Neutral sphingomyelinase-3 mediates TNF-stimulated oxidant activity in skeletal muscle

Neutral sphingomyelinase regulates mechanotransduction in human engineered cardiac tissues and mouse hearts

Cardiovascular disease is the leading cause of death in the USA and is known to be exacerbated by elevated mechanical stress from hypertension. Caveolae are plasma membrane structures that buffer mechanical stress but have been found to be reduced in pathological conditions associated with chronically stretched myocardium. To explore the physiological implications of the loss of caveolae, we used human engineered cardiac tissue (ECT) constructs, composed of human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes and hiPSC-derived cardiac fibroblasts, to develop a long-term cyclic stretch protocol that recapitulates the effects of hypertension on caveolae expression, membrane tension, and the β-adrenergic response. Leveraging this new stretch protocol, we identified neutral sphingomyelinases (nSMase) as mechanoregulated mediators of caveolae loss, ceramide production and the blunted β-adrenergic response in this human cardiac model. Specifically, in our ECT model, nSMase inhibition via GW4869 prevented stretch-induced loss of caveolae-like structures, mitigated nSMase-dependent ceramide production, and maintained the ECT contractile kinetic response to isoprenaline. These findings are correlated with a blood lipidomic analysis in middle-aged and older adults, which revealed an increase of the circulating levels of ceramides in adults with hypertension. Furthermore, we found that conduction slowing from increased pressure loading in mouse left ventricle was abolished in the context of nSMase inhibition. Collectively, these findings identify nSMase as a potent drug target for mitigating stretch-induced effects on cardiac function. KEY POINTS: We have developed a new stretch protocol for human engineered cardiac tissue that recapitulates changes in plasma membrane morphology observed in animal models of pressure/volume overload. Stretch of engineered cardiac tissue induces activation of neutral sphingomyelinase (nSMase), generation of ceramide, and disassembly of caveolae. Activation of nSMase blunts cardiac β-adrenergic contractile kinetics and mediates stretch-induced slowing of conduction and upstroke velocity. Circulating ceramides are increased in adults with hypertension, highlighting the clinical relevance of stretch-induced nSMase activity.

Two novel cases of biallelic SMPD4 variants with brain structural abnormalities

Sphingomyelin phosphodiesterase 4 (SMPD4) encodes a member of the Mg2+-dependent, neutral sphingomyelinase family that catalyzes the hydrolysis of the phosphodiester bond of sphingomyelin to form phosphorylcholine and ceramide. Recent studies have revealed that biallelic loss-of-function variants of SMPD4 cause syndromic neurodevelopmental disorders characterized by microcephaly, congenital arthrogryposis, and structural brain anomalies. In this study, three novel loss-of-function SMPD4 variants were identified using exome sequencing (ES) in two independent patients with developmental delays, microcephaly, seizures, and brain structural abnormalities. Patient 1 had a homozygous c.740_741del, p.(Val247Glufs*21) variant and showed profound intellectual disability, hepatomegaly, a simplified gyral pattern, and a thin corpus callosum without congenital dysmorphic features. Patient 2 had a compound heterozygous nonsense c.2124_2125del, p.(Phe709*) variant and splice site c.1188+2dup variant. RNA analysis revealed that the c.1188+2dup variant caused exon 13 skipping, leading to a frameshift (p.Ala406Ser*6). In vitro transcription analysis using minigene system suggested that mRNA transcribed from mutant allele may be degraded by nonsense-mediated mRNA decay system. He exhibited diverse manifestations, including growth defects, muscle hypotonia, respiratory distress, arthrogryposis, insulin-dependent diabetes mellitus, sensorineural hearing loss, facial dysmorphism, and various brain abnormalities, including cerebral atrophy, hypomyelination, and cerebellar hypoplasia. Here, we review previous literatures and discuss the phenotypic diversity of SMPD4-related disorders.

m6A modification-tuned sphingolipid metabolism regulates postnatal liver development in male mice

Different organs undergo distinct transcriptional, epigenetic and physiological alterations that guarantee their functional maturation after birth. However, the roles of epitranscriptomic machineries in these processes have remained elusive. Here we demonstrate that expression of RNA methyltransferase enzymes Mettl3 and Mettl14 gradually declines during postnatal liver development in male mice. Liver-specific Mettl3 deficiency causes hepatocyte hypertrophy, liver injury and growth retardation. Transcriptomic and N6-methyl-adenosine (m⁶A) profiling identify the neutral sphingomyelinase, Smpd3, as a target of Mettl3. Decreased decay of Smpd3 transcripts due to Mettl3 deficiency results in sphingolipid metabolism rewiring, characterized by toxic ceramide accumulation and leading to mitochondrial damage and elevated endoplasmic reticulum stress. Pharmacological Smpd3 inhibition, Smpd3 knockdown or Sgms1 overexpression that counteracts Smpd3 can ameliorate the abnormality of Mettl3-deficent liver. Our findings demonstrate that Mettl3-N⁶-methyl-adenosine fine-tunes sphingolipid metabolism, highlighting the pivotal role of an epitranscriptomic machinery in coordination of organ growth and the timing of functional maturation during postnatal liver development.

