Changes in Northern Elephant Seal Skeletal Muscle Following Thirty Days of Fasting and Reduced Activity

Abstract

Northern elephant seals (NES, Mirounga angustirostris) undergo an annual molt during which they spend ∼40 days fasting on land with reduced activity and lose approximately one-quarter of their body mass. Reduced activity and muscle load in stereotypic terrestrial mammalian models results in decreased muscle mass and capacity for force production and aerobic metabolism. However, the majority of lost mass in fasting female NES is from fat while muscle mass is largely preserved. Although muscle mass is preserved, potential changes to the metabolic and contractile capacity are unknown. To assess potential changes in NES skeletal muscle during molt, we collected muscle biopsies from 6 adult female NES before the molt and after ∼30 days at the end of the molt. Skeletal muscle was assessed for respiratory capacity using high resolution respirometry, and RNA was extracted to assess changes in gene expression. Despite a month of reduced activity, fasting, and weight loss, skeletal muscle respiratory capacity was preserved with no change in OXPHOS respiratory capacity. Molt was associated with 162 upregulated genes including those favoring lipid metabolism. We identified 172 downregulated genes including those coding for ribosomal proteins and genes associated with skeletal muscle force transduction and glucose metabolism. Following ∼30 days of molt, NES skeletal muscle metabolic capacity is preserved although mechanotransduction may be compromised. In the absence of exercise stimulus, fasting-induced shifts in muscle metabolism may stimulate pathways associated with preserving the mass and metabolic capacity of slow oxidative muscle.

Keywords: elephant seal; fasting; lipid metabolism; muscle atrophy; skeletal muscle.

FIGURE 1

Northern elephant seal skeletal muscle respiratory flux before and after approximately 1 month during the yearly molt. Error bars represent standard deviation. High resolution respirometry was used to assess (A) respiratory states of leak (L_n and L_Omy), OXPHOS (OXP), ETS, CI, and CII. (B) Ratios of respiratory rates were calculated to assess the relative contribution of leak respiration to OXPHOS (L_n/OXPHOS = CCR_Ln), the relative contribution of CI to OXPHOS (CI/OXPHOS = SCR_CI), the relative contribution of CII to OXPHOS (CII/OXPHOS = SCR_CII), and relative contribution of OXPHOS to ETS (OXPHOS/ETS = FCR). Statistical significance (p < 0.05) is denoted by *.

FIGURE 2

Following 1 month of reduced activity and fasting during the annual molt, NES skeletal muscle demonstrated differential gene expression in 642 genes including 313 downregulated genes and 328 upregulated genes (p_FDR < 0.05). (A) Volcano plot of fold change and p-value for upregulated and downregulated genes with significantly upregulated genes in red and significantly downregulated genes in blue. (B) Enrichr pathway analysis using significantly altered genes with ≥ 1 log₂ fold change in gene expression (162 upregulated genes and 172 downregulated genes) was used to identify significantly upregulated (red) and 7 downregulated (blue) metabolic pathways. Bar length is presented as the Enrichr combined score calculated as the log of the p-value from the Fisher exact test multiplied by the z-score of the deviation from the expected rank. The p-value and ratio of the number of altered genes to the total number of pathway genes is noted for each. (C) There were 5 downregulated genes identified in the “Glycolysis/gluconeogenesis” pathway and 6 downregulated genes identified in the “HIF-1 signaling” pathway. The 9 downregulated genes in the “focal adhesion” pathway and 6 downregulated genes in the “ECM-receptor interaction” pathways had considerable overlap and were primarily related to skeletal muscle structural integrity.