Elevated whole muscle phosphatidylcholine: phosphatidylethanolamine ratio coincides with reduced SERCA activity in murine overloaded plantaris muscles

Vitamin D, Calbindin, and calcium signaling: Unraveling the Alzheimer's connection

Calcium is a ubiquitous second messenger that is indispensable in regulating neurotransmission and memory formation. A precise intracellular calcium level is achieved through the concerted action of calcium channels, and calcium exerts its effect by binding to an array of calcium-binding proteins, including calmodulin (CAM), calcium-calmodulin complex-dependent protein kinase-II (CAMK-II), calbindin (CAL), and calcineurin (CAN). Calbindin orchestrates a plethora of signaling events that regulate synaptic transmission and depolarizing signals. Vitamin D, an endogenous fat-soluble metabolite, is synthesized in the skin upon exposure to ultraviolet B radiation. It modulates calcium signaling by increasing the expression of the calcium-sensing receptor (CaSR), stimulating phospholipase C activity, and regulating the expression of calcium channels such as TRPV6. Vitamin D also modulates the activity of calcium-binding proteins, including CAM and calbindin, and increases their expression. Calbindin, a high-affinity calcium-binding protein, is involved in calcium buffering and transport in neurons. It has been shown to inhibit apoptosis and caspase-3 activity stimulated by presenilin 1 and 2 in AD. Whereas CAM, another calcium-binding protein, is implicated in regulating neurotransmitter release and memory formation by phosphorylating CAN, CAMK-II, and other calcium-regulated proteins. CAMK-II and CAN regulate actin-induced spine shape changes, which are further modulated by CAM. Low levels of both calbindin and vitamin D are attributed to the pathology of Alzheimer's disease. Further research on vitamin D via calbindin-CAMK-II signaling may provide newer insights, revealing novel therapeutic targets and strategies for treatment.

Decreased sarcoplasmic reticulum phospholipids in human skeletal muscle are associated with metabolic syndrome

Metabolic syndrome affects more than one in three adults and is associated with increased risk of diabetes, cardiovascular disease, and all-cause mortality. Muscle insulin resistance is a major contributor to the development of the metabolic syndrome. Studies in mice have linked skeletal muscle sarcoplasmic reticulum (SR) phospholipid composition to sarcoplasmic/endoplasmic reticulum Ca2+-ATPase activity and insulin sensitivity. To determine if the presence of metabolic syndrome alters specific phosphatidylcholine (PC) and phosphatidylethanolamine (PE) species in human SR, we compared SR phospholipid composition in skeletal muscle from sedentary subjects with metabolic syndrome and sedentary control subjects without metabolic syndrome. Both total PC and total PE were significantly decreased in skeletal muscle SR of sedentary metabolic syndrome patients compared with sedentary controls, particularly in female participants, but there was no difference in the PC:PE ratio between groups. Total SR PC levels, but not total SR PE levels or PC:PE ratio, were significantly negatively correlated with BMI, waist circumference, total fat, visceral adipose tissue, triglycerides, fasting insulin, and homeostatic model assessment for insulin resistance. These findings are consistent with the existence of a relationship between skeletal muscle SR PC content and insulin resistance in humans.

Skeletal Muscle Consequences of Phosphatidylethanolamine Synthesis Deficiency

The maintenance of phospholipid homeostasis is increasingly being implicated in metabolic health. Phosphatidylethanolamine (PE) is the most abundant phospholipid on the inner leaflet of cellular membranes, and we have previously shown that mice with a heterozygous ablation of the PE synthesizing enzyme, Pcyt2 (Pcyt2+/-), develop obesity, insulin resistance, and NASH. Skeletal muscle is a major determinant of systemic energy metabolism, making it a key player in metabolic disease development. Both the total PE levels and the ratio of PE to other membrane lipids in skeletal muscle are implicated in insulin resistance; however, the underlying mechanisms and the role of Pcyt2 regulation in this association remain unclear. Here, we show how reduced phospholipid synthesis due to Pcyt2 deficiency causes Pcyt2+/- skeletal muscle dysfunction and metabolic abnormalities. Pcyt2+/- skeletal muscle exhibits damage and degeneration, with skeletal muscle cell vacuolization, disordered sarcomeres, mitochondria ultrastructure irregularities and paucity, inflammation, and fibrosis. There is intramuscular adipose tissue accumulation, and major disturbances in lipid metabolism with impaired FA mobilization and oxidation, elevated lipogenesis, and long-chain fatty acyl-CoA, diacylglycerol, and triacylglycerol accumulation. Pcyt2+/- skeletal muscle exhibits perturbed glucose metabolism with elevated glycogen content, impaired insulin signaling, and reduced glucose uptake. Together, this study lends insight into the critical role of PE homeostasis in skeletal muscle metabolism and health with broad implications on metabolic disease development.