Cross-Regulation of the Cellular Redox System, Oxygen, and Sphingolipid Signalling

Redox-active mediators are now appreciated as powerful molecules to regulate cellular dynamics such as viability, proliferation, migration, cell contraction, and relaxation, as well as gene expression under physiological and pathophysiological conditions. These molecules include the various reactive oxygen species (ROS), and the gasotransmitters nitric oxide (NO∙), carbon monoxide (CO), and hydrogen sulfide (H2S). For each of these molecules, direct targets have been identified which transmit the signal from the cellular redox state to a cellular response. Besides these redox mediators, various sphingolipid species have turned out as highly bioactive with strong signalling potential. Recent data suggest that there is a cross-regulation existing between the redox mediators and sphingolipid molecules that have a fundamental impact on a cell’s fate and organ function. This review will summarize the effects of the different redox-active mediators on sphingolipid signalling and metabolism, and the impact of this cross-talk on pathophysiological processes. The relevance of therapeutic approaches will be highlighted.

Sphingomyelinase activity promotes atrophy and attenuates force in human muscle fibres and is elevated in heart failure patients

BACKGROUND

Activation of sphingomyelinase (SMase) as a result of a general inflammatory response has been implicated as a mechanism underlying disease-related loss of skeletal muscle mass and function in several clinical conditions including heart failure. Here, for the first time, we characterize the effects of SMase activity on human muscle fibre contractile function and assess skeletal muscle SMase activity in heart failure patients.

METHODS

The effects of SMase on force production and intracellular Ca²⁺ handling were investigated in single intact human muscle fibres. Additional mechanistic studies were performed in single mouse toe muscle fibres. RNA sequencing was performed in human muscle bundles exposed to SMase. Intramuscular SMase activity was measured from heart failure patients (n = 61, age 69 ± 0.8 years, NYHA III-IV, ejection fraction 25 ± 1.0%, peak VO₂ 14.4 ± 0.6 mL × kg × min) and healthy age-matched control subjects (n = 10, age 71 ± 2.2 years, ejection fraction 60 ± 1.2%, peak VO₂ 25.8 ± 1.1 mL × kg × min). SMase activity was related to circulatory factors known to be associated with progression and disease severity in heart failure.

RESULTS

Sphingomyelinase reduced muscle fibre force production (-30%, P < 0.05) by impairing sarcoplasmic reticulum (SR) Ca²⁺ release (P < 0.05) and reducing myofibrillar Ca²⁺ sensitivity. In human muscle bundles exposed to SMase, RNA sequencing analysis revealed 180 and 291 genes as up-regulated and down-regulated, respectively, at a FDR of 1%. Gene-set enrichment analysis identified 'proteasome degradation' as an up-regulated pathway (average fold-change 1.1, P = 0.008), while the pathway 'cytoplasmic ribosomal proteins' (average fold-change 0.8, P < 0.0001) and factors involving proliferation of muscle cells (average fold-change 0.8, P = 0.0002) where identified as down-regulated. Intramuscular SMase activity was ~20% higher (P < 0.05) in human heart failure patients than in age-matched healthy controls and was positively correlated with markers of disease severity and progression, and with several circulating inflammatory proteins, including TNF-receptor 1 and 2. In a longitudinal cohort of heart failure patients (n = 6, mean follow-up time 2.5 ± 0.2 years), SMase activity was demonstrated to increase by 30% (P < 0.05) with duration of disease.

CONCLUSIONS

The present findings implicate activation of skeletal muscle SMase as a mechanism underlying human heart failure-related loss of muscle mass and function. Moreover, our findings strengthen the idea that SMase activation may underpin disease-related loss of muscle mass and function in other clinical conditions, acting as a common patophysiological mechanism for the myopathy often reported in diseases associated with a systemic inflammatory response.

Vascular Smooth Muscle Cell Neutral Sphingomyelinase 2 in the Release of Exosomes and Vascular Calcification

Vascular calcification (VC) is the pathological precipitation of calcium salts in the walls of blood vessels. It is a risk factor for cardiovascular events and their associated mortality. VC can be observed in a variety of cardiovascular diseases and is most prominent in diseases that are associated with dysregulated mineral homeostasis such as in chronic kidney disease. Local factors and mechanisms underlying VC are still incompletely understood, but it is appreciated that VC is a multifactorial process in which vascular smooth muscle cells (VSMCs) play an important role. VSMCs participate in VC by releasing extracellular vesicles (EVs), the extent, composition, and propensity to calcify of which depend on VSMC phenotype and microenvironment. Currently, no targeted therapy is available to treat VC. In-depth knowledge of molecular players of EV release and the understanding of their mechanisms constitute a vital foundation for the design of pharmacological treatments to combat VC effectively. This review highlights our current knowledge of VSMCs in VC and focuses on the biogenesis of exosomes and the role of the neutral Sphingomyelinase 2 (nSMase2).

