TAK1 regulates skeletal muscle mass and mitochondrial function

Secondary interactions in ubiquitin-binding domains achieve linkage or substrate specificity

Small ubiquitin-binding domains (UBDs) recognize small surface patches on ubiquitin with weak affinity, and it remains a conundrum how specific cellular responses may be achieved. Npl4-type zinc-finger (NZF) domains are ∼30 amino acid, compact UBDs that can provide two ubiquitin-binding interfaces, imposing linkage specificity to explain signaling outcomes. We here comprehensively characterize the linkage preference of human NZF domains. TAB2 prefers Lys6 and Lys63 linkages phosphorylated on Ser65, explaining why TAB2 recognizes depolarized mitochondria. Surprisingly, most NZF domains do not display chain linkage preference, despite conserved, secondary interaction surfaces. This suggests that some NZF domains may specifically bind ubiquitinated substrates by simultaneously recognizing substrate and an attached ubiquitin. We show biochemically and structurally that the NZF1 domain of the E3 ligase HOIPbinds preferentially to site-specifically ubiquitinated forms of NEMO and optineurin. Thus, despite their small size, UBDs may impose signaling specificity via multivalent interactions with ubiquitinated substrates.

The IRE1α/XBP1 signaling axis drives myoblast fusion in adult skeletal muscle

Skeletal muscle regeneration involves a signaling network that regulates the proliferation, differentiation, and fusion of muscle precursor cells to injured myofibers. IRE1α, one of the arms of the unfolded protein response, regulates cellular proteostasis in response to ER stress. Here, we demonstrate that inducible deletion of IRE1α in satellite cells of mice impairs skeletal muscle regeneration through inhibiting myoblast fusion. Knockdown of IRE1α or its downstream target, X-box protein 1 (XBP1), also inhibits myoblast fusion during myogenesis. Transcriptome analysis revealed that knockdown of IRE1α or XBP1 dysregulates the gene expression of molecules involved in myoblast fusion. The IRE1α-XBP1 axis mediates the gene expression of multiple profusion molecules, including myomaker (Mymk). Spliced XBP1 (sXBP1) transcription factor binds to the promoter of Mymk gene during myogenesis. Overexpression of myomaker in IRE1α-knockdown cultures rescues fusion defects. Inducible deletion of IRE1α in satellite cells also inhibits myoblast fusion and myofiber hypertrophy in response to functional overload. Collectively, our study demonstrates that IRE1α promotes myoblast fusion through sXBP1-mediated up-regulation of the gene expression of multiple profusion molecules, including myomaker.

mTORC1 and BMP-Smad1/5 regulation of serum-stimulated myotube hypertrophy: a role for autophagy

Protein synthesis regulation is critical for skeletal muscle hypertrophy, yet other established cellular processes are necessary for growth-related cellular remodeling. Autophagy has a well-acknowledged role in muscle quality control, but evidence for its role in myofiber hypertrophy remains equivocal. Both mammalian target of rapamycin complex I (mTORC1) and bone morphogenetic protein (BMP)-Smad1/5 (Sma and Mad proteins from Caenorhabditis elegans and Drosophila, respectively) signaling are reported regulators of myofiber hypertrophy; however, gaps remain in our understanding of how this regulation is integrated with growth processes and autophagy regulation. Therefore, we investigated the mTORC1 and Smad1/5 regulation of protein synthesis and autophagy flux during serum-stimulated myotube growth. Chronic serum stimulation experiments were performed on day 5 differentiated C2C12 myotubes incubated in differentiation medium [2% horse serum (HS)] or growth medium [5% fetal bovine serum (FBS)] for 48 h. Rapamycin or LDN193189 was dosed for 48 h to inhibit mTORC1 and BMP-Smad1/5 signaling, respectively. Acute serum stimulation was examined in day 7 differentiated myotubes. Protein synthesis was measured by puromycin incorporation. Bafilomycin A1 and immunoblotting for LC3B were used to assess autophagy flux. Chronic serum stimulation increased myotube diameter 22%, total protein 21%, total RNA 100%, and Smad1/5 phosphorylation 404% and suppressed autophagy flux. Rapamycin, but not LDN193189, blocked serum-induced myotube hypertrophy and the increase in total RNA. Acute serum stimulation increased protein synthesis 111%, Smad1/5 phosphorylation 559%, and rpS6 phosphorylation 117% and suppressed autophagy flux. Rapamycin increased autophagy flux during acute serum stimulation. These results provide evidence for mTORC1, but not BMP-Smad1/5, signaling being required for serum-induced myotube hypertrophy and autophagy flux by measuring LC3BII/I expression. Further investigation is warranted to examine the role of autophagy flux in myotube hypertrophy.NEW & NOTEWORTHY The present study demonstrates that myotube hypertrophy caused by chronic serum stimulation requires mammalian target of rapamycin complex 1 (mTORC1) signaling but not bone morphogenetic protein (BMP)-Smad1/5 signaling. The suppression of autophagy flux was associated with serum-induced myotube hypertrophy and mTORC1 regulation of autophagy flux by measuring LC3BII/I expression. Rapamycin is widely investigated for beneficial effects in aging skeletal muscle and sarcopenia; our results provide evidence that rapamycin can regulate autophagy-related signaling during myotube growth, which could benefit skeletal muscle functional and metabolic health.

Thbs1 regulates skeletal muscle mass in a TGFβ-Smad2/3-ATF4-dependent manner

Loss of muscle mass is a feature of chronic illness and aging. Here, we report that skeletal muscle-specific thrombospondin-1 transgenic mice (Thbs1 Tg) have profound muscle atrophy with age-dependent decreases in exercise capacity and premature lethality. Mechanistically, Thbs1 activates transforming growth factor β (TGFβ)-Smad2/3 signaling, which also induces activating transcription factor 4 (ATF4) expression that together modulates the autophagy-lysosomal pathway (ALP) and ubiquitin-proteasome system (UPS) to facilitate muscle atrophy. Indeed, myofiber-specific inhibition of TGFβ-receptor signaling represses the induction of ATF4, normalizes ALP and UPS, and partially restores muscle mass in Thbs1 Tg mice. Similarly, myofiber-specific deletion of Smad2 and Smad3 or the Atf4 gene antagonizes Thbs1-induced muscle atrophy. More importantly, Thbs1-/- mice show significantly reduced levels of denervation- and caloric restriction-mediated muscle atrophy, along with blunted TGFβ-Smad3-ATF4 signaling. Thus, Thbs1-mediated TGFβ-Smad3-ATF4 signaling in skeletal muscle regulates tissue rarefaction, suggesting a target for atrophy-based muscle diseases and sarcopenia with aging.

Inflammatory Cytokine-Induced Muscle Atrophy and Weakness Can Be Ameliorated by an Inhibition of TGF-β-Activated Kinase-1

Chronic inflammation causes muscle wasting. Because most inflammatory cytokine signals are mediated via TGF-β-activated kinase-1 (TAK1) activation, inflammatory cytokine-induced muscle wasting may be ameliorated by the inhibition of TAK1 activity. The present study was undertaken to clarify whether TAK1 inhibition can ameliorate inflammation-induced muscle wasting. SKG/Jcl mice as an autoimmune arthritis animal model were treated with a small amount of mannan as an adjuvant to enhance the production of TNF-α and IL-1β. The increase in these inflammatory cytokines caused a reduction in muscle mass and strength along with an induction of arthritis in SKG/Jcl mice. Those changes in muscle fibers were mediated via the phosphorylation of TAK1, which activated the downstream signaling cascade via NF-κB, p38 MAPK, and ERK pathways, resulting in an increase in myostatin expression. Myostatin then reduced the expression of muscle proteins not only via a reduction in MyoD1 expression but also via an enhancement of Atrogin-1 and Murf1 expression. TAK1 inhibitor, LL-Z1640-2, prevented all the cytokine-induced changes in muscle wasting. Thus, TAK1 inhibition can be a new therapeutic target of not only joint destruction but also muscle wasting induced by inflammatory cytokines.