An Investigation of the Altered Textural Property in Woody Breast Myopathy Using an Integrative Omics Approach

Woody breast (WB) is a myopathy observed in broiler Pectoralis major (PM) characterized by its tough and rubbery texture with greater level of calcium content. The objective of this study was to investigate the functionality/integrity of WB sarcoplasmic reticulum (SR), which may contribute to the elevated calcium content observed in WB and other factors that may influence WB texture. Fourteen Ross line broiler PM [7 severe WB and 7 normal (N)] were selected, packaged, and frozen at −20°C at 8 h postmortem from a commercial processing plant. Samples were used to measure pH, sarcomere length, proteolysis, calpain activity, collagenase activity, collagen content, collagen crosslinks density, and connective tissue peak transitional temperature. Exudate was also collected from each sample to evaluate free calcium concentration. The SR fraction of the samples was separated and utilized for proteomic and lipidomic analysis. The WB PM had a higher pH, shorter sarcomeres, lower % of intact troponin-T, more autolyzed μ/m calpain, more activated collagenase, greater collagen content, greater mature collagen crosslinks density, and higher connective tissue peak transitional temperature than the N PM (p ≤ 0.05). Exudate from WB PM had higher levels of free calcium than those from N PM (p < 0.05). Proteomics data revealed an upregulation of calcium transport proteins and a downregulation of proteins responsible for calcium release (p < 0.05) in WB SR. Interestingly, there was an upregulation of phospholipase A2 (PLA2), and cholinesterase exhibited a 7.6-fold increase in WB SR (p < 0.01). Lipidomics data revealed WB SR had less relative % of phosphatidylcholine (PC) and more lysophosphatidylcholine (LPC; p < 0.05). The results indicated that upregulation of calcium transport proteins and downregulation of calcium-release proteins in WB SR may be the muscle’s attempt to regulate this proposed excessive signaling of calcium release due to multiple factors, such as upregulation of PLA2 resulting in PC hydrolysis and presence of cholinesterase inhibitors in the system prolonging action potential. In addition, the textural abnormality of WB may be the combined effects of shorter sarcomere length and more collagen with greater crosslink density being deposited in the broiler PM.

Ryanodine receptor leak triggers fiber Ca2+ redistribution to preserve force and elevate basal metabolism in skeletal muscle

Muscle contraction depends on tightly regulated Ca²⁺ release. Aberrant Ca²⁺ leak through ryanodine receptor 1 (RyR1) on the sarcoplasmic reticulum (SR) membrane can lead to heatstroke and malignant hyperthermia (MH) susceptibility, as well as severe myopathy. However, the mechanism by which Ca²⁺ leak drives these pathologies is unknown. Here, we investigate the effects of four mouse genotypes with increasingly severe RyR1 leak in skeletal muscle fibers. We find that RyR1 Ca²⁺ leak initiates a cascade of events that cause precise redistribution of Ca²⁺ among the SR, cytoplasm, and mitochondria through altering the Ca²⁺ permeability of the transverse tubular system membrane. This redistribution of Ca²⁺ allows mice with moderate RyR1 leak to maintain normal function; however, severe RyR1 leak with RYR1 mutations reduces the capacity to generate force. Our results reveal the mechanism underlying force preservation, increased ATP metabolism, and susceptibility to MH in individuals with gain-of-function RYR1 mutations.

Low-dose lithium feeding increases the SERCA2a-to-phospholamban ratio, improving SERCA function in murine left ventricles

NEW FINDINGS

What is the central question of this study? Inhibition of glycogen synthase kinase-3 (GSK3) has been shown to improve cardiac SERCA2a function. Lithium can inhibit GSK3, but therapeutic doses used in treating bipolar disorder can have toxic effects. It has not been determined whether subtherapeutic doses of lithium can improve cardiac SERCA function. What is the main finding and its importance? Using left ventricles from wild-type mice, we found that subtherapeutic lithium feeding for 6 weeks decreased GSK3 activity and increased cardiac SERCA function compared with control-fed mice. These findings warrant the investigation of low-dose lithium feeding in preclinical models of cardiomyopathy and heart failure to determine the therapeutic benefit of GSK3 inhibition.