Ca2+-activated sphingomyelin scrambling and turnover mediate ESCRT-independent lysosomal repair

Lysosomes are vital organelles vulnerable to injuries from diverse materials. Failure to repair or sequester damaged lysosomes poses a threat to cell viability. Here we report that cells exploit a sphingomyelin-based lysosomal repair pathway that operates independently of ESCRT to reverse potentially lethal membrane damage. Various conditions perturbing organelle integrity trigger a rapid calcium-activated scrambling and cytosolic exposure of sphingomyelin. Subsequent metabolic conversion of sphingomyelin by neutral sphingomyelinases on the cytosolic surface of injured lysosomes promotes their repair, also when ESCRT function is compromised. Conversely, blocking turnover of cytosolic sphingomyelin renders cells more sensitive to lysosome-damaging drugs. Our data indicate that calcium-activated scramblases, sphingomyelin, and neutral sphingomyelinases are core components of a previously unrecognized membrane restoration pathway by which cells preserve the functional integrity of lysosomes.

Dysregulation of multiple signaling pathways: A possible cause of cerebral palsy

Cerebral palsy (CP) is a lifelong disability characterized by the impairment of brain functions that result in improper posture and abnormal motor patterns. Understanding this brain abnormality and the role of genetic, epigenetic, and non-genetic factors such as signaling pathway dysregulation and cytokine dysregulation in the pathogenesis of CP is a complex process. Hypoxic-ischemic injury and prematurity are two well-known contributors of CP. Like in the case of other neurodevelopmental disorders such as intellectual disability and autism, the genomic constituents in CP are highly complex. The neuroinflammation that is triggered by maternal cytokine response plays a critical role in the pathogenesis of fetal inflammation response, which is one of the contributing factors of CP, and it continues even after the birth of children suffering from CP. Canonical Wnt signaling pathway is important for the development of mammalian fetal brain and it regulates distinct processes including neurogenesis. The glycogen synthase kinase-3 (GSK-3) antagonistic activity in the Wnt signaling pathway plays a crucial role in neurogenesis and neural development. In this review, we investigated several genetic and non-genetic pathways that are involved in the pathogenesis of CP and their regulation, impairment, and implications for causing CP during embryonic growth and developmental period. Investigating the role of these pathways help to develop novel therapeutic interventions and biomarkers for early diagnosis and treatment. This review also helps us to comprehend the mechanical approach of various signaling pathways, as well as their consequences and relevance in the understanding of CP.

Proximity Ligation Mapping of Microcephaly Associated SMPD4 Shows Association with Components of the Nuclear Pore Membrane

SMPD4 is a neutral sphingomyelinase implicated in a specific type of congenital microcephaly. Although not intensively studied, SMPD4 deficiency has also been found to cause cell division defects. This suggests a role for SMPD4 in cell-cycle and differentiation. In order to explore this role, we used proximity ligation to identify the partners of SMPD4 in vivo in HEK293T cells. We found that these partners localize near the endoplasmic reticulum (ER) and the nuclear membrane. Using mass spectrometry, we could identify these partners and discovered that SMPD4 is closely associated with several nucleoporins, including NUP35, a nucleoporin directly involved in pore membrane curvature and pore insertion. This suggests that SMPD4 may play a role in this process.

Characterization of neutral sphingomyelinase activity and isoform expression in rodent skeletal muscle mitochondria

Skeletal muscle is composed of fiber types that differ in mitochondrial content, antioxidant capacity, and susceptibility to apoptosis. Ceramides have been linked to oxidative stress-mediated apoptotic intracellular signalling and the enzyme neutral sphingomyelinase (nSMase) is, in part, responsible for generating these ceramides through the hydrolysis of sphingomyelin. Despite the role of ceramides in mediating apoptosis, there is a gap in the literature regarding nSMase in skeletal muscle mitochondria. This study aimed to characterize total nSMase activity and individual isoform expression in isolated subsarcolemmal (SS) mitochondria from soleus, diaphragm, plantaris, and extensor digitorum longus (EDL). Total nSMase activity did not differ between muscle types. nSMase2 content was detectable in all muscles and higher in EDL, soleus, and plantaris compared to diaphragm whereas nSMase3 was undetectable in all muscles. Finally, total nSMase activity positively correlated to nSMase2 protein content in soleus but not the other muscles. These findings suggest that nSMase associated with SS mitochondria may play a role in intracellular signalling processes involving ceramides in skeletal muscle and nSMase2 may be the key isoform, specifically in slow twitch muscle like soleus. Further studies are needed to fully elucidate the specific contribution of nSMase, along with the role of the various isoforms and mitochondrial subpopulation in generating mitochondrial ceramides in skeletal muscle, and its potential effects on mediating apoptosis.

