2 ± 1.4 μgO₂ g^-1 h^-1). Within the thermal window of 22–38°C H. scabra sustained positive aerobic capacity, with assumed optimal performance range between 29–31.5°C (13.85–18.7 μgO₂ g^-1 h^-1). Between 39–41°C H. scabra decreased respiration progressively, while gene expression levels of Hsp70 increased significantly at 41°C, indicating prioritization of heat shock response (HSR) and homeostatic disruption. At the cold end (17–22°C) homeostatic disruption was visible through incrementally increasing energetic expenses to fuel basal maintenance costs, but no Hsp70 overexpression occurred. TMMR, TSMR and TAS proved to be reliable metrics, similar to the traditional energetic key parameters MMR, SMR and AS, to determine a specific aerobic performance window for the sluggish bottom dwelling species H. scabra. In addition, the linkage between respiration physiology and molecular defense mechanisms showed valuable analytical synergies in terms of mechanistic prioritization as response to thermal stress. Overall, this study will help to define lethal temperatures for aquaculture and to predict the effects of environmental stress, such as ocean warming, in H. scabra.

A comparative experimental approach to ecotoxicology in shallow-water and deep-sea holothurians suggests similar behavioural responses

Exploration of deep-sea mineral resources is burgeoning, raising concerns regarding ecotoxicological impacts on deep-sea fauna. Assessing toxicity in deep-sea species is technologically challenging, which promotes interest in establishing shallow-water ecotoxicological proxy species. However, the effects of temperature and hydrostatic pressure on toxicity, and how adaptation to deep-sea environmental conditions might moderate these effects, are unknown. To address these uncertainties we assessed behavioural and physiological (antioxidant enzyme activity) responses to exposure to copper-spiked artificial sediments in a laboratory experiment using a shallow-water holothurian (Holothuria forskali), and in an in situ experiment using a deep-sea holothurian (Amperima sp.). Both species demonstrated sustained avoidance behaviour, evading contact with contaminated artificial sediment. However, A. sp. demonstrated sustained avoidance of 5mgl^-1 copper-contaminated artificial sediment whereas H. forskali demonstrated only temporary avoidance of 5mgl^-1 copper-contaminated artificial sediment, suggesting that H. forskali may be more tolerant of metal exposure over 96h. Nonetheless, the acute behavioural response appears consistent between the shallow-water species and the deep-sea species, suggesting that H. forskali may be a suitable ecotoxicological proxy for A. sp. in acute (≤24h) exposures, which may be representative of deep-sea mining impacts. No antioxidant response was observed in either species, which was interpreted to be the consequence of avoiding copper exposure. Although these data suggest that shallow-water taxa may be suitable ecotoxicological proxies for deep-sea taxa, differences in methodological and analytical approaches, and in sex and reproductive stage of experimental subjects, require caution in assessing the suitability of H. forskali as an ecotoxicological proxy for A. sp. Nonetheless, avoidance behaviour may have bioenergetic consequences that affect growth and/or reproductive output, potentially impacting fecundity and/or offspring fitness, and thus influencing source-sink dynamics and persistence of wider deep-sea populations.

Cellular responses and HSP70 expression during wound healing in Holothuria tubulosa (Gmelin, 1788)

Wound repair is a key event in the regeneration mechanisms of echinoderms. We studied, at the behavioural, cellular and molecular levels, the wound healing processes in Holothuria tubulosa after injuries to the body wall. The experiments were performed for periods of up to 72 h, and various coelomocyte counts, as well as the expression of heat shock proteins (HS27, HSP70 and HSP90), were recorded. Dermal wound healing was nearly complete within 72 h. In the early stages, we observed the injured animals twisting their bodies to keep their injuries on the surface of the water for the extrusion of the buccal pedicles. At the cellular level, we found time-dependent variations in the circulating coelomocyte counts. After injury, in particular, we observed a significant increase in spherule cells at 2.5 h post-injury. Using the western blot method, we observed and reported that the wounds produced, compared with controls, a significant increase in HSP27 and HSP70 expression in coelomocytes, whereas HSP70 was increased in scar tissue and HSP90 was increased only in cell-free coelomic fluid. These results highlight that the wounds were responsible for the stress condition with the induction of cellular and biochemical responses.

Expression of pluripotency factors in echinoderm regeneration

Cell dedifferentiation is an integral component of post-traumatic regeneration in echinoderms. As dedifferentiated cells become multipotent, we asked if this spontaneous broadening of developmental potential is associated with the action of the same pluripotency factors (known as Yamanaka factors) that were used to induce pluripotency in specialized mammalian cells. In this study, we investigate the expression of orthologs of the four Yamanaka factors in regeneration of two different organs, the radial nerve cord and the digestive tube, in the sea cucumber Holothuria glaberrima. All four pluripotency factors are expressed in uninjured animals, although their expression domains do not always overlap. In regeneration, the expression levels of the four genes were not regulated in a coordinated way, but instead showed different dynamics for individual genes and also were different between the radial nerve and the gut. SoxB1, the ortholog of the mammalian Sox2, was drastically downregulated in the regenerating intestine, suggesting that this factor is not required for dedifferentiation/regeneration in this organ. On the other hand, during the early post-injury stage, Myc, the sea cucumber ortholog of c-Myc, was significantly upregulated in both the intestine and the radial nerve cord and is therefore hypothesized to play a central role in dedifferentiation/regeneration of various tissue types.

