Rainbow smelt: the unusual case of cryoprotection by sustained glycerol production in an aquatic animal

Abstract

Rainbow smelt flourish at -1.8 °C, the freezing point of sea water. An antifreeze protein contributes to freeze point depression but, more importantly, cryoprotection is due to an elevation in osmotic pressure, by the accumulation of glycerol. The lower the water temperature, the higher the plasma glycerol with levels recorded as high as 400 mmol l(-1). Glycerol freely diffuses out in direct relation to the glycerol concentration and fish may lose as much as 15% of their glycerol reserve per day. Glycerol levels decrease from a maximum in February/March while water temperature is still sub-zero. The decrease in glycerol may respond to a photoperiod signal as opposed to initiation which is triggered by low temperature. The initial increase in glycerol level is supported by liver glycogen but high sustained glycerol level is dependent upon dietary carbohydrate and protein. The metabolic pathways leading to glycerol involve flux from glycogen/glucose to the level of dihydroxyacetone phosphate (DHAP) via the initial part of glycolysis and from amino acids via a truncated gluconeogenesis again to the level of DHAP. DHAP in turn is converted to glycerol 3-phosphate (G3P) and then directly to glycerol. The key to directing DHAP to G3P is a highly active glycerol 3-P dehydrogenase. G3P is converted directly to glycerol via G3P phosphatase, the rate-limiting step in the process. The transition to glycerol production is associated with increased activities of enzymes at key loci in the top part of glycogenolysis/glycolysis. Curtailment of the final section of glycolysis may reside at the level of pyruvate oxidation with an inactivation of pyruvate dehydrogenase (PDH) driven by increased levels of PDH kinase. Enzymes associated with amino acid trafficking are elevated as is the pivotal enzyme phosphoenolpyruvate carboxykinase.

Hepatic transcriptome of the freeze-tolerant Cope’s gray treefrog, Dryophytes chrysoscelis: responses to cold acclimation and freezing

“…Approximately 0.1% of the total transcripts were identified as non-coding functional RNAs upon searching the Rfam database [33] using Blast2GO (Table 3). Of note, the transcriptome of D. chrysoscelis included both C/D box and H/ACA box small nucleolar RNAs and 20 different microRNAs: let-7 and miR- 7,10,15,16,22,23,24,27,29,30,101,122,142,144,145,194,214,221, and 363.…”

“…Upon initiation of freezing, treefrogs mobilize cryoprotective glucose and glycerol derived from liver glycogen catabolism [14][15][16][17]. These species also accumulate cryoprotective glycerol and urea during cold acclimation in anticipation of freezing [5,13,[18][19][20][21] a response similar to that observed in freezetolerant insects [22] and cold-tolerant fish [23]. In D. chrysoscelis, the anticipatory accumulation of cryoprotectant occurs concurrently with changes in kidney function [5] and adjustments in aquaglyceroporin expression and localization [7,20], which likely facilitate permeation of these solutes into various tissues.…”

Enzymatic capacities of metabolic fuel use in cuttlefish (Sepia officinalis) and responses to food deprivation: insight into the metabolic organization and starvation survival strategy of cephalopods

“…Muscle PEPCK also could be involved in cataplerosis (Yang et al 2009) or in supplying carbon to glycerol production for lipid synthesis, considering the high activities of G3PDH in mantle and systemic heart ( Fig. 5) (potentially analogous to the glycerol-producing liver of rainbow smelt; Driedzic 2015).…”

Elevated osmolytes in rainbow smelt: the effects of urea, glycerol and trimethylamine oxide on muscle contractile properties

“…−1.8°C; Raymond, 1995). Smelt not only survive but are also remarkably active under these conditions (Driedzic, 2015). To avoid freezing, these fish display a variety of thermal acclimation responses related to prevention of freezing in sub-zero temperatures (Driedzic and Ewart, 2004).…”

“…Another element to the thermal acclimation response in these smelt is the accumulation of anti-freeze proteins (AFPs) and a variety of osmolytes to prevent freezing in sub-zero temperatures (Driedzic, 2015). When exposed to cold temperatures (both in a laboratory setting and naturally during the winter months), smelt show increased levels of glycerol, urea, trimethylamine N-oxide (TMAO) and AFPs in their plasma, muscle and other tissues (Raymond, 1998;Treberg et al, 2002;Liebscher et al, 2006).…”