Regulation of Death Receptor Signaling by S-Palmitoylation and Detergent-Resistant Membrane Micro Domains-Greasing the Gears of Extrinsic Cell Death Induction, Survival, and Inflammation

Death-receptor-mediated signaling results in either cell death or survival. Such opposite signaling cascades emanate from receptor-associated signaling complexes, which are often formed in different subcellular locations. The proteins involved are frequently post-translationally modified (PTM) by ubiquitination, phosphorylation, or glycosylation to allow proper spatio-temporal regulation/recruitment of these signaling complexes in a defined cellular compartment. During the last couple of years, increasing attention has been paid to the reversible cysteine-centered PTM S-palmitoylation. This PTM regulates the hydrophobicity of soluble and membrane proteins and modulates protein:protein interaction and their interaction with distinct membrane micro-domains (i.e., lipid rafts). We conclude with which functional and mechanistic roles for S-palmitoylation as well as different forms of membrane micro-domains in death-receptor-mediated signal transduction were unraveled in the last two decades.

Chromophobe renal cell carcinoma: New genetic and metabolic insights

Chromophobe renal cell carcinoma (chRCC) represents 5% of all RCC. ChRCC appears to arise from the distal nephron, in contrast to clear cell RCC that arises from the proximal nephron. ChRCC is distinctive because the tumor cells contain abundant abnormal mitochondria, and frequently have loss of one copy of multiple chromosomes, with a low rate of somatic mutations. Here, we focus on recent discoveries related to genetic and metabolic factors that may promote the progression of chRCC and summarize how these findings may relate to the prognosis and treatment of chRCC.

Exploring the Therapeutic Landscape of Sphingomyelinases

Sphingosine, ceramide, sphingosine-1-phosphate, and other related sphingolipids have emerged as important bioactive molecules involved in a variety of key cellular processes such as cell growth, differentiation, apoptosis, exosome release, and inter- and intracellular cell communication, making the pathways of sphingolipid metabolism a key domain in maintaining cell homeostasis (Hannun and Obeid, Trends Biochem Sci 20:73-77, 1995; Hannun and Obeid, Nat Rev Mol Cell Biol 9:139-150, 2008; Kosaka et al., J Biol Chem 288:10849-10859, 2013). Various studies have determined that these pathways play a central role in regulating intracellular production of ceramide and the other bioactive sphingolipids and hence are an important component of signaling in various diseases such as cancer, diabetes, and neurodegenerative and cardiovascular diseases (Chaube et al., Biochim Biophys Acta 1821:313-323, 2012; Clarke et al., Adv Enzyme Regul 51:51-58, 2011b; Horres and Hannun, Neurochem Res 37:1137-1149, 2012). In this chapter, we discuss one of the major enzyme classes in producing ceramide, sphingomyelinases (SMases), from a biochemical and structural perspective with an emphasis on their applicability as therapeutic targets.

Loss of SMPD4 Causes a Developmental Disorder Characterized by Microcephaly and Congenital Arthrogryposis

Sphingomyelinases generate ceramide from sphingomyelin as a second messenger in intracellular signaling pathways involved in cell proliferation, differentiation, or apoptosis. Children from 12 unrelated families presented with microcephaly, simplified gyral pattern of the cortex, hypomyelination, cerebellar hypoplasia, congenital arthrogryposis, and early fetal/postnatal demise. Genomic analysis revealed bi-allelic loss-of-function variants in SMPD4, coding for the neutral sphingomyelinase-3 (nSMase-3/SMPD4). Overexpression of human Myc-tagged SMPD4 showed localization both to the outer nuclear envelope and the ER and additionally revealed interactions with several nuclear pore complex proteins by proteomics analysis. Fibroblasts from affected individuals showed ER cisternae abnormalities, suspected for increased autophagy, and were more susceptible to apoptosis under stress conditions, while treatment with siSMPD4 caused delayed cell cycle progression. Our data show that SMPD4 links homeostasis of membrane sphingolipids to cell fate by regulating the cross-talk between the ER and the outer nuclear envelope, while its loss reveals a pathogenic mechanism in microcephaly.

Changes in Membrane Ceramide Pools in Rat Soleus Muscle in Response to Short-Term Disuse

Lipid raft disruption is an early event during skeletal muscle unloading. Ceramide (Cer) serves as a signaling lipid that can contribute to lipid raft disturbance and muscle atrophy. Using biochemical and fluorescent approaches, the distribution of Cer and related molecules in the rat soleus muscle subjected to 12 h of hindlimb suspension (HS) was studied. HS led to upregulation of TNFα receptor 1 (TNFR1), Cer-producing enzymes, and acid and neutral sphingomyelinase (SMase) in detergent-resistant membranes (lipid rafts), which was accompanied by an increase in Cer and a decrease in sphingomyelin in this membrane fraction. Fluorescent labeling indicated increased Cer in the sarcoplasm as well as the junctional (synaptic) and extrajunctional compartments of the suspended muscles. Also, a loss of membrane asymmetry (a hallmark of membrane disturbance) was induced by HS. Pretreatment with clomipramine, a functional inhibitor of acid SMase, counteracted HS-mediated changes in the Cer/sphingomyelin ratio and acid SMase abundance as well as suppressed Cer accumulation in the intracellular membranes of junctional and extrajunctional regions. However, the elevation of plasma membrane Cer and disturbance of the membrane asymmetry were suppressed only in the junctional compartment. We suggest that acute HS leads to TNFR1 and SMase upregulation in the lipid raft fraction and deposition of Cer throughout the sarcolemma and intracellularly. Clomipramine-mediated downregulation of acid SMase can suppress Cer accumulation in all compartments, excluding the extrajunctional plasma membrane.

