Difference in hepatic metallothionein content in Antarctic red-blooded and haemoglobinless fish: undetectable metallothionein levels in haemoglobinless fish is accompanied by accumulation of untranslated metallothionein mRNA

Abstract

Icefish (family Channichthyidae, suborder Nothothenioidei) are a group of Antarctic fish that have evolved unique phenotypes in order to adapt to the environment in which they live. Besides the lack of haemoglobin and the drastic reduction in the number of erythrocyte-like cells, another striking feature of the icefish is that their liver is devoid of metallothionein. These cysteine-rich heavy-metal-binding proteins are usually present in large amounts in a large variety of organisms, from bacteria to mammals. Despite the failure to detect appreciable levels of metallothionein in icefish liver, a cDNA encoding metallothionein was produced from total RNA by reverse transcriptase PCR. The icefish metallothionein showed high percentage identity with metallothionein from Trematomus bernachii, a red-blooded Antarctic fish in which a normal content of hepatic metallothionein was found. Steady-state mRNA levels were assessed in fish liver by high-stringency hybridization of the metallothionein probe with total RNA. The results showed that icefish livers retain large amounts of untranslated metallothionein mRNA. The stability of the icefish transcript might be correlated with the lack of specific motifs in the untranslated 3' ends of mRNA.

Evidence for enhanced bioavailability of trace elements in the marine ecosystem of Deception Island, a volcano in Antarctica

“…Low trace element levels in the liver, typically the site of numerous oxygen-demanding enzymatic processes involving cleansing the blood of undesired constituent (including trace elements), indicates that in C. gunnari the liver does not seem to play a trace element detoxification role. Accordingly, metallothioneins (metal-binding proteins) are not present in liver tissue despite their genes being expressed in this tissue (Scudiero et al, 1997). C. gunnari flesh has a different structure and physiology than in red-blooded fishes, with lower aerobic metabolic capacities, higher mitochondrial densities, larger oxidative muscle fibers, and higher lipid content (OÕBrien, Skilbeck, Sidell, & Egginton, 2003).…”

Fish and mammalian metallothioneins: a comparative study

“…With respect to mammalian metallothioneins, fish metallothioneins display a number of distinctive features in the primary structure, including the displacement of a cysteine residue located in the carboxyterminal half of the molecule (occupying position 54 in the alignment shown in Fig. 1) and a lower number of lysine residues juxtaposed to cysteines (Scudiero et al, 1997). The main consequence of the displacement of the banomalousQ cysteine residue is the drastically different orientation of the motif K50-G51-K52-T53 of fish metallothionein with respect to the corresponding K50-G51-A52-A53 motif of mouse metallothionein (see Fig.…”

Metal detoxification and homeostasis in Antarctic Notothenioids. A comparative survey on evolution, expression and functional properties of fish and mammal metallothioneins

“…Using icefish MT cDNA as a probe, it was possible to estimate MT mRNA expression by Northern blotting of total RNA extracted from liver of both the red-blooded T. bernacchii and hemoglobinless C. hamatus. The results showed that large amounts of MT mRNA were present in the red-blooded species as well as in icefish (Scudiero et al 1997). Hence, the presence of a transcript in icefish without any appreciable amount of protein suggests that MT deficiency in icefish is not the consequence of gene inactivation as in the case of hemoglobin genes (Cocca et al 1995).…”