TAK1 inhibition mitigates intracerebral hemorrhage-induced brain injury through reduction of oxidative stress and neuronal pyroptosis via the NRF2 signaling pathway

Introduction

Intracerebral hemorrhage (ICH) often triggers oxidative stress through reactive oxygen species (ROS). Transforming growth factor-β-activated kinase 1 (TAK1) plays a pivotal role in regulating oxidative stress and inflammation across various diseases. 5Z-7-Oxozeaenol (OZ), a specific inhibitor of TAK1, has exhibited therapeutic effects in various conditions. However, the impact of OZ following ICH and its underlying molecular mechanisms remain elusive. This study aimed to explore the possible role of OZ in ICH and its underlying mechanisms by inhibiting oxidative stress-mediated pyroptosis.

Methods

Adult male Sprague-Dawley rats were subjected to an ICH model, followed by treatment with OZ. Neurobehavioral function, blood-brain barrier integrity, neuronal pyroptosis, and oxidative stress markers were assessed using various techniques including behavioral tests, immunofluorescence staining, western blotting, transmission electron microscopy, and biochemical assays.

Results

Our study revealed that OZ administration significantly inhibited phosphorylated TAK1 expression post-ICH. Furthermore, TAK1 blockade by OZ attenuated blood-brain barrier (BBB) disruption, neuroinflammation, and oxidative damage while enhancing neurobehavioral function. Mechanistically, OZ administration markedly reduced ROS production and oxidative stress by facilitating nuclear factor-erythroid 2-related factor 2 (NRF2) nuclear translocation. This was accompanied by a subsequent suppression of the NOD-like receptor protein 3 (NLRP3) activation-mediated inflammatory cascade and neuronal pyroptosis.

Discussion

Our findings highlight that OZ alleviates brain injury and oxidative stress-mediated pyroptosis via the NRF2 pathway. Inhibition of TAK1 emerges as a promising approach for managing ICH.

Kongensin a attenuates intervertebral disc degeneration by inhibiting TAK1-mediated PANoptosis of nucleus pulposus cells

Low back pain (LBP) is most commonly caused by intervertebral disc degeneration (IVDD). Pyroptosis, apoptosis, and necroptosis are crucial in IVDD pathogenesis; however, possible simultaneous occurrence in IVDD and co-regulation between the pathways and the regulatory mechanisms have not been investigated. PANoptosis is a regulated cell death (RCD) pathway with the key characteristics of pyroptosis, apoptosis, and necroptosis. This study revealed that tert-butyl hydroperoxide (TBHP) altered the expression of key proteins involved in PANoptosis in nucleus pulposus cells (NPCs). Furthermore, the natural product Kongensin A (KA), which has potential anti-necrotic and anti-inflammatory properties, inhibited PANoptosis. TAK1, often referred to as mitogen-activated protein kinase kinase kinase 7 (Map3k7), is a key regulator of innate immunity, cell death, inflammation, and cellular homeostasis; however, the physiological roles and regulatory mechanisms underlying IVDD remain unclear. In this study, we discovered that KA can upregulate TAK1 expression in NPCs, -which inhibits PANoptosis by suppressing oxidative stress. In conclusion, our results suggest that KA inhibits PANoptosis and delays IVDD progression in NPCs by upregulating TAK1 expression to maintain mitochondrial redox balance. Consequently, targeting TAK1 may be a promising therapeutic approach for IVDD therapy.

Takinib inhibits microglial M1 polarization and oxidative damage after subarachnoid hemorrhage by targeting TAK1-dependent NLRP3 inflammasome signaling pathway

Transforming growth factor-β-activated kinase 1 (TAK1) positively regulates oxidative stress and inflammation in different diseases. Takinib, a novel and specific TAK1 inhibitor, has beneficial effects in a variety of disorders. However, the effects of takinib on early brain injury (EBI) after subarachnoid hemorrhage (SAH) and the underlying molecular mechanisms remain unknown. Our study showed that takinib administration significantly inhibited phosphorylated TAK1 expression after SAH. In addition, takinib suppressed M1 microglial polarization and promoted M2 microglial polarization. Furthermore, blockade of TAK1 by takinib reduced neuroinflammation, oxidative damage, brain edema, and neuronal apoptosis, and improved neurological behavior after SAH. Mechanistically, we revealed that TAK1 inhibition by takinib mitigated reactive oxygen species (ROS) production and ROS-mediated nod-like receptor pyrin domain-containing protein 3 (NLRP3) inflammasome activation. In contrast, NLRP3 activation by nigericin abated the neuroprotective effects of takinib against EBI after SAH. In general, our study demonstrated that takinib could protect against EBI by targeting TAK1-ROS-NLRP3 inflammasome signaling. Inhibition of TAK1 might be a promising option in the management of SAH.

Transient changes to metabolic homeostasis initiate mitochondrial adaptation to endurance exercise

Endurance exercise is well established to increase mitochondrial content and function in skeletal muscle, a process termed mitochondrial biogenesis. Current understanding is that exercise initiates skeletal muscle mitochondrial remodeling via modulation of cellular nutrient, energetic and contractile stress pathways. These subtle changes in the cellular milieu are sensed by numerous transduction pathways that serve to initiate and coordinate an increase in mitochondrial gene transcription and translation. The result of these acute signaling events is the promotion of growth and assembly of mitochondria, coupled to a greater capacity for aerobic ATP provision in skeletal muscle. The aim of this review is to highlight the acute metabolic events induced by endurance exercise and the subsequent molecular pathways that sense this transient change in cellular homeostasis to drive mitochondrial adaptation and remodeling.

Targeted regulation of TAK1 counteracts dystrophinopathy in a DMD mouse model

Muscular dystrophies make up a group of genetic neuromuscular disorders that involve severe muscle wasting. TGF-β-activated kinase 1 (TAK1) is an important signaling protein that regulates cell survival, growth, and inflammation. TAK1 has been recently found to promote myofiber growth in the skeletal muscle of adult mice. However, the role of TAK1 in muscle diseases remains poorly understood. In the present study, we have investigated how TAK1 affects the progression of dystrophic phenotype in the mdx mouse model of Duchenne muscular dystrophy (DMD). TAK1 is highly activated in the dystrophic muscle of mdx mice during the peak necrotic phase. While targeted inducible inactivation of TAK1 inhibits myofiber injury in young mdx mice, it results in reduced muscle mass and contractile function. TAK1 inactivation also causes loss of muscle mass in adult mdx mice. By contrast, forced activation of TAK1 through overexpression of TAK1 and TAB1 induces myofiber growth without having any deleterious effect on muscle histopathology. Collectively, our results suggest that TAK1 is a positive regulator of skeletal muscle mass and that targeted regulation of TAK1 can suppress myonecrosis and ameliorate disease progression in DMD.