Palmitoylation is required for TNF-R1 signaling

BACKGROUND

Binding of tumor necrosis factor (TNF) to TNF-receptor 1 (TNF-R1) can induce either cell survival or cell death. The selection between these diametrically opposed effects depends on the subcellular location of TNF-R1: plasma membrane retention leads to survival, while endocytosis leads to cell death. How the respective TNF-R1 associated signaling complexes are recruited to the distinct subcellular location is not known. Here, we identify palmitoylation of TNF-R1 as a molecular mechanism to achieve signal diversification.

METHODS

Human monocytic U937 cells were analyzed. Palmitoylated proteins were enriched by acyl resin assisted capture (AcylRAC) and analyzed by western blot and mass spectrometry. Palmitoylation of TNF-R1 was validated by metabolic labeling. TNF induced depalmitoylation and involvement of APT2 was analyzed by enzyme activity assays, pharmacological inhibition and shRNA mediated knock-down. TNF-R1 palmitoylation site analysis was done by mutated TNF-R1 expression in TNF-R1 knock-out cells. Apoptosis (nuclear DNA fragmentation, caspase 3 assays), NF-κB activation and TNF-R1 internalization were used as biological readouts.

RESULTS

We identify dynamic S-palmitoylation as a new mechanism that controls selective TNF signaling. TNF-R1 itself is constitutively palmitoylated and depalmitoylated upon ligand binding. We identified the palmitoyl thioesterase APT2 to be involved in TNF-R1 depalmitoylation and TNF induced NF-κB activation. Mutation of the putative palmitoylation site C248 interferes with TNF-R1 localization to the plasma membrane and thus, proper signal transduction.

CONCLUSIONS

Our results introduce palmitoylation as a new layer of dynamic regulation of TNF-R1 induced signal transduction at a very early step of the TNF induced signaling cascade. Understanding the underlying mechanism may allow novel therapeutic options for disease treatment in future.

Probing compartment-specific sphingolipids with targeted bacterial sphingomyelinases and ceramidases

Sphingolipids contribute to the regulation of cell and tissue homeostasis, and disorders of sphingolipid metabolism lead to diseases such as inflammation, stroke, diabetes, and cancer. Sphingolipid metabolic pathways involve an array of enzymes that reside in specific subcellular organelles, resulting in the formation of many diverse sphingolipids with distinct molecular species based on the diversity of the ceramide (Cer) structure. In order to probe compartment-specific metabolism of sphingolipids in this study, we analyzed the Cer and SM species preferentially produced in the inner plasma membrane (PM), Golgi apparatus, ER, mitochondria, nucleus, and cytoplasm by using compartmentally targeted bacterial SMases and ceramidases. The results showed that the length of the acyl chain of Cer becomes longer according to the progress of Cer from synthesis in the ER to the Golgi apparatus, then to the PM. These findings suggest that each organelle shows different properties of SM-derived Cers consistent with its emerging distinct functions in vitro and in vivo.

Muscle transcriptome signature and gene regulatory network analysis in two divergent lines of a hilly bovine species Mithun (Bos frontalis)

A massive bovine, Bos frontalis, also known as Mithun or Gayal, found at higher altitude is very promising meat and milk animal. For candidate gene and marker discovery, RNA-seq data was generated from longissimus dorsi muscle tissues with Illumina-HiSeq. Such markers can be used in future for genetic gain of traits like feed conversion efficiency (FCE) and average daily gain (ADG). Analysis revealed 297differentially expressed genes (DEGs) having 173 up and 124 down-regulated unigenes. Extensive conservation was found in genic region while comparing with Bos taurus. Analysis revealed 57 pathways having 112 enzymes, 72 transcriptional factors and cofactors, 212 miRNAs regulating 71 DEGs, 25,855 SSRs, mithun-specific 104,822 variants and 7288 indels, gene regulatory network (GRN) having 24 hub-genes and transcriptional factors regulating cell proliferation, immune tolerance and myogenesis. This is first report of muscle transcriptome depicting candidate genes with GRN controlling FCE and ADG. Reported putative molecular markers, candidate genes and hub proteins can be valuable genomic resources for association studies in genetic improvement programme.

Small-hairpin RNA and pharmacological targeting of neutral sphingomyelinase prevent diaphragm weakness in rats with heart failure and reduced ejection fraction