Organochlorine pollutants in Western Antarctic Peninsula sediments and benthic deposit feeders

Sediments and benthic deposit feeding holothurians were collected near the Palmer Long Term Ecological Research grid during the austral winter of 2008. Polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs) were measured in Western Antarctic Peninsula continental shelf sediments, porewater, and benthic biota. Concentrations and fluxes in sediments decreased sharply away from the tip of the peninsula toward its interior. Sedimentary PCB fluxes were an order of magnitude lower than reported elsewhere, supporting the notion of a pristiner Antarctic environment. Hexa-chlorinated biphenyls dominated (40-100%) the PCB profiles in the sediments, while trichlorinated biphenyl 28 was the most abundant PCB congener in the porewater. PCB and OCP concentrations in holothurians were comparable to concentrations in other low trophic level biota in the Antarctic food web (i.e., krill). The partitioning of PCBs and OCPs between the sediments and porewater can be explained by a dual-mode model, which included both organic carbon and black carbon as partitioning media. Alternatively, a simpler one-parameter prediction assuming coal tar-like organic carbon performed equally well in explaining porewater concentrations The majorities of PCBs (63-94%) in the Western Antarctic Peninsula sediments were bound to black carbon or recalcitrant tar-like organic carbon, thereby lowering porewater concentrations. PCBs and OCPs in the holothurians were in equilibrium with those in the porewater.

Common cellular events occur during wound healing and organ regeneration in the sea cucumber Holothuria glaberrima

BACKGROUND

All animals possess some type of tissue repair mechanism. In some species, the capacity to repair tissues is limited to the healing of wounds. Other species, such as echinoderms, posses a striking repair capability that can include the replacement of entire organs. It has been reported that some mechanisms, namely extracellular matrix remodeling, appear to occur in most repair processes. However, it remains unclear to what extent the process of organ regeneration, particularly in animals where loss and regeneration of complex structures is a programmed natural event, is similar to wound healing. We have now used the sea cucumber Holothuria glaberrima to address this question.

RESULTS

Animals were lesioned by making a 3-5 mm transverse incision between one of the longitudinal muscle pairs along the bodywall. Lesioned tissues included muscle, nerve, water canal and dermis. Animals were allowed to heal for up to four weeks (2, 6, 12, 20, and 28 days post-injury) before sacrificed. Tissues were sectioned in a cryostat and changes in cellular and tissue elements during repair were evaluated using classical dyes, immmuohistochemistry and phalloidin labeling. In addition, the temporal and spatial distribution of cell proliferation in the animals was assayed using BrdU incorporation. We found that cellular events associated with wound healing in H. glaberrima correspond to those previously shown to occur during intestinal regeneration. These include: (1) an increase in the number of spherule-containing cells, (2) remodeling of the extracellular matrix, (3) formation of spindle-like structures that signal dedifferentiation of muscle cells in the area flanking the lesion site and (4) intense cellular division occurring mainly in the coelomic epithelium after the first week of regeneration.

CONCLUSION

Our data indicate that H. glaberrima employs analogous cellular mechanisms during wound healing and organ regeneration. Thus, it is possible that regenerative limitations in some organisms are due either to the absence of particular mechanisms associated with repair or the inability of activating the repair process in some tissues or stages.

Exogonadal oogenesis in a temperate holothurian

Unusual structures were detected on the visceral peritoneum of the ovarian tubules in about 5%-10% of female sea cucumbers (Cucumaria frondosa) collected off Newfoundland, eastern Canada. The condition varied from mild to severe, with localized castration observed in the most heavily affected tubule sections. Investigation of the structures using histology, transmission electron microscopy (TEM), and gene analysis revealed that they were oocytes at different stages of development, growing singly or in groups of up to six. Their size and composition were consistent with those of oocytes found in the lumen of the ovaries, although "exogonadal" oocytes were devoid of a vitelline coat and presented few cortical granules. TEM sections suggest that the atypical oocytes emerged from the peritoneum and grew toward the coelomic cavity, and that they were not in direct contact with the basal lamina or the inner germinal layers. Similar masses have been observed in C. frondosa from the Gulf of St. Lawrence (Québec, Canada) and the Barents Sea (Russia), and in C. japonica from Russia and Psolus fabricii from Canada. The possibility that exogonadal oogenesis is attributable to anthropogenic disturbances should be investigated even though some of the affected specimens originate from presumably pristine locations.

