Sarcolipin Signaling Promotes Mitochondrial Biogenesis and Oxidative Metabolism in Skeletal Muscle

Abstract

The major objective of this study was to understand the molecular basis of how sarcolipin uncoupling of SERCA regulates muscle oxidative metabolism. Using genetically engineered sarcolipin (SLN) mouse models and primary muscle cells, we demonstrate that SLN plays a crucial role in mitochondrial biogenesis and oxidative metabolism in muscle. Loss of SLN severely compromised muscle oxidative capacity without affecting fiber-type composition. Mice overexpressing SLN in fast-twitch glycolytic muscle reprogrammed mitochondrial phenotype, increasing fat utilization and protecting against high-fat diet-induced lipotoxicity. We show that SLN affects cytosolic Ca²⁺ transients and activates the Ca²⁺/calmodulin-dependent protein kinase II (CamKII) and PGC1α axis to increase mitochondrial biogenesis and oxidative metabolism. These studies provide a fundamental framework for understanding the role of sarcoplasmic reticulum (SR)-Ca²⁺ cycling as an important factor in mitochondrial health and muscle metabolism. We propose that SLN can be targeted to enhance energy expenditure in muscle and prevent metabolic disease.

Keywords: Ca(2+) signaling; CamKII; PGC1α; SERCA; lipotoxicity; mitochondrial biogenesis; oxidative metabolism; primary muscle myotubes; sarcolipin; skeletal muscle.

Figure 1.. SLN Regulates the Mitochondrial Phenotype and Oxidative Metabolism during Neonatal Skeletal Muscle Development

(A–H) Neonatal muscle development. (A) SLN protein expression in neonatal quadriceps and gastrocnemius of WT mice. (B) The expression level of SERCA1a, SERCA2a, and calsequestrin 1 and 2 (CASQ1 and CASQ2) in 10-day-old WT and Sln-KO quadriceps muscle. (C) Decreased fatty acid (palmitoylcarnitine) oxidation in Sln-KO muscle. PC, palmitoylcarnitine; M, malate; Glut, glutamate; Succ, succinate (n = 5). (D) Decrease in mtDNA content. (E) Succinate dehydrogenase (SDH) activity staining in 10-day-old WT and Sln-KO quadriceps muscle. (F) Decreased expression of mitochondrial electron transport chain (ETC) proteins in Sln-KO muscle. (G) Decreased expression of enzymes involved in fat mobilization (LPL, lipoprotein lipase), fatty acid transport (CPT1-M, carnitine palmitoyltransferase-1 mitochondrial), β-oxidation enzymes (LCAD, long-chain acyl-CoA dehydrogenase; HADHB; 3-ketoacyl-CoA thiolase, acetyl-CoA acyltransferase, or beta-ke-tothiolase), adenine nucleotide translocator (ANT), and citrate synthase. (H) Upregulation of major glycolytic enzymes and higher levels of phosphorylated 5’ adenosine monophosphate-activated protein (AMP) kinase (pAMPK) in Sln-KO muscle. (I–O) Adult soleus muscle from WT and Sln-KO mice. (I) mtDNA content is not altered in Sln-KO soleus. (J) ETC protein expression are unchanged. (K) Sln-KO soleus shows decreased expression of proteins involved in fat mobilization, fatty acid transport, β-oxidation enzymes, and citrate synthase. (L) Decreased fatty acid oxidation in Sln-KO soleus muscle. P, palmitoylCoA; C, carnitine; M, malate. (M) Levels of glycolytic enzymes are not altered. (N) SDH activity staining of adult WT and Sln-KO soleus muscle. (O) Immunostaining with myosin isoform-specific antibodies reveals that muscle fiber composition is not affected in the Sln-KO soleus muscle (n = 4). Yellow, myosin ATPase type I; red, myosin ATPase type IIa; green, myosin ATPase type IIb; black, myosin ATPase type x. Data are shown as mean ± SEM. *p < 0.01, **p < 0.001, ***p < 0.0001, t test.