Heart failure with reduced ejection fraction (HFREF) increases neutral sphingomyelinase (NSMase) activity and mitochondrial reactive oxygen species (ROS) emission and causes diaphragm weakness. We tested whether a systemic pharmacological NSMase inhibitor or short-hairpin RNA (shRNA) targeting NSMase isoform 3 (NSMase3) would prevent diaphragm abnormalities induced by HFREF caused by myocardial infarction. In the pharmacological intervention, we used intraperitoneal injection of GW4869 or vehicle. In the genetic intervention, we injected adeno-associated virus serotype 9 (AAV9) containing shRNA targeting NSMase3 or a scrambled sequence directly into the diaphragm. We also studied acid sphingomyelinase-knockout mice. GW4869 prevented the increase in diaphragm ceramide content, weakness, and tachypnea caused by HFREF. For example, maximal specific forces (in N/cm²) were vehicle [sham 31 ± 2 and HFREF 26 ± 2 ( P < 0.05)] and GW4869 (sham 31 ± 2 and HFREF 31 ± 1). Respiratory rates were (in breaths/min) vehicle [sham 61 ± 3 and HFREF 84 ± 11 ( P < 0.05)] and GW4869 (sham 66 ± 2 and HFREF 72 ± 2). AAV9-NSMase3 shRNA prevented heightening of diaphragm mitochondrial ROS and weakness [in N/cm², AAV9-scrambled shRNA: sham 31 ± 2 and HFREF 27 ± 2 ( P < 0.05); AAV9-NSMase3 shRNA: sham 30 ± 1 and HFREF 30 ± 1] but displayed tachypnea. Both wild-type and ASMase-knockout mice with HFREF displayed diaphragm weakness. Our study suggests that activation of NSMase3 causes diaphragm weakness in HFREF, presumably through accumulation of ceramide and elevation in mitochondrial ROS. Our data also reveal a novel inhibitory effect of GW4869 on tachypnea in HFREF likely mediated by changes in neural control of breathing.

Caveolae-mediated effects of TNF-α on human skeletal muscle cells

Chronic diseases are characterized by the production of pro-inflammatory cytokines such than TNF-α and are frequently correlated with muscle wasting conditions. Among the pleiotropic effects of TNF-α within the cell, its binding to TNFR1 receptor has been shown to activate sphingomyelinases leading to the production of ceramides. Sphingomyelinases and TNF receptor have been localized within caveolae which are specialized RAFT enriched in cholesterol and sphingolipids. Because of their inverted omega shape, maintained by the oligomerization of specialized proteins, caveolins and cavins, caveolae serve as membrane reservoir therefore providing mechanical protection to plasma membranes. Although sphingolipids metabolites, caveolins and TNF-α/TNFR1 have been shown to independently interfere with muscle physiology, no data have clearly demonstrated their concerted action on muscle cell regeneration. In this context, our study aimed at studying the molecular mechanisms induced by TNF-α at the level of caveolae in LHCN-M2 human muscle satellite cells. Here we showed that TNF-α-induced production of ROS and nSMase activation requires caveolin. More strikingly, we have demonstrated that TNF-α induces the formation of additional caveolae at the plasma membrane of myoblasts. Furthermore, TNF-α prevents myoblast fusion suggesting that inflammation could modulate caveolae organization/function and satellite cell function.

Neutral Sphingomyelinases in Cancer: Friend or Foe?

For many years, neutral sphingomyelinases (N-SMases) were long thought to be anticancer enzymes owing to their roles as key producers of ceramide linked to apoptosis, growth arrest, and the chemotherapeutic response. However, in recent years, with the cloning of multiple isoforms and with new information on their cellular roles, particularly for nSMase2, a more complex picture is emerging suggesting that N-SMases have both pro- and anticancer roles. In this chapter, we will summarize current knowledge on N-SMase expression in cancer and the roles of N-SMase activity and specific isoforms in cancer-relevant biologies. We will also discuss what we see as the major challenges ahead for research into N-SMases in cancer.

The HBx gene of hepatitis B virus can influence hepatic microenvironment via exosomes by transferring its mRNA and protein

The cellular secretory vesicles known as 'exosomes' have emerged as key player in intercellular transport and communication between different eukaryotic in order to maintain body homeostasis. Many pathogenic viruses utilize exosome pathway to efficiently transfer bioactive components from infected cells to naïve cells. Here, we show that HBx can tweak the exosome biogenesis machinery both by enhancing neutral sphingomyelinase2 activity as well as by interacting with exosomal biomarkers such as neutral sphingomyelinase2, CD9 and CD81. The nano particle tracking analysis revealed enhanced secretion of exosomes by the HBx-expressing cells while confocal studies confirmed the co-localization of HBx with CD9 and CD63. Importantly, we observed the encapsulation of HBx mRNA and protein in these exosomes besides some other qualitative changes. The exosomal cargo secreted by HBx-expressing cells had a profound effect on the recipient hepatic cells including creation of a milieu conducive for cellular-transformation. Thus, the present study unfolds a novel role of HBx in intercellular communication by facilitating horizontal transfer of viral gene products and other host factors via exosomes in order to support viral spread and pathogenesis.

Disease-Induced Skeletal Muscle Atrophy and Fatigue

Numerous health problems, including acute critical illness, cancer, diseases associated with chronic inflammation, and neurological disorders, often result in skeletal muscle weakness and fatigue. Disease-related muscle atrophy and fatigue is an important clinical problem because acquired skeletal muscle weakness can increase the duration of hospitalization, result in exercise limitation, and contribute to a poor quality of life. Importantly, skeletal muscle atrophy is also associated with increased morbidity and mortality of patients. Therefore, improving our understanding of the mechanism(s) responsible for skeletal muscle weakness and fatigue in patients is a required first step to develop clinical protocols to prevent these skeletal muscle problems. This review will highlight the consequences and potential mechanisms responsible for skeletal muscle atrophy and fatigue in patients experiencing acute critical illness, cancer, chronic inflammatory diseases, and neurological disorders.