AMP-activated protein kinase: An energy sensor and survival mechanism in the reinstatement of metabolic homeostasis

Cells are programmed to favorably respond towards the nutrient availability by adapting their metabolism to meet energy demands. AMP-activated protein kinase (AMPK) is a highly conserved serine/threonine energy-sensing kinase. It gets activated upon a decrease in the cellular energy status as reflected by an increased AMP/ATP ratio, ADP, and also during the conditions of glucose starvation without change in the adenine nucelotide ratio. AMPK functions as a centralized regulator of metabolism, acting at cellular and physiological levels to circumvent the metabolic stress by restoring energy balance. This review intricately highlights the integrated signaling pathways by which AMPK gets activated allosterically or by multiple non-canonical upstream kinases. AMPK activates the ATP generating processes (e.g., fatty acid oxidation) and inhibits the ATP consuming processes that are non-critical for survival (e.g., cell proliferation, protein and triglyceride synthesis). An integrated signaling network with AMPK as the central effector regulates all the aspects of enhanced stress resistance, qualified cellular housekeeping, and energy metabolic homeostasis. Importantly, the AMPK mediated amelioration of cellular stress and inflammatory responses are mediated by stimulation of transcription factors such as Nrf2, SIRT1, FoxO and inhibition of NF-κB serving as main downstream effectors. Moreover, many lines of evidence have demonstrated that AMPK controls autophagy through mTOR and ULK1 signaling to fine-tune the metabolic pathways in response to different cellular signals. This review also highlights the critical involvement of AMPK in promoting mitochondrial health, and homeostasis, including mitophagy. Loss of AMPK or ULK1 activity leads to aberrant accumulation of autophagy-related proteins and defective mitophagy thus, connecting cellular energy sensing to autophagy and mitophagy.

Emerging role of TAK1 in the regulation of skeletal muscle mass

Maintenance of skeletal muscle mass and strength throughout life is crucial for heathy living and longevity. Several signaling pathways have been implicated in the regulation of skeletal muscle mass in adults. TGF-β-activated kinase 1 (TAK1) is a key protein, which coordinates the activation of multiple signaling pathways. Recently, it was discovered that TAK1 is essential for the maintenance of skeletal muscle mass and myofiber hypertrophy following mechanical overload. Forced activation of TAK1 in skeletal muscle causes hypertrophy and attenuates denervation-induced muscle atrophy. TAK1-mediated signaling in skeletal muscle promotes protein synthesis, redox homeostasis, mitochondrial health, and integrity of neuromuscular junctions. In this article, we have reviewed the role and potential mechanisms through which TAK1 regulates skeletal muscle mass and growth. We have also proposed future areas of research that could be instrumental in exploring TAK1 as therapeutic target for improving muscle mass in various catabolic conditions and diseases.

Rpl3l gene deletion in mice reduces heart weight over time

Introduction: The ribosomal protein L3-like (RPL3L) is a heart and skeletal muscle-specific ribosomal protein and paralogue of the more ubiquitously expressed RPL3 protein. Mutations in the human RPL3L gene are linked to childhood cardiomyopathy and age-related atrial fibrillation, yet the function of RPL3L in the mammalian heart remains unknown. Methods and Results: Here, we observed that mouse cardiac ventricles express RPL3 at birth, where it is gradually replaced by RPL3L in adulthood but re-expressed with induction of hypertrophy in adults. Rpl3l gene-deleted mice were generated to examine the role of this gene in the heart, although Rpl3l -/- mice showed no overt changes in cardiac structure or function at baseline or after pressure overload hypertrophy, likely because RPL3 expression was upregulated and maintained in adulthood. mRNA expression analysis and ribosome profiling failed to show differences between the hearts of Rpl3l null and wild type mice in adulthood. Moreover, ribosomes lacking RPL3L showed no differences in localization within cardiomyocytes compared to wild type controls, nor was there an alteration in cardiac tissue ultrastructure or mitochondrial function in adult Rpl3l -/- mice. Similarly, overexpression of either RPL3 or RPL3L with adeno-associated virus -9 in the hearts of mice did not cause discernable pathology. However, by 18 months of age Rpl3l -/- null mice had significantly smaller hearts compared to wild type littermates. Conclusion: Thus, deletion of Rpl3l forces maintenance of RPL3 expression within the heart that appears to fully compensate for the loss of RPL3L, although older Rpl3l -/- mice showed a mild but significant reduction in heart weight.

Targeted ablation of Fn14 receptor improves exercise capacity and inhibits neurogenic muscle atrophy

Skeletal muscle atrophy is a prevalent complication in multiple chronic diseases and disuse conditions. Fibroblast growth factor-inducible 14 (Fn14) is a member of the TNF receptor superfamily and a bona fide receptor of the TWEAK cytokine. Accumulating evidence suggests that Fn14 levels are increased in catabolic conditions as well as during exercise. However, the role of Fn14 in the regulation of skeletal muscle mass and function remains poorly understood. In this study, through the generation of novel skeletal muscle-specific Fn14-knockout mice, we have investigated the muscle role of Fn14 in the regulation of exercise capacity and denervation-induced muscle atrophy. Our results demonstrate that there was no difference in skeletal muscle mass between control and muscle-specific Fn14-knockout mice. Nevertheless, the deletion of Fn14 in skeletal muscle significantly improved exercise capacity and resistance to fatigue. This effect of Fn14 deletion is associated with an increased proportion of oxidative myofibers and higher capillaries number per myofiber in skeletal muscle. Furthermore, our results demonstrate that targeted deletion of Fn14 inhibits denervation-induced muscle atrophy in adult mice. Deletion of Fn14 reduced the expression of components of the ubiquitin-proteasome system and non-canonical NF-kappa B signaling in denervated skeletal muscle, as well as increased the phosphorylation of Akt kinase and FoxO3a transcription factor. Collectively, our results demonstrate that targeted inhibition of Fn14 improves exercise tolerance and inhibits denervation-induced muscle atrophy in adult mice.

Role of Muscle LIM Protein in Mechanotransduction Process

The induction of protein synthesis is crucial to counteract the deconditioning of neuromuscular system and its atrophy. In the past, hormones and cytokines acting as growth factors involved in the intracellular events of these processes have been identified, while the implications of signaling pathways associated with the anabolism/catabolism ratio in reference to the molecular mechanism of skeletal muscle hypertrophy have been recently identified. Among them, the mechanotransduction resulting from a mechanical stress applied to the cell appears increasingly interesting as a potential pathway for therapeutic intervention. At present, there is an open question regarding the type of stress to apply in order to induce anabolic events or the type of mechanical strain with respect to the possible mechanosensing and mechanotransduction processes involved in muscle cells protein synthesis. This review is focused on the muscle LIM protein (MLP), a structural and mechanosensing protein with a LIM domain, which is expressed in the sarcomere and costamere of striated muscle cells. It acts as a transcriptional cofactor during cell proliferation after its nuclear translocation during the anabolic process of differentiation and rebuilding. Moreover, we discuss the possible opportunity of stimulating this mechanotransduction process to counteract the muscle atrophy induced by anabolic versus catabolic disorders coming from the environment, aging or myopathies.

Potential role of recombinant growth differentiation factor 11 in Alzheimer’s disease treatment

Alzheimer's disease (AD) is a neurodegenerative disease closely related to the accumulation of beta-amyloid (Aβ) plaques. Growth differentiation factor 11 (GDF11) is one of the proteins that play a role in the aggravation of AD. Decreased concentration of GDF11 disrupts regenerative nervous system, blood vessels, and various vital systems. Low levels of GDF11 with age can be overcome with recombinant GDF11 (rGDF11) to rejuvenate the regenerative effect. Based on research results, rGDF11 enhance the proliferation rate of neuronal precursor cells as well as angiogenesis. rGDF11 can replace lost levels of GDF11, overcome astrogliosis and activation of nerve cell microglia. Therapeutic effect of rGDF11 leads to an improved prognosis in AD patients by neurogenesis and angiogenesis. The prospects of rGDF11 in the treatment of AD have great potential for further research in the future.