Figure 2.. Transgenic Overexpression of SLN in Glycolytic Muscle (Tibialis Anterior) Programs Mitochondria to Increase Fatty Acid Metabolism and Protects from High-Fat Diet-Induced Lipotoxicity

(A–D) WT and Sln^OE mice maintained on regular chow diet. (A) mtDNA copy number (n = 5). (B) ETC protein expression. (C) Expression level of fatty acid transporters (CD36 and CPT1-M), β-oxidation enzymes (LCAD and HADHB), ANT1/2, and TFAM in Sln^OE muscle is greater than in WT. (D) Increased fatty acid oxidation in Sln^OE muscle. (E–L) Sln^OE, WT, and Sln-KO mice fed on a high-fat diet (HFD) for 12 weeks. (E) mtDNA copy number (n = 5). (F) ETC protein expression. (G) Regulators of oxidative metabolism. (H) Increased fatty acid oxidation in Sln^OE muscle oxidation (n = 5). (I) Oil red O staining of Sln^OE TA muscle shows no lipid accumulation. (J) Soleus muscle from Sln-KO mice showing increased lipid accumulation. (K) Sln^OE muscle shows lower levels of ceramides, DAG (diacylglyceride), and acylcarnitines (n = 4). (L) Higher rate of glucose uptake and clearance in TA muscle of Sln^OE mice (n = 5). Data are shown as mean ± SEM. *p < 0.01, **p < 0.001, ***p < 0.0001, t test.

Figure 3.. SLN Recruits PGC1α to Increase Mitochondrial Biogenesis

(A) Primary muscle myotubes derived from satellite cell culture. (B) Protein levels of SLN during WT primary myotube differentiation. (C) Expression level of SLN and SR proteins, SERCA, and CASQ in primary myotubes. (D) mtDNA copy number. (E) Mitochondrial OXPHOS protein levels during myotube differentiation. (F) Fatty acid-stimulated oxygen consumption in myotubes. (G) Oxygen consumption rate (OCR) in Sln-KO myotubes. (H–J) Adenoviral SLN gene transfer rescued muscle mitochondrial content (H and I) and OCR (J) in Sln-KO myotubes. (K) PGC1α expression in WT and Sln-KO myotubes following adenoviral gene transfer. (L) PGC1α gene expression during WT and Sln-KO myotube differentiation. (M) Protein expression level of PGC1α, PPARδ, and TFAM in myotubes. (N–P) mtDNA copy number (N), ETC protein expression (O), and OCR (P) following knockdown of PGC1α and/or adenoviral SLN gene transfer in myotubes. (Q) Rescue of mtDNA content in Sln-KO myotubes by adenoviral PGC1α gene transfection.

Figure 4.. SLN-Mediated Increase in Mitochondrial Biogenesis Depends on SR-Ca²⁺ Cycling, Activation of CamKII, and Recruitment of the PGC1α Axis

(A) Adenoviral-mediated expression of SLN in myotubes. (B) Myoplasmic Ca²⁺ as detected by Fluo-4 signal after caffeine administration. (C) Ca²⁺ transients after activation with caffeine, as indicated by both time measurement (in seconds) and curve slope analysis (in int/ms). (D) Activation of SR-Ca²⁺ cycling by caffeine treatment (3.5 mM to promote SR-Ca²⁺ release) increases CamKII phosphorylation and Mef2c expression in WT myotubes. (E) Increased mtDNA content. (F) PGC1α expression in caffeine-treated WT myotubes. (G) Caffeine treatment induced expression of proteins involved in fatty acid metabolism in WT myotubes, but not in Sln-KO myotubes. (H) Caffeine increases OCR in WT myotubes. (I) In hibition of SR-Ca²⁺ cycling by dantrolene treatment (10 μM to block SR-Ca²⁺ release) decreased CamKII phosphorylation and Mef2c expression. (J) mtDNA content. (K) PGC1α expression followed by dantrolene treatment. (L) Dantrolene treatment decreased expression of mitochondrial transcriptional regulators and metabolic proteins. (M) OCR in myotubes. (N) Inhibition of CamKII activity by KN93 treatment (10 nM) decreased CamKII phosphorylation, whereas adenoviral SLN gene transfer in KO myotubes rescuedthe phosphorylation status in CamKII. (O and P) mtDNA content (O) and OCR (P) in myotubes following KN93 pretreatment. (Q) Schematic representation of how sarcolipin signals to increase mitochondrial biogenesis.