Distinct profiles of expressed sequence tags during intestinal regeneration in the sea cucumber Holothuria glaberrima

Repair and regeneration are key processes for tissue maintenance, and their disruption may lead to disease states. Little is known about the molecular mechanisms that underline the repair and regeneration of the digestive tract. The sea cucumber Holothuria glaberrima represents an excellent model to dissect and characterize the molecular events during intestinal regeneration. To study the gene expression profile, cDNA libraries were constructed from normal, 3-day, and 7-day regenerating intestines of H. glaberrima. Clones were randomly sequenced and queried against the nonredundant protein database at the National Center for Biotechnology Information. RT-PCR analyses were made of several genes to determine their expression profile during intestinal regeneration. A total of 5,173 sequences from three cDNA libraries were obtained. About 46.2, 35.6, and 26.2% of the sequences for the normal, 3-days, and 7-days cDNA libraries, respectively, shared significant similarity with known sequences in the protein database of GenBank but only present 10% of similarity among them. Analysis of the libraries in terms of functional processes, protein domains, and most common sequences suggests that a differential expression profile is taking place during the regeneration process. Further examination of the expressed sequence tag dataset revealed that 12 putative genes are differentially expressed at significant level (R > 6). Experimental validation by RT-PCR analysis reveals that at least three genes (unknown C-4677-1, melanotransferrin, and centaurin) present a differential expression during regeneration. These findings strongly suggest that the gene expression profile varies among regeneration stages and provide evidence for the existence of differential gene expression.

Spherulocytes in the echinodermHolothuria glaberrimaand their involvement in intestinal regeneration

The holothuroid echinoderm Holothuria glaberrima can regenerate its intestine after a process of evisceration. Spherule-containing cells, the spherulocytes, appear to be associated with intestinal regeneration. We have used histochemistry and immunocytochemistry to characterize these cells and their role in the regeneration process. Spherulocytes are 10-20 microm in diameter with an acrocentric nucleus and spherule-like structures within their cytoplasm. They are found in the connective tissue of the intestine and mesentery of noneviscerated and regenerating animals. During the second week of regeneration, the number of spherulocytes in the regenerating intestine increases and a dramatic change in their morphology occurs. Together with the morphological change, the immunohistochemical labeling of the cells also changes; the antibodies not only recognize the spherule structures but also label the cellular cytoplasm in a more homogeneous pattern. Moreover, immunohistochemical labeling also appears to be dispersed within the extracellular matrix, suggesting that the cells are liberating their vesicular contents. Spherulocytes are found in other tissues of H. glaberrima, always associated with the connective tissue component. Our data strongly suggest that spherulocytes are involved in intestinal regeneration but their specific role remains undetermined. In summary, our data expand our knowledge of the cellular events associated with regeneration processes in echinoderms and provide for comparisons with similar processes in vertebrates.

Tensilin-like stiffening protein fromHolothuria leucospilotadoes not induce the stiffest state of catch connective tissue

The dermis of sea cucumbers is a catch connective tissue or mutable connective tissue that exhibits large changes in mechanical properties. A stiffening protein, tensilin, has been isolated from the sea cucumber Cucumaria frondosa. We purified a similar protein, H-tensilin, from Holothuria leucospilota, which belongs to a different family to C. frondosa. H-tensilin appeared as a single band with an apparent molecular mass of 34 kDa on SDS-PAGE. No sugar chain was detected. Tryptic fragments of the protein had homology to known tensilin. H-tensilin aggregated isolated collagen fibrils in vitro in a buffer containing 0.5 mol l–1 NaCl with or without 10 mmol l–1 Ca2+. The activity of H-tensilin was quantitatively studied by dynamic mechanical tests on the isolated dermis. H-tensilin increased stiffness of the dermis in the soft state, induced by Ca2+-free artificial seawater, to a level comparable to that of the standard state, which was the state found in the dermis rested in artificial seawater with normal ionic condition. H-tensilin decreased the energy dissipation ratio of the soft dermis to a level comparable to that of the standard state. When H-tensilin was applied on the dermis in the standard state, it did not alter stiffness nor dissipation ratio. The subsequent application of artificial seawater in which the potassium concentration was raised to 100 mmol l–1increased stiffness by one order of magnitude. These findings suggest that H-tensilin is involved in the changes from the soft state to the standard state and that some stiffening factors other than tensilin are necessary for the changes from the standard to the stiff state.

Transdifferentiation in holothurian gut regeneration

It has recently been shown that the whole spectrum of cell types constituting a multicellular organism can be generated from stem cells. Our study provides an example of an alternative mechanism of tissue repair. Injection of distilled water into the coelomic cavity of the holothurian Eupentacta fraudatrix results in the loss of the whole digestive tract, except the cloaca. The new gut reforms from two separate rudiments. One rudiment appears at the anterior end of the body and extends posteriorly. The second rudiment grows anteriorly from the cloaca. In the anterior rudiment, the luminal epithelium (normally derived from endoderm) develops de novo through direct transdifferentiation of the coelomic epithelial cells (mesodermal in origin). In the posterior rudiment, the luminal epithelium originates from the lining epithelium of the cloaca. After 27 days, the two rudiments come into contact and fuse to form a continuous digestive tube lined with a fully differentiated luminal epithelium. Thus in this species, the luminal epithelia of the anterior and posterior gut rudiments develop from two different cell sources-i.e., from the mesodermally derived mesothelium and the endodermally derived epithelium of the cloacal lining, respectively. Our data suggest that differentiated cells of echinoderms are capable of transdifferentiation into other cell types.