Identification and characterization of the mitochondrial targeting sequence and mechanism in human citrate synthase

Stress response silencing by an E3 ligase mutated in neurodegeneration

Stress response pathways detect and alleviate adverse conditions to safeguard cell and tissue homeostasis, yet their prolonged activation induces apoptosis and disrupts organismal health1-3. How stress responses are turned off at the right time and place remains poorly understood. Here we report a ubiquitin-dependent mechanism that silences the cellular response to mitochondrial protein import stress. Crucial to this process is the silencing factor of the integrated stress response (SIFI), a large E3 ligase complex mutated in ataxia and in early-onset dementia that degrades both unimported mitochondrial precursors and stress response components. By recognizing bifunctional substrate motifs that equally encode protein localization and stability, the SIFI complex turns off a general stress response after a specific stress event has been resolved. Pharmacological stress response silencing sustains cell survival even if stress resolution failed, which underscores the importance of signal termination and provides a roadmap for treating neurodegenerative diseases caused by mitochondrial import defects.

H55 N variation in citrate synthase leads to decrement in the enzyme activity and transport rate to mitochondria in HEI-OC1 cells

A/J mouse is a typical animal model of age-related deafness. Previous studies have shown that the mice suffer from progressive hearing loss and degeneration of cochlear cells, and a variation of H55 N in citrate synthase (CS) causes about 40% the hearing loss. CS is a key enzyme in the tricarboxylic acid cycle, which is transported from cytoplasm to mitochondria after synthesis, sorted by the mitochondrial targeting sequence (MTS). To explore the mechanism of CS (H55 N) variation in affecting its function, HEI-OC1 cells were infected with lentivirus particles to express CS-Flag or CS(H55 N)-Flag. The results showed that H55 N variation in CS, as purified by co-immunoprecipitation, decreased the enzyme activity by about 50%. Confocal microscope co-localization indicated that the CS (H55 N) variation led to a decrement in its mitochondrial content. Western blot also showed the amount of CS(H55 N)-Flag was more than that of CS(WT)-Flag in the cytosol. The results suggest H55 N variation in CS lead to decrement of its enzyme activity and targeting transport to mitochondria. We therefore conclude that decrement in CS activity and mitochondrial delivery contributes to the degeneration of cochlear cells and thus the hearing loss in A/J mice.

Recent Advancements in Mitochondria-Targeted Nanoparticle Drug Delivery for Cancer Therapy

Mitochondria are critical subcellular organelles that produce most of the adenosine triphosphate (ATP) as the energy source for most eukaryotic cells. Moreover, recent findings show that mitochondria are not only the "powerhouse" inside cells, but also excellent targets for inducing cell death via apoptosis that is mitochondria-centered. For several decades, cancer nanotherapeutics have been designed to specifically target mitochondria with several targeting moieties, and cause mitochondrial dysfunction via photodynamic, photothermal, or/and chemo therapies. These strategies have been shown to augment the killing of cancer cells in a tumor while reducing damage to its surrounding healthy tissues. Furthermore, mitochondria-targeting nanotechnologies have been demonstrated to be highly efficacious compared to non-mitochondria-targeting platforms both in vitro and in vivo for cancer therapies. Moreover, mitochondria-targeting nanotechnologies have been intelligently designed and tailored to the hypoxic and slightly acidic tumor microenvironment for improved cancer therapies. Collectively, mitochondria-targeting may be a promising strategy for the engineering of nanoparticles for drug delivery to combat cancer.

Levels of Coenzyme Q10 and Several COQ Proteins in Human Astrocytoma Tissues Are Inversely Correlated with Malignancy

In a previous study, we reported the alterations of primary antioxidant enzymes and decreased citrate synthase (CS) activities in different grades of human astrocytoma tissues. Here, we further investigated coenzyme Q₁₀ (CoQ₁₀) levels and protein levels of polyprenyl diphosphate synthase subunit (PDSS2) and several COQ proteins required for CoQ₁₀ biosynthesis in these tissues. We found that the level of endogenous CoQ₁₀, but not of exogenous α-tocopherol, was higher in nontumor controls than in all grades of astrocytoma tissues. The levels of COQ3, COQ5, COQ6, COQ7, COQ8A, and COQ9, but not of COQ4, were lower in Grade IV astrocytoma tissues than in controls or low-grade (Grades I and II) astrocytomas, but PDSS2 levels were higher in astrocytoma tissues than in controls. Correlation analysis revealed that the levels of CoQ₁₀ and COQ proteins were negatively correlated with malignancy degree and positively correlated with CS activity, whereas PDSS2 level was positively correlated with malignancy. Moreover, lower level of mitochondrial DNA-encoded cytochrome c oxidase subunit 2 was not only associated with a higher malignancy degree but also with lower level of all COQ proteins detected. The results revealed that mitochondrial abnormalities are associated with impaired CoQ₁₀ maintenance in human astrocytoma progression.

Upregulation of Fibrinogen-Like 1 Expression Contributes to Reducing the Progression of Preeclampsia

Fibrinogen-like 1 (FGL1) is involved in liver injury and liver regeneration, but its role in placenta and preeclampsia (PE) remains unclear. We assessed FGL1 expression in serum and placenta from L-NAME-induced PE-like mouse and in women with (n