Proteomic and physiological responses of leopard sharks (Triakis semifasciata) to salinity change

Abstract

SUMMARYPartially euryhaline elasmobranchs may tolerate physiologically challenging, variable salinity conditions in estuaries as a tradeoff to reduce predation risk or to gain access to abundant food resources. To further understand these trade-offs and to evaluate the underlying mechanisms, we examined the responses of juvenile leopard sharks to salinity changes using a suite of measurements at multiple organizational levels: gill and rectal gland proteomes (using 2-D gel electrophoresis and tandem mass spectrometry), tissue biochemistry (Na + /K + -ATPase, caspase 3/7 and chymotrypsin-like proteasome activities), organismal physiology (hematology, plasma composition, muscle moisture) and individual behavior. Our proteomics results reveal coordinated molecular responses to low salinity -several of which are common to both rectal gland and gill -including changes in amino acid and inositol (i.e. osmolyte) metabolism, energy metabolism and proteins related to transcription, translation and protein degradation. Overall, leopard sharks employ a strategy of maintaining plasma urea, ion concentrations and Na + /K + -ATPase activities in the short-term, possibly because they rarely spend extended periods in low salinity conditions in the wild, but the sharks osmoconform to the surrounding conditions by 3 weeks. We found no evidence of apoptosis at the time points tested, while both tissues exhibited proteomic changes related to the cytoskeleton, suggesting that leopard sharks remodel existing osmoregulatory epithelial cells and activate physiological acclimatory responses to solve the problems posed by low salinity exposure. The behavioral measurements reveal increased activity in the lowest salinity in the short-term, while activity decreased in the lowest salinity in the long-term. Our data suggest that physiological/behavioral trade-offs are involved in using estuarine habitats, and pathway modeling implicates tumor necrosis factor  (TNF) as a key node of the elasmobranch hyposmotic response network. Supplementary material available online at

Salinity-induced activation of the myo-inositol biosynthesis pathway in tilapia gill epithelium

“…Interestingly, the stability of myo-inositol may be affected by environmental salinity. For instance, myo-inositol degradation appears to be upregulated in euryhaline sharks when they are exposed to dilute seawater, which is consistent with the need to lower compatible osmolyte levels during hypo-osmotic stress (Dowd et al, 2010). Furthermore, the metabolite myo-inositol was shown to increase in tilapia tissues during exposure to elevated salinity, which is consistent with MIB pathway activation by salinity stress (Fiess et al, 2007).…”

“…Effects of salinity stress on mRNA and protein abundances have been documented for many genes, and this mechanism of regulation represents a major pillar of salinity stress responses in fish and other organisms (Fiol et al, 2006;Evans and Somero, 2008;Dowd et al, 2010). However, alternative mechanisms such as post-translational modification (PTM) and alternative splicing also play crucial roles in salinity stress responses.…”

Tilapia (Oreochromis mossambicus) brain cells respond to hyperosmotic challenge by inducingmyo-inositol biosynthesis

“…Significant upregulation or downregulation of IMPA1 mRNA under hyper-and hypo-osmotic challenge, respectively, has been reported in other teleost fishes (Evans and Somero, 2008;Kalujnaia et al, 2010;Whitehead et al, 2012). In leopard sharks, an osmoconforming species, inositol-related proteins have also been reported to be regulated in rectal gland and gill tissues in response to hypo-osmotic challenge (Dowd et al, 2010). Under acute acclimation, MIPS splice variants and IMPA1 displayed peak induction around 16 h, which closely corresponds with the highest inorganic ion concentration (Na + and Cl -) and plasma osmolality.…”

Proteomics of hyposaline stress in blue mussel congeners (genusMytilus): implications for biogeographic range limits in response to climate change

“…Proteins involved in small GTPase and cytoskeletal pathways were enriched in osmoregulatory tissues of sharks (Lee et al, 2006). Several of the proteins representing energy metabolism in Mytilus were also found in the rectal glands of sharks in response to a feeding-associated salt load (Dowd et al, 2008), but shark gill tissue showed a number of proteasome isoforms in response to salinity change (Dowd et al, 2010), a response that was almost absent in Mytilus. Our results indicate that these cellular processes play an important role in setting tolerance limits towards hyposaline stress.…”