Supraphysiological activation of TAK1 promotes skeletal muscle growth and mitigates neurogenic atrophy

Skeletal muscle mass is regulated through coordinated activation of multiple signaling pathways. TAK1 signalosome has been found to be activated in various conditions of muscle atrophy and hypertrophy. However, the role and mechanisms by which TAK1 regulates skeletal muscle mass remain less understood. Here, we demonstrate that supraphysiological activation of TAK1 in skeletal muscle of adult mice stimulates translational machinery, protein synthesis, and myofiber growth. TAK1 causes phosphorylation of elongation initiation factor 4E (eIF4E) independent of mTOR. Inactivation of TAK1 disrupts neuromuscular junction morphology and causes deregulation of Smad signaling. Using genetic approaches, we demonstrate that TAK1 prevents excessive loss of muscle mass during denervation. TAK1 favors the nuclear translocation of Smad4 and cytoplasmic retention of Smad6. TAK1 is also required for the phosphorylation of eIF4E in denervated skeletal muscle. Collectively, our results demonstrate that TAK1 supports skeletal muscle growth and prevents neurogenic muscle atrophy in adult mice.

TGF-β-activated kinase 1 (TAK1) is essential for the maintenance of skeletal muscle mass through incompletely understood mechanisms. Here the authors show that supraphysiological activation of TAK1 leads to muscle hypertrophy through the elongation initiation factor 4E (eIF4E) involved in protein synthesis, and prevents denervation-induced atrophy in mice.

Chronic Activation of AMPK Induces Mitochondrial Biogenesis through Differential Phosphorylation and Abundance of Mitochondrial Proteins in Dictyostelium discoideum

Mitochondrial biogenesis is a highly controlled process that depends on diverse signalling pathways responding to cellular and environmental signals. AMP-activated protein kinase (AMPK) is a critical metabolic enzyme that acts at a central control point in cellular energy homeostasis. Numerous studies have revealed the crucial roles of AMPK in the regulation of mitochondrial biogenesis; however, molecular mechanisms underlying this process are still largely unknown. Previously, we have shown that, in cellular slime mould Dictyostelium discoideum, the overexpression of the catalytic α subunit of AMPK led to enhanced mitochondrial biogenesis, which was accompanied by reduced cell growth and aberrant development. Here, we applied mass spectrometry-based proteomics of Dictyostelium mitochondria to determine the impact of chronically active AMPKα on the phosphorylation state and abundance of mitochondrial proteins and to identify potential protein targets leading to the biogenesis of mitochondria. Our results demonstrate that enhanced mitochondrial biogenesis is associated with variations in the phosphorylation levels and abundance of proteins related to energy metabolism, protein synthesis, transport, inner membrane biogenesis, and cellular signalling. The observed changes are accompanied by elevated mitochondrial respiratory activity in the AMPK overexpression strain. Our work is the first study reporting on the global phosphoproteome profiling of D. discoideum mitochondria and its changes as a response to constitutively active AMPK. We also propose an interplay between the AMPK and mTORC1 signalling pathways in controlling the cellular growth and biogenesis of mitochondria in Dictyostelium as a model organism.

Canonical and Non-canonical TGFβ Signaling Activate Autophagy in an ULK1-Dependent Manner

The mechanism(s) in which transforming growth factor beta 1 (TGFβ) modulates autophagy in cancer remain unclear. Here, we characterized the TGFβ signaling pathways that induce autophagy in non-small cell lung cancer cells, using cells lines stably expressing GFP-LC3-RFP-LC3ΔG constructs that measure autophagic flux. We demonstrated that TGFβ1 increases Unc 51-like kinase 1 (ULK1) protein levels, 5' adenosine monophosphate-activated protein kinase (AMPK)-dependent ULK1 phosphorylation at serine (S) 555 and ULK1 complex formation but decreases mechanistic target of rapamycin (mTOR) activity on ULK1. Further analysis revealed that the canonical Smad4 pathway and the non-canonical TGFβ activated kinase 1/tumor necrosis factor receptor-associated factor 6/P38 mitogen activated protein kinase (TAK1-TRAF6-P38 MAPK) pathway are important for TGFβ1-induced autophagy. The TAK1-TRAF6-P38 MAPK pathway was essential for downregulating mTOR S2448 phosphorylation, ULK1 S555 phosphorylation and autophagosome formation. Furthermore, although siRNA-mediated Smad4 silencing did not alter mTOR-dependent ULK1 S757 phosphorylation, it did reduce AMPK-dependent ULK1 S555 phosphorylation and autophagosome formation. Additionally, Smad4 silencing and inhibiting the TAK1-TRAF6-P38 MAPK pathway decreased autophagosome-lysosome co-localization in the presence of TGFβ. Our results suggest that the Smad4 and TAK1-TRAF6-P38 MAPK signaling pathways are essential for TGFβ-induced autophagy and provide specific targets for the inhibition of TGFβ in tumor cells that utilize autophagy in their epithelial-mesenchymal transition program.

MyD88-mediated signaling intercedes in neurogenic muscle atrophy through multiple mechanisms

Skeletal muscle atrophy is a debilitating complication of many chronic disease states and disuse conditions including denervation. However, molecular and signaling mechanisms of muscle wasting remain less understood. Here, we demonstrate that the levels of several toll-like receptors (TLRs) and their downstream signaling adaptor, myeloid differentiation primary response 88 (MyD88), are induced in skeletal muscle of mice in response to sciatic nerve denervation. Muscle-specific ablation of MyD88 mitigates denervation-induced skeletal muscle atrophy in mice. Targeted ablation of MyD88 suppresses the components of ubiquitin-proteasome system, autophagy, and FOXO transcription factors in skeletal muscle during denervation. We also found that specific inhibition of MyD88 reduces the activation of canonical nuclear factor-kappa (NF-κB) pathway and expression of receptors for inflammatory cytokines in denervated muscle. In contrast, inhibition of MyD88 stimulates the activation of non-canonical NF-κB signaling in denervated skeletal muscle. Ablation of MyD88 also inhibits the denervation-induced increase in phosphorylation of AMPK without having any effect on the phosphorylation of mTOR. Moreover, targeted ablation of MyD88 inhibits the activation of a few components of the unfolded protein response (UPR) pathways, especially X-box protein 1 (XBP1). Importantly, myofiber-specific ablation of XBP1 mitigates denervation-induced skeletal muscle atrophy in mice. Collectively, our experiments suggest that TLR-MyD88 signaling mediates skeletal muscle wasting during denervation potentially through the activation of canonical NF-κB signaling, AMPK and UPR pathways.

Structural basis for specific recognition of K6-linked polyubiquitin chains by the TAB2 NZF domain

TAK1-binding protein 2 (TAB2) has generally been considered to bind specifically to K63-linked polyubiquitin chains via its C-terminal Npl4 zinc-finger (NZF) domain. However, a recent study showed that the NZF domain of TAB2 (TAB2-NZF) could also interact with K6-linked polyubiquitin chains. Here, we report the crystal structure of TAB2-NZF in complex with K6-linked diubiquitin (K6-Ub₂) at 1.99-Å resolution. TAB2-NZF simultaneously interacts with the distal and proximal ubiquitin moieties of K6-Ub₂. By comparing the structures of TAB2-NZF in complex with K6-Ub₂ and with K63-linked diubiquitin (K63-Ub₂), we reveal that the binding mechanism of TAB2-NZF with K6-Ub₂ is similar to that with K63-Ub₂, except for the flexible C-terminal region of the distal ubiquitin. Therefore, we conclude that the C-terminal flexibility of the distal ubiquitin contributes to the dual specificity of TAB2-NZF toward K6- and K63-linked ubiquitin chains. This study provides important insights into the functions of K6-linked ubiquitin chains, which are currently unclear.

Catalpol counteracts the pathology in a mouse model of Duchenne muscular dystrophy by inhibiting the TGF-β1/TAK1 signaling pathway

Duchenne muscular dystrophy (DMD) is a progressive neuromuscular disease caused by a mutation in the gene encoding the dystrophin protein. Catalpol is an iridoid glycoside found in Chinese herbs with anti-inflammatory, anti-oxidant, anti-apoptotic, and hypoglycemic activities that can protect against muscle wasting. In the present study we investigated the effects of catalpol on DMD. Aged Dystrophin-deficient (mdx) mice (12 months old) were treated with catalpol (100, 200 mg·kg-1·d-1, ig) for 6 weeks. At the end of the experiment, the mice were sacrificed, and gastrocnemius (GAS), tibialis anterior (TA), extensor digitorum longus (EDL), soleus (SOL) muscles were collected. We found that catalpol administration dose-dependently increased stride length and decreased stride width in Gait test. Wire grip test showed that the time of wire grip and grip strength were increased. We found that catalpol administration dose-dependently alleviated skeletal muscle damage, evidenced by reduced plasma CK and LDH activity as well as increased the weight of skeletal muscles. Catalpol administration had no effect on dystrophin expression, but exerted anti-inflammatory effects. Furthermore, catalpol administration dose-dependently decreased tibialis anterior (TA) muscle fibrosis, and inhibited the expression of TGF-β1, TAK1 and α-SMA. In primary myoblasts from mdx mice, knockdown of TAK1 abolished the inhibitory effects of catalpol on the expression levels of TGF-β1 and α-SMA. In conclusion, catalpol can restore skeletal muscle strength and alleviate skeletal muscle damage in aged mdx mice, thus may provide a novel therapy for DMD. Catalpol attenuates muscle fibrosis by inhibiting the TGF-β1/TAK1 signaling pathway.

In vitro assessment of anti-fibrotic drug activity does not predict in vivo efficacy in murine models of Duchenne muscular dystrophy

AIM

Fibrosis is the most common complication from chronic diseases, and yet no therapy capable of mitigating its effects is available. Our goal is to unveil specific signaling regulating the fibrogenic process and to identify potential small molecule candidates that block fibrogenic differentiation of fibro/adipogenic progenitors.

METHOD

We performed a large-scale drug screen using muscle-resident fibro/adipogenic progenitors from a mouse model expressing EGFP under the Collagen1a1 promotor. We first confirmed that the EGFP was expressed in response to TGFβ1 stimulation in vitro. Then we treated cells with TGFβ1 alone or with drugs from two libraries of known compounds. The drugs ability to block the fibrogenic differentiation was quantified by imaging and flow cytometry. From a two-rounds screening, positive hits were tested in vivo in the mice model for the Duchenne Muscular Dystrophy (mdx mice). The histopathology of the muscles was assessed with picrosirius red (fibrosis) and laminin staining (myofiber size).

KEY FINDINGS

From the in vitro drug screening, we identified 21 drugs and tested 3 in vivo on the mdx mice. None of the three drugs significantly improved muscle histopathology.

SIGNIFICANCE

The in vitro drug screen identified various efficient compounds, none of them strongly inhibited fibrosis in skeletal muscle of mdx mice. To explain these observations, we hypothesize that in Duchenne Muscular Dystrophy, in which fibrosis is a secondary event due to chronic degeneration and inflammation, the drugs tested could have adverse effect on regeneration or inflammation, balancing off any positive effects and leading to the absence of significant results.

Confounding Roles of ER Stress and the Unfolded Protein Response in Skeletal Muscle Atrophy

Skeletal muscle is an essential organ, responsible for many physiological functions such as breathing, locomotion, postural maintenance, thermoregulation, and metabolism. Interestingly, skeletal muscle is a highly plastic tissue, capable of adapting to anabolic and catabolic stimuli. Skeletal muscle contains a specialized smooth endoplasmic reticulum (ER), known as the sarcoplasmic reticulum, composed of an extensive network of tubules. In addition to the role of folding and trafficking proteins within the cell, this specialized organelle is responsible for the regulated release of calcium ions (Ca2+) into the cytoplasm to trigger a muscle contraction. Under various stimuli, such as exercise, hypoxia, imbalances in calcium levels, ER homeostasis is disturbed and the amount of misfolded and/or unfolded proteins accumulates in the ER. This accumulation of misfolded/unfolded protein causes ER stress and leads to the activation of the unfolded protein response (UPR). Interestingly, the role of the UPR in skeletal muscle has only just begun to be elucidated. Accumulating evidence suggests that ER stress and UPR markers are drastically induced in various catabolic stimuli including cachexia, denervation, nutrient deprivation, aging, and disease. Evidence indicates some of these molecules appear to be aiding the skeletal muscle in regaining homeostasis whereas others demonstrate the ability to drive the atrophy. Continued investigations into the individual molecules of this complex pathway are necessary to fully understand the mechanisms.

TAK1 inhibition improves myoblast differentiation and alleviates fibrosis in a mouse model of Duchenne muscular dystrophy

BACKGROUND

Transforming growth factor-β-activated kinase 1 (TAK1) plays a key role in regulating fibroblast and myoblast proliferation and differentiation. However, the TAK1 changes associated with Duchenne muscular dystrophy (DMD) are poorly understood, and it remains unclear how TAK1 regulation could be exploited to aid the treatment of this disease.

METHODS

Muscle biopsies were obtained from control donors or DMD patients for diagnosis (n = 6 per group, male, 2-3 years, respectively). Protein expression of phosphorylated TAK1 was measured by western blot and immunofluorescence analysis. In vivo overexpression of TAK1 was performed in skeletal muscle to assess whether TAK1 is sufficient to induce or aggravate atrophy and fibrosis. To explore whether TAK1 inhibition protects against muscle damage, mdx (loss of dystrophin) mice were treated with adeno-associated virus (AAV)-short hairpin TAK1 (shTAK1) or NG25 (a TAK1 inhibitor). Serum analysis, skeletal muscle performance and histology, muscle contractile function, and gene and protein expression were performed.

RESULTS

We found that TAK1 was activated in the dystrophic muscles of DMD patients (n = 6, +72.2%, P < 0.001), resulting in fibrosis ( +65.9% for fibronectin expression, P < 0.001) and loss of muscle fibres (-32.5%, P < 0.01). Moreover, TAK1 was activated by interleukin-1β, tumour necrosis factor-α, and transforming growth factor-β1 (P < 0.01). Overexpression of TAK1 by AAV vectors further aggravated fibrosis (n = 8, +39.6% for hydroxyproline content, P < 0.01) and exacerbated muscle wasting (-31.6%, P < 0.01) in mdx mice; however, these effects were reversed in mdx mice by treatment with AAV-short hairpin TAK1 (shTAK1) or NG25 (a TAK1 inhibitor). The molecular mechanism underlying these effects may be related to the prevention of TAK1-mediated transdifferentiation of myoblasts into fibroblasts, thereby reducing fibrosis and increasing myoblast differentiation.

CONCLUSIONS

Our findings show that TAK1 activation exacerbated fibrosis and muscle degeneration and that TAK1 inhibition can improve whole-body muscle quality and the function of dystrophic skeletal muscle. Thus, TAK1 inhibition may constitute a novel therapy for DMD.

TAK1 Mediates ROS Generation Triggered by the Specific Cephalosporins through Noncanonical Mechanisms

It is known that a wide variety of antibacterial agents stimulate generation of reactive oxygen species (ROS) in mammalian cells. However, its mechanisms are largely unknown. In this study, we unexpectedly found that transforming growth factor-β (TGF-β)-activated kinase 1 (TAK1) is involved in the generation of mitochondrial ROS (mtROS) initiated by cefotaxime (CTX), one of specific antibacterial cephalosporins that can trigger oxidative stress-induced cell death. TAK1-deficient macrophages were found to be sensitive to oxidative stress-induced cell death stimulated by H₂O₂. Curiously, however, TAK1-deficient macrophages exhibited strong resistance to oxidative stress-induced cell death stimulated by CTX. Microscopic analysis revealed that CTX-induced ROS generation was overridden by knockout or inhibition of TAK1, suggesting that the kinase activity of TAK1 is required for CTX-induced ROS generation. Interestingly, pharmacological blockade of the TAK1 downstream pathways, such as nuclear factor-κB (NF-κB) and mitogen-activated protein kinase (MAPK) pathways, did not affect the CTX-induced ROS generation. In addition, we observed that CTX promotes translocation of TAK1 to mitochondria. Together, these observations suggest that mitochondrial TAK1 mediates the CTX-induced mtROS generation through noncanonical mechanisms. Thus, our data demonstrate a novel and atypical function of TAK1 that mediates mtROS generation triggered by the specific cephalosporins.

A potential therapeutic effect of catalpol in Duchenne muscular dystrophy revealed by binding with TAK1

BACKGROUND

Duchenne muscular dystrophy (DMD) is a progressive muscle disease caused by the loss of dystrophin, which results in inflammation, fibrosis, and the inhibition of myoblast differentiation in skeletal muscle. Catalpol, an iridoid glycoside, improves skeletal muscle function by enhancing myogenesis; it has potential to treat DMD. We demonstrate the positive effects of catalpol in dystrophic skeletal muscle.

METHODS

mdx (loss of dystrophin) mice (n = 18 per group) were treated with catalpol (200 mg/kg) for six consecutive weeks. Serum analysis, skeletal muscle performance and histology, muscle contractile function, and gene and protein expression were performed. Molecular docking and ligand-target interactions, RNA interference, immunofluorescence, and plasmids transfection were utilized to explore the protective mechanism in DMD by which catalpol binding with transforming growth factor-β-activated kinase 1 (TAK1) in skeletal muscle.

RESULTS

Six weeks of catalpol treatment improved whole-body muscle health in mdx mice, which was characterized by reduced plasma creatine kinase (n = 18, -35.1%, P < 0.05) and lactic dehydrogenase (n = 18, -10.3%, P < 0.05) activity. These effects were accompanied by enhanced grip strength (n = 18, +25.4%, P < 0.05) and reduced fibrosis (n = 18, -29.0% for hydroxyproline content, P < 0.05). Moreover, catalpol treatment protected against muscle fatigue and promoted muscle recovery in the tibialis anterior (TA) and diaphragm (DIA) muscles (n = 6, +69.8%, P < 0.05 and + 74.8%, P < 0.001, respectively), which was accompanied by enhanced differentiation in primary myoblasts from DMD patients (n = 6, male, mean age: 4.7 ± 1.9 years) and mdx mice. In addition, catalpol eliminated p-TAK1 overexpression in mdx mice (n = 12, -21.3%, P < 0.05) and primary myoblasts. The catalpol-induced reduction in fibrosis and increased myoblast differentiation resulted from the inhibition of TAK1 phosphorylation, leading to reduced myoblast trans-differentiation into myofibroblasts. Catalpol inhibited the phosphorylation of TAK1 by binding to TAK1, possibly at Asp-206, Thr-208, Asn-211, Glu-297, Lys-294, and Tyr-293.

CONCLUSIONS

Our findings show that catalpol and TAK1 inhibitors substantially improve whole-body muscle health and the function of dystrophic skeletal muscles and may provide a novel therapy for DMD.

TAK1 preserves skeletal muscle mass and mitochondrial function through redox homeostasis

Skeletal muscle atrophy is debilitating consequence of a large number of chronic disease states, aging, and disuse conditions. Skeletal muscle mass is regulated through coordinated activation of a number of signaling cascades. Transforming growth factor-β activated kinase 1 (TAK1) is a central kinase that mediates the activation of multiple signaling pathways in response to various growth factors, cytokines, and microbial products. Accumulating evidence suggests that TAK1 promotes skeletal muscle growth and essential for the maintenance of muscle mass in adults. Targeted inactivation of TAK1 leads to severe muscle wasting and kyphosis in mice. However, the mechanisms by which TAK1 prevents loss of muscle mass remain poorly understood. Through generation of inducible skeletal muscle-specific Tak1-knockout mice, we demonstrate that targeted ablation of TAK1 disrupts redox signaling leading to the accumulation of reactive oxygen species and loss of skeletal muscle mass and contractile function. Suppression of oxidative stress using Trolox improves muscle contractile function and inhibits the activation of catabolic signaling pathways in Tak1-deficient muscle. Moreover, Trolox inhibits the activation of ubiquitin-proteasome system and autophagy markers in skeletal muscle of Tak1-deficient mice. Furthermore, inhibition of oxidative stress using Trolox prevents the slow-to-fast type fiber transition and improves mitochondrial respiration in skeletal muscle of Tak1-deficient mice. Overall, our results demonstrate that TAK1 maintains skeletal muscle mass and health through redox homeostasis.

Implications of miRNAs on TGF-β/TAK1/mTOR pathway in mediating the renoprotective effects of pentoxifylline against cisplatin-induced nephrotoxicity in rats

Cisplatin (CIS)-mediated nephrotoxicity is induced via transforming growth factor-beta (TGF-β) and TGF-β-activated kinase (TAK1). TGF-β and TAK1 are known to interact with microRNA-let-7b and microRNA-26b, respectively. Additionally, TGF-β1 is reported to down-regulate the autophagy marker microtubule-associated protein 1 light chain 3-II (LC3-II) through upregulation of microRNA-34a. Pentoxifylline (PTX) anti-inflammatory effects are mediated via suppressing TGF-β and regulating mammalian target of rapamycin (mTOR). The current study aimed to investigate the involvement of microRNAs let-7b, 26b, and 34a, and the modulating impact of PTX on CIS-induced nephrotoxicity. Moreover, we aimed at examining the ability of PTX to interact with TGF-β receptor-1 (TGFβR-1), and TAK1, and examine its ability to downgrade the previously reported toxicities. Hence, the expression of the aforementioned microRNAs, and protein levels of TGFβR-1, TGF-β1, TAK1, mTOR, LC3-II, and NF-κB were assessed. Molecular docking studies of PTX on TGFβR-1 and TAK1 were also executed. CIS induced TGF-β1, with down-regulation of microRNA-let-7b and -26b, and up-regulation of microRNA-34a. TGFβR-1, TAK1, and mTOR levels were increased, while LC3-II level was decreased. PTX significantly protected renal cells against CIS-induced changes as indicated by reverting the level of the investigated parameters, while exhibiting an antagonistic effect on TGFβR-1 and TAK1. Our results postulate a possible role of epigenetic regulation of CIS-induced nephrotoxicity through the investigated microRNAs proposing them as potential future targets for controlling this serious toxicity. PTX was able to shield CIS-induced toxicity possibly through blocking TGF-β pathway, while promoting autophagy in a TAK1 independent manner with the involvement of the examined microRNAs.

The PI3K-AKT-mTOR Pathway and Prostate Cancer: At the Crossroads of AR, MAPK, and WNT Signaling

Oncogenic activation of the phosphatidylinositol-3-kinase (PI3K), protein kinase B (PKB/AKT), and mammalian target of rapamycin (mTOR) pathway is a frequent event in prostate cancer that facilitates tumor formation, disease progression and therapeutic resistance. Recent discoveries indicate that the complex crosstalk between the PI3K-AKT-mTOR pathway and multiple interacting cell signaling cascades can further promote prostate cancer progression and influence the sensitivity of prostate cancer cells to PI3K-AKT-mTOR-targeted therapies being explored in the clinic, as well as standard treatment approaches such as androgen-deprivation therapy (ADT). However, the full extent of the PI3K-AKT-mTOR signaling network during prostate tumorigenesis, invasive progression and disease recurrence remains to be determined. In this review, we outline the emerging diversity of the genetic alterations that lead to activated PI3K-AKT-mTOR signaling in prostate cancer, and discuss new mechanistic insights into the interplay between the PI3K-AKT-mTOR pathway and several key interacting oncogenic signaling cascades that can cooperate to facilitate prostate cancer growth and drug-resistance, specifically the androgen receptor (AR), mitogen-activated protein kinase (MAPK), and WNT signaling cascades. Ultimately, deepening our understanding of the broader PI3K-AKT-mTOR signaling network is crucial to aid patient stratification for PI3K-AKT-mTOR pathway-directed therapies, and to discover new therapeutic approaches for prostate cancer that improve patient outcome.

Withaferin A ameliorates ovarian cancer-induced cachexia and proinflammatory signaling

BACKGROUND

Ovarian cancer is the fifth leading cause of cancer-related deaths amongst women in the United States. Cachexia is the primary cause of death in approximately 30% of cancer patients, and is often evidenced in ovarian cancer patients. We tested the steroidal lactone Withaferin A to examine if it could ameliorate ovarian cancer-induced cachexia.

METHODS

Six-week-old severely immunodeficient female mice were xenografted with the ovarian cancer cell line A2780 followed by treatment with Withaferin A or vehicle. Changes in functional grip strength were assessed on a weekly basis. Postmortem, H&E staining was performed on skeletal muscle sections and immunofluorescent immunohistochemistry was performed on skeletal muscle and tumor sections. The levels of NF-κB-related proinflammatory cytokines were assessed in the xenografted tumors and in resident host skeletal muscle.

RESULTS

Xenografting of the A2780 cell line resulted in a significant rate of mortality, which was attenuated by a therapeutic dosage of Withaferin A. Mice that received vehicle treatment following xenografting exhibited functional muscle decline over the course of the study. The therapeutic dosage Withaferin A treatment attenuated this reduction in grip strength, whereas the supratherapeutic dosage of Withaferin A was found to be toxic/lethal and demonstrated a further decline in functional muscle strength and an increased rate of mortality on par with vehicle treatment. At a histological level, the vehicle treated tumor-bearing mice exhibited a profound reduction in myofibrillar cross-sectional area compared to the vehicle treated tumor-free control group. The atrophic changes induced by the xenografted tumor were significantly ameliorated by treatment with Withaferin A. The combination of functional muscle weakening and induction of myofibrillar atrophy corroborate a cachectic phenotype, which was functionally rescued by Withaferin A. Further, treatment completely abolished the slow-to-fast myofiber type conversion observed in the settings of cancer-induced cachexia. In both host resident skeletal muscle and the xenografted tumors, we report an increase in NF-κB-related proinflammatory cytokines that was reversed by Withaferin A treatment. Finally, we demonstrated that Withaferin A significantly downregulates cytosolic and nuclear levels of phospho-p65, the active canonical NF-κB transcription factor, in xenografted tumors.

CONCLUSIONS

Cumulatively, our results demonstrate a previously overlooked role of Withaferin A in a xenograft model of ovarian cancer. We propose mechanisms by which Withaferin A reduces NF-κB-dependent pro-inflammatory cytokine production leading to an attenuation of the cachectic phenotype in an i.p. xenograft model of ovarian cancer.

GDF11 Implications in Cancer Biology and Metabolism. Facts and Controversies

Growth Differentiation Factor 11 (GDF11), a member of the super family of the Transforming Growth Factor β, has gained more attention in the last few years due to numerous reports regarding its functions in other systems, which are different to those related to differentiation and embryonic development, such as age-related muscle dysfunction, skin biology, metabolism, and cancer. GDF11 is expressed in many tissues, including skeletal muscle, pancreas, kidney, nervous system, and retina, among others. GDF11 circulating levels and protein content in tissues are quite variable and are affected by pathological conditions or age. Although, GDF11 biology had a lot of controversies, must of them are only misunderstandings regarding the variability of its responses, which are independent of the tissue, grade of cellular differentiation or pathologies. A blunt fact regarding GDF11 biology is that its target cells have stemness feature, a property that could be found in certain adult cells in health and in disease, such as cancer cells. This review is focused to present and analyze the recent findings in the emerging research field of GDF11 function in cancer and metabolism, and discusses the controversies surrounding the biology of this atypical growth factor.

IKKα Kinase Regulates the DNA Damage Response and Drives Chemo-resistance in Cancer

Phosphorylated IKKα(p45) is a nuclear active form of the IKKα kinase that is induced by the MAP kinases BRAF and TAK1 and promotes tumor growth independent of canonical NF-κB signaling. Insights into the sources of IKKα(p45) activation and its downstream substrates in the nucleus remain to be defined. Here, we discover that IKKα(p45) is rapidly activated by DNA damage independent of ATM-ATR, but dependent on BRAF-TAK1-p38-MAPK, and is required for robust ATM activation and efficient DNA repair. Abolishing BRAF or IKKα activity attenuates ATM, Chk1, MDC1, Kap1, and 53BP1 phosphorylation, compromises 53BP1 and RIF1 co-recruitment to sites of DNA lesions, and inhibits 53BP1-dependent fusion of dysfunctional telomeres. Furthermore, IKKα or BRAF inhibition synergistically enhances the therapeutic potential of 5-FU and irinotecan to eradicate chemotherapy-resistant metastatic human tumors in vivo. Our results implicate BRAF and IKKα kinases in the DDR and reveal a combination strategy for cancer treatment.

The MAP3K7-mTOR Axis Promotes the Proliferation and Malignancy of Hepatocellular Carcinoma Cells

Targeted therapy is currently limited for patients with hepatocellular carcinoma (HCC) due to the lack of suitable targets. Kinases play pivotal roles in many cellular biological processes, whereas dysregulation of kinases may lead to various diseases, particularly cancer. However, the role of kinases in HCC malignancy remains unclear. In this study, we employed a kinome small interfering RNA (siRNA) library, comprising 710 kinase-related genes, to screen whether any kinases were essential for cell proliferation in various HCC cell lines. Through a kinome siRNA library screening, we found that MAP3K7 was a crucial gene for HCC cell proliferation. Pharmacological or genetic ablation of MAP3K7 diminished the growth, migration, and invasion of HCC cells, including primary HCC cells. Stable knockdown of MAP3K7 attenuated tumor formation in a spheroid cell culture model and tumor xenograft mouse model. In addition, silencing MAP3K7 reduced the phosphorylation and expression of mammalian target of rapamycin (mTOR) in HCC cells. MAP3K7 expression was positively correlated with mTOR expression in tumors of patients with HCC. Higher co-expression of MAP3K7 and mTOR was significantly associated with poor prognosis of HCC. Taken together, our results revealed that the MAP3K7-mTOR axis might promote tumorigenesis and malignancy, which provides a potential marker or therapeutic target for HCC patients.

MyD88 is required for satellite cell-mediated myofiber regeneration in dystrophin-deficient mdx mice

Duchenne muscular dystrophy (DMD), caused by mutations in the dystrophin gene, leads to severe muscle wasting and eventual death of the afflicted individuals, primarily due to respiratory failure. Deficit in myofiber regeneration, potentially due to an exhaustion of satellite cells, is one of the major pathological features of DMD. Myeloid differentiation primary response 88 (MyD88) is an adaptor protein that mediates activation of various inflammatory pathways in response to signaling from Toll-like receptors and interleukin-1 receptor. MyD88 also regulates cellular survival, proliferation and differentiation in a cell-autonomous manner. However, the role of MyD88 in satellite stem cell homeostasis and function in dystrophic muscle remains unknown. In this study, we demonstrate that tamoxifen-inducible deletion of MyD88 in satellite cells causes loss of skeletal muscle mass and strength in the mdx mouse model of DMD. Satellite cell-specific deletion of MyD88 inhibits myofiber regeneration and stimulates fibrogenesis in dystrophic muscle of mdx mice. Deletion of MyD88 also reduces the number of satellite cells and inhibits their fusion with injured myofibers in dystrophic muscle of mdx mice. Ablation of MyD88 in satellite cells increases the markers of M2 macrophages without having any significant effect on M1 macrophages and expression of inflammatory cytokines. Finally, we found that satellite cell-specific deletion of MyD88 leads to aberrant activation of Notch and Wnt signaling in skeletal muscle of mdx mice. Collectively, our results demonstrate that MyD88-mediated signaling in satellite cells is essential for the regeneration of injured myofibers in dystrophic muscle of mdx mice.

Mitochondria-associated lactate dehydrogenase is not a biologically significant contributor to bioenergetic function in murine striated muscle

Previous studies indicate that mitochondria-localized lactate dehydrogenase (mLDH) might be a significant contributor to metabolism. In the heart, the presence of mLDH could provide cardiac mitochondria with a higher capacity to generate reducing equivalents directly available for respiration, especially during exercise when circulating lactate levels are high. The purpose of this study was to test the hypothesis that mLDH contributes to striated muscle bioenergetic function. Mitochondria isolated from murine cardiac and skeletal muscle lacked an appreciable ability to respire on lactate in the absence or presence of exogenous NAD⁺. Although three weeks of treadmill running promoted physiologic cardiac growth, mitochondria isolated from the hearts of acutely exercised or exercise-adapted mice showed no further increase in lactate oxidation capacity. In all conditions tested, cardiac mitochondria respired at >20-fold higher levels with provision of pyruvate compared with lactate. Similarly, skeletal muscle mitochondria showed little capacity to respire on lactate. Protease protection assays of isolated cardiac mitochondria confirmed that LDH is not localized within the mitochondrion. We conclude that mLDH does not contribute to cardiac bioenergetics in mice.

The Role of AMPK in the Regulation of Skeletal Muscle Size, Hypertrophy, and Regeneration

AMPK (5’-adenosine monophosphate-activated protein kinase) is heavily involved in skeletal muscle metabolic control through its regulation of many downstream targets. Because of their effects on anabolic and catabolic cellular processes, AMPK plays an important role in the control of skeletal muscle development and growth. In this review, the effects of AMPK signaling, and those of its upstream activator, liver kinase B1 (LKB1), on skeletal muscle growth and atrophy are reviewed. The effect of AMPK activity on satellite cell-mediated muscle growth and regeneration after injury is also reviewed. Together, the current data indicate that AMPK does play an important role in regulating muscle mass and regeneration, with AMPKα1 playing a prominent role in stimulating anabolism and in regulating satellite cell dynamics during regeneration, and AMPKα2 playing a potentially more important role in regulating muscle degradation during atrophy.

PERK regulates skeletal muscle mass and contractile function in adult mice

Skeletal muscle mass is regulated by the coordinated activation of several anabolic and catabolic pathways. The endoplasmic reticulum (ER) is a major site of protein folding and a reservoir for calcium ions. Accretion of misfolded proteins or depletion in calcium concentration causes stress in the ER, which leads to the activation of a signaling network known as the unfolded protein response (UPR). In the present study, we investigated the role of the protein kinase R-like endoplasmic reticulum kinase (PERK) arm of the UPR in the regulation of skeletal muscle mass and function in naive conditions and in a mouse model of cancer cachexia. Our results demonstrate that the targeted inducible deletion of PERK reduces skeletal muscle mass, strength, and force production during isometric contractions. Deletion of PERK also causes a slow-to-fast fiber type transition in skeletal muscle. Furthermore, short hairpin RNA-mediated knockdown or pharmacologic inhibition of PERK leads to atrophy in cultured myotubes. While increasing the rate of protein synthesis, the targeted deletion of PERK leads to the increased expression of components of the ubiquitin-proteasome system and autophagy in skeletal muscle. Ablation of PERK also increases the activation of calpains and deregulates the gene expression of the members of the FGF19 subfamily. Furthermore, the targeted deletion of PERK increases muscle wasting in Lewis lung carcinoma tumor-bearing mice. Our findings suggest that the PERK arm of the UPR is essential for the maintenance of skeletal muscle mass and function in adult mice.-Gallot, Y. S., Bohnert, K. R., Straughn, A. R., Xiong, G., Hindi, S. M., Kumar, A. PERK regulates skeletal muscle mass and contractile function in adult mice.

Transmission Electron Microscopy for Analysis of Mitochondria in Mouse Skeletal Muscle

Skeletal muscle is the most abundant tissue in the human body and regulates a variety of functions including locomotion and whole-body metabolism. Skeletal muscle has a plethora of mitochondria, the organelles that are essential for aerobic generation of ATP which provides the chemical energy to fuel vital functions such as contraction. The number of mitochondria in skeletal muscle and their function decline with normal aging and in various neuromuscular diseases and in catabolic conditions such as cancer, starvation, denervation, and immobilization. Moreover, compromised mitochondrial function is also associated with metabolic disorders including type 2 diabetes mellitus. It is now clear that maintaining mitochondrial content and function in skeletal muscle is vital for sustained health throughout the lifespan. While a number of staining methods are available to study mitochondria, transmission electron microscopy (TEM) is still the most important method to study mitochondrial structure and health in skeletal muscle. It provides critical information about mitochondrial content, cristae density, organization, formation of autophagosomes, and any other abnormalities commonly observed in various disease conditions. In this article, we describe a detailed protocol for sample preparation and analysis of mouse skeletal muscle mitochondria by TEM.