ABSTRACT

The sarco(endo)plasmic reticulum Ca²⁺ -ATPase (SERCA) pump is responsible for regulating calcium (Ca²⁺ ) within myocytes, with SERCA2a being the dominant isoform in cardiomyocytes. Its inhibitor, phospholamban (PLN), acts by decreasing the affinity of SERCA for Ca²⁺ . Changes in the SERCA2a:PLN ratio can cause Ca²⁺ dysregulation often seen in patients with dilated cardiomyopathy and heart failure. The enzyme glycogen synthase kinase-3 (GSK3) is known to downregulate SERCA function by decreasing the SERCA2a:PLN ratio. In this study, we sought to determine whether feeding mice low-dose lithium, a natural GSK3 inhibitor, would improve left ventricular SERCA function by altering the SERCA2a:PLN ratio. To this end, male wild-type C57BL/6J mice were fed low-dose lithium via drinking water (10 mg kg^-1  day^-1 LiCl for 6 weeks) and left ventricles were harvested. GSK3 activity was significantly reduced in LiCl-fed versus control-fed mice. The apparent affinity of SERCA for Ca²⁺ was also increased (pCa₅₀ ; control, 6.09 ± 0.03 versus LiCl, 6.26 ± 0.04, P < 0.0001) along with a 2.0-fold increase in SERCA2a:PLN ratio in LiCl-fed versus control-fed mice. These findings suggest that low-dose lithium supplementation can improve SERCA function by increasing the SERCA2a:PLN ratio. Future studies in murine preclinical models will determine whether GSK3 inhibition via low-dose lithium could be a potential therapeutic strategy for dilated cardiomyopathy and heart failure.

An association analysis for 14 candidate genes mapping to meat quality quantitative trait loci in a Duroc pig population reveals that the ATP1A2 genotype is highly associated with muscle electric conductivity

In previous GWAS carried out in a Duroc commercial line (Lipgen population), we detected on pig chromosomes 3, 4 and 14 several QTL for gluteus medius muscle redness (GM a*), electric conductivity in the longissimus dorsi muscle (LD CE) and vaccenic acid content in the LD muscle (LD C18:1 n - 7), respectively. We have genotyped, in the Lipgen population, 19 SNPs mapping to 14 genes located within these QTL. Subsequently, association analyses have been performed. After correction for multiple testing, two SNPs in the TGFBRAP1 (rs321173745) and SELENOI (rs330820437) genes were associated with GM a*, whereas ACADSB (rs81449951) and GPR26 (rs343087568) genotypes displayed significant associations with LD vaccenic content. Moreover, the polymorphisms located at the ATP1A2 (rs344748241), ATP8B2 (rs81382410) and CREB3L4 (rs321278469 and rs330133789) genes showed significant associations with LD CE. We made a second round of association analyses including the SNPs mentioned above as well as other SNPs located in the chromosomes to which they map. After performing a correction for multiple testing, the only association that remained significant at the chromosome-wide level was that between the ATP1A2 genotype and LD CE. From a functional point of view, this association is meaningful because this locus encodes a subunit of the Na⁺ /K⁺ -ATPase responsible for maintaining an electrochemical gradient across the plasma membrane.

Components of activation heat in skeletal muscle

Activation heat (q_A) production by muscle is the thermal accompaniment of the release of Ca²⁺ from the sarcoplasmic reticulum (SR) into the cytoplasm, its interactions with regulatory proteins and other cytoplasmic Ca²⁺ buffers and its return to the SR. The contribution of different Ca²⁺-related reactions to q_A is difficult to determine empirically and therefore, for this study, a mathematical model was developed to describe Ca²⁺ movements and accompanying thermal changes in muscle fibres in response to stimulation. The major sources of heat within a few milliseconds of the initiation of Ca²⁺ release are Ca²⁺ binding to Tn and Pv. Ca²⁺ binding to ATP produces a relatively small amount of heat. Ca²⁺ dissociation from ATP and Tn, with heat absorption, are of similar time course to the decline of force. In muscle lacking Pv (e.g. mouse soleus), Ca²⁺ is then rapidly pumped into the SR. In muscles with Pv, Ca²⁺ that dissociates from Tn and ATP binds to Pv and then dissociates slowly (over 10 s of seconds) and is then pumped into the SR; the net effect of these two processes is heat absorption. It is proposed that this underlies Hill's "negative delayed heat". After all the Ca²⁺ is returned to the SR, q_A is proportional to the amount of Ca²⁺ released into the cytoplasm. In muscles with Pv this is 20-60 s after Ca²⁺ release; in muscles without Pv, all Ca²⁺ is returned to the SR soon after the end of force relaxation.