Patients experiencing statin-induced myalgia exhibit a unique program of skeletal muscle gene expression following statin re-challenge

Statins, the 3-hydroxy-3-methyl-glutaryl (HMG)-CoA reductase inhibitors, are widely prescribed for treatment of hypercholesterolemia. Although statins are generally well tolerated, up to ten percent of statin-treated patients experience myalgia symptoms, defined as muscle pain without elevated creatinine phosphokinase (CPK) levels. Myalgia is the most frequent reason for discontinuation of statin therapy. The mechanisms underlying statin myalgia are not clearly understood. To elucidate changes in gene expression associated with statin myalgia, we compared profiles of gene expression in skeletal muscle biopsies from patients with statin myalgia who were undergoing statin re-challenge (cases) versus those of statin-tolerant controls. A robust separation of case and control cohorts was revealed by Principal Component Analysis of differentially expressed genes (DEGs). To identify putative gene expression and metabolic pathways that may be perturbed in skeletal muscles of patients with statin myalgia, we subjected DEGs to Ingenuity Pathways (IPA) and DAVID (Database for Annotation, Visualization and Integrated Discovery) analyses. The most prominent pathways altered by statins included cellular stress, apoptosis, cell senescence and DNA repair (TP53, BARD1, Mre11 and RAD51); activation of pro-inflammatory immune response (CXCL12, CST5, POU2F1); protein catabolism, cholesterol biosynthesis, protein prenylation and RAS-GTPase activation (FDFT1, LSS, TP53, UBD, ATF2, H-ras). Based on these data we tentatively conclude that persistent myalgia in response to statins may emanate from cellular stress underpinned by mechanisms of post-inflammatory repair and regeneration. We also posit that this subset of individuals is genetically predisposed to eliciting altered statin metabolism and/or increased end-organ susceptibility that lead to a range of statin-induced myopathies. This mechanistic scenario is further bolstered by the discovery that a number of single nucleotide polymorphisms (e.g., SLCO1B1, SLCO2B1 and RYR2) associated with statin myalgia and myositis were observed with increased frequency among patients with statin myalgia.

Skeletal Muscle Alterations Are Exacerbated in Heart Failure With Reduced Compared With Preserved Ejection Fraction: Mediated by Circulating Cytokines?

BACKGROUND

A greater understanding of the different underlying mechanisms between patients with heart failure with reduced (HFrEF) and with preserved (HFpEF) ejection fraction is urgently needed to better direct future treatment. However, although skeletal muscle impairments, potentially mediated by inflammatory cytokines, are common in both HFrEF and HFpEF, the underlying cellular and molecular alterations that exist between groups are yet to be systematically evaluated. The present study, therefore, used established animal models to compare whether alterations in skeletal muscle (limb and respiratory) were different between HFrEF and HFpEF, while further characterizing inflammatory cytokines.

METHODS AND RESULTS

Rats were assigned to (1) HFrEF (ligation of the left coronary artery; n=8); (2) HFpEF (high-salt diet; n=10); (3) control (con: no intervention; n=7). Heart failure was confirmed by echocardiography and invasive measures. Soleus tissue in HFrEF, but not in HFpEF, showed a significant increase in markers of (1) muscle atrophy (ie, MuRF1, calpain, and ubiquitin proteasome); (2) oxidative stress (ie, higher nicotinamide adenine dinucleotide phosphate oxidase but lower antioxidative enzyme activities); (3) mitochondrial impairments (ie, a lower succinate dehydrogenase/lactate dehydrogenase ratio and peroxisome proliferator-activated receptor-γ coactivator-1α expression). The diaphragm remained largely unaffected between groups. Plasma concentrations of circulating cytokines were significantly increased in HFrEF for tumor necrosis factor-α, whereas interleukin-1β and interleukin-12 were higher in HFpEF.

CONCLUSIONS

Our findings suggest, for the first time, that skeletal muscle alterations are exacerbated in HFrEF compared with HFpEF, which predominantly reside in limb, rather than in respiratory, muscle. This disparity may be mediated, in part, by the different circulating inflammatory cytokines that were elevated between HFpEF and HFrEF.

Regulation of sphingomyelin metabolism

Sphingolipids (SFs) represent a large class of lipids playing diverse functions in a vast number of physiological and pathological processes. Sphingomyelin (SM) is the most abundant SF in the cell, with ubiquitous distribution within mammalian tissues, and particularly high levels in the Central Nervous System (CNS). SM is an essential element of plasma membrane (PM) and its levels are crucial for the cell function. SM content in a cell is strictly regulated by the enzymes of SM metabolic pathways, which activities create a balance between SM synthesis and degradation. The de novo synthesis via SM synthases (SMSs) in the last step of the multi-stage process is the most important pathway of SM formation in a cell. The SM hydrolysis by sphingomyelinases (SMases) increases the concentration of ceramide (Cer), a bioactive molecule, which is involved in cellular proliferation, growth and apoptosis. By controlling the levels of SM and Cer, SMSs and SMases maintain cellular homeostasis. Enzymes of SM cycle exhibit unique properties and diverse tissue distribution. Disturbances in their activities were observed in many CNS pathologies. This review characterizes the physiological roles of SM and enzymes controlling SM levels as well as their involvement in selected pathologies of the Central Nervous System, such as ischemia/hypoxia, Alzheimer disease (AD), Parkinson disease (PD), depression, schizophrenia and Niemann Pick disease (NPD).

Role of Sphingolipids and Metabolizing Enzymes in Hematological Malignancies

Sphingolipids such as ceramide, sphingosine-1-phosphate and sphingomyelin have been emerging as bioactive lipids since ceramide was reported to play a role in human leukemia HL-60 cell differentiation and death. Recently, it is well-known that ceramide acts as an inducer of cell death, that sphingomyelin works as a regulator for microdomain function of the cell membrane, and that sphingosine-1-phosphate plays a role in cell survival/proliferation. The lipids are metabolized by the specific enzymes, and each metabolite could be again returned to the original form by the reverse action of the different enzyme or after a long journey of many metabolizing/synthesizing pathways. In addition, the metabolites may serve as reciprocal bio-modulators like the rheostat between ceramide and sphingosine-1-phosphate. Therefore, the change of lipid amount in the cells, the subcellular localization and the downstream signal in a specific subcellular organelle should be clarified to understand the pathobiological significance of sphingolipids when extracellular stimulation induces a diverse of cell functions such as cell death, proliferation and migration. In this review, we focus on how sphingolipids and their metabolizing enzymes cooperatively exert their function in proliferation, migration, autophagy and death of hematopoetic cells, and discuss the way developing a novel therapeutic device through the regulation of sphingolipids for effectively inhibiting cell proliferation and inducing cell death in hematological malignancies such as leukemia, malignant lymphoma and multiple myeloma.

Proteomic analysis of media from lung cancer cells reveals role of 14-3-3 proteins in cachexia

Aims: At the time of diagnosis, 60% of lung cancer patients present with cachexia, a severe wasting syndrome that increases morbidity and mortality. Tumors secrete multiple factors that contribute to cachectic muscle wasting, and not all of these factors have been identified. We used Orbitrap electrospray ionization mass spectrometry to identify novel cachexia-inducing candidates in media conditioned with Lewis lung carcinoma cells (LCM). Results: One-hundred and 58 proteins were confirmed in three biological replicates. Thirty-three were identified as secreted proteins, including 14-3-3 proteins, which are highly conserved adaptor proteins known to have over 200 binding partners. We confirmed the presence of extracellular 14-3-3 proteins in LCM via western blot and discovered that LCM contained less 14-3-3 content than media conditioned with C2C12 myotubes. Using a neutralizing antibody, we depleted extracellular 14-3-3 proteins in myotube culture medium, which resulted in diminished myosin content. We identified the proposed receptor for 14-3-3 proteins, CD13, in differentiated C2C12 myotubes and found that inhibiting CD13 via Bestatin also resulted in diminished myosin content. Conclusions: Our novel findings show that extracellular 14-3-3 proteins may act as previously unidentified myokines and may signal via CD13 to help maintain muscle mass.

Mitochondria dysfunction in lung cancer-induced muscle wasting in C2C12 myotubes

AIMS

Cancer cachexia is a syndrome which results in severe loss of muscle mass and marked fatigue. Conditioned media from cachexia-inducing cancer cells triggers metabolic dysfunction in skeletal muscle, including decreased mitochondrial respiration, which may contribute to fatigue. We hypothesized that Lewis lung carcinoma conditioned medium (LCM) would impair the mitochondrial electron transport chain (ETC) and increase production of reactive oxygen species, ultimately leading to decreased mitochondrial respiration. We incubated C2C12 myotubes with LCM for 30 min, 2, 4, 24 or 48 h. We measured protein content by western blot; oxidant production by 2',7'-dichlorofluorescin diacetate (DCF), 4-amino-5-methylamino-2',7'-difluorofluorescein diacetate (DAF), and MitoSox; cytochrome c oxidase activity by oxidation of cytochrome c substrate; and oxygen consumption rate (OCR) of intact myotubes by Seahorse XF Analyzer.

RESULTS

LCM treatment for 2 or 24 h decreased basal OCR and ATP-related OCR, but did not alter the content of mitochondrial complexes I, III, IV and V. LCM treatment caused a transient rise in reactive oxygen species (ROS). In particular, mitochondrial superoxide (MitoSOX) was elevated at 2 h. 4-Hydroxynonenal, a marker of oxidative stress, was elevated in both cytosolic and mitochondrial fractions of cell lysates after LCM treatment.

CONCLUSION

These data show that lung cancer-conditioned media alters electron flow in the ETC and increases mitochondrial ROS production, both of which may ultimately impair aerobic metabolism and decrease muscle endurance.