Chronic social stress alters protein metabolism in juvenile rainbow trout, Oncorhynchus mykiss

Abstract

When confined in pairs, juvenile rainbow trout (Oncorhynchus mykiss) form dominance hierarchies in which subordinate fish exhibit characteristic physiological changes including reduced growth rates and chronically elevated plasma cortisol concentrations. We hypothesized that alterations in protein metabolism contribute to the reduced growth rate of socially stressed trout, and predicted that subordinate trout would exhibit reduced rates of protein synthesis coupled with increases in protein degradation. Protein metabolism was assessed in dominant and subordinate fish after 4 days of social interaction, and in fish that were separated after 4 days of interaction for a 4 days recovery period, to determine whether effects on protein metabolism recovered when social stress was alleviated. Protein metabolism was assessed in liver and white muscle by measuring the fractional rate of protein synthesis and markers of protein degradation. In the white muscle of subordinate fish, protein synthesis was inhibited and activities of the ubiquitin-proteasome pathway (UPP) and the autophagy lysosomal system (ALS) were elevated. By contrast, the liver of subordinate fish exhibited increased rates of protein synthesis and activation of the ALS. When allowed to recover from chronic social stress for 4 days, differences in protein metabolism observed in white muscle of subordinate fish during the interaction period disappeared. In liver, protein synthesis returned to baseline levels during recovery from social stress, but markers of protein degradation did not. Collectively, these data support the hypothesis that inhibition of muscle protein synthesis coupled with increases in muscle protein breakdown contribute to the reduced growth rates of subordinate rainbow trout.

Keywords: Autophagy lysosomal system; Cortisol; Fractional rate of protein synthesis; Salmonid; Social hierarchy; Ubiquitin proteasome pathway.

Fig. 1

Specific enrichment (%) of the phenylalanine free-pool of liver (a) and white muscle (b), and the protein pool of liver (c) and white muscle (d), in relation to time (min) post injection of the tracer in rainbow trout (Oncorhynchus mykiss). Values are means ± SEM with N = 3 per time point. The equations for the regression lines for free-pool enrichment (S_a) are a S_a (%) = − 0.04923 × time + 42.16, R² = 0.46, P = 0.0057, and b S_a (%) = − 0.01736 × time + 31.56, R² = 0.05, P = 0.4037. The equations for the regression lines for protein pool enrichment (S_b) are c S_b (%) = 0.002265 × time + 0.1114, R² = 0.71, P < 0.0001, and d S_b (%) = 5.421e-005 × time + 0.01016, R² = 0.45, P = 0.0058

Fig. 2

Fractional rates of protein synthesis in the liver (a) and white muscle (b) of dominant, subordinate and sham-treated rainbow trout (Oncorhynchus mykiss) at 4 days of interaction period or following 4 days of recovery from a 4-days period of social interaction. Values are means + SEM with N = 5 for dominant fish in the interaction treatment, and N = 6 for all other groups. Within a treatment (interaction or recovery), groups that share a letter are not significantly different from one another. An asterisk indicates a significant difference between interaction and recovery values within a social status category (2-way ANOVA, see text for details)

Fig. 3

Relative mRNA abundance of cathepsin D (a), cathepsin L (b), and mafbx (c) in liver of dominant, subordinate and sham-treated rainbow trout (Oncorhynchus mykiss) at 4 days of social interaction or following 4 days of recovery from a 4-days interaction period. Values are expressed relative to the sham group sampled at 4 days of interaction. Values are means + SEM with N = 4–6 for all groups. Social status categories that share a letter are not significantly different from one another (2-way ANOVA, see text for details)

Fig. 4

Relative mRNA abundance of cathepsin D (a), cathepsin L (b), mafbx (c) and murf1 (d) in white muscle of dominant, subordinate and sham-treated rainbow trout (Oncorhynchus mykiss) at 4 days of social interaction or following 4 days of recovery from a 4-days interaction period. Values are expressed relative to the sham group sampled at 4 days of interaction. Values are means + SEM with N = 5–6 for all groups. Within a treatment (interaction or recovery), groups that share a letter are not significantly different from one another. An asterisk indicates a significant difference between interaction and recovery values within a social status category (2-way ANOVA, see text for details)

Fig. 5

Relative abundance of polyubiquitinated proteins in the liver (a) and white muscle (b) of dominant, subordinate and sham-treated rainbow trout (Oncorhynchus mykiss) at 4 days of social interaction or following 4 days of recovery from a 4-days interaction period. Values are expressed relative to the sham group sampled at 4 days of interaction. Values are means + SEM with N = 5–6 for all groups. In panel a, social status categories that share a letter are not significantly different from one another and an asterisk indicates a significant difference between interaction and recovery values (2-way ANOVA, see text for details). In panel b, groups within a treatment (interaction or recovery) that share a letter are not significantly different from one another. An asterisk indicates a significant difference between interaction and recovery values within a social status category (2-way ANOVA, see text for details)

Fig. 6

Maximal chymotrypsin-like activity of the 20S proteasome (20S proteasome) in the liver (a) and white muscle (b) of dominant, subordinate and sham-treated rainbow trout (Oncorhynchus mykiss) at 4 days of social interaction or following 4 days of recovery from a 4-days interaction period. Values are means + SEM with N = 4–6 for all groups. In panel a, no significant differences were detected (2-way ANOVA, see text for details). In panel b, social status categories that share a letter are not significantly different from one another (2-way ANOVA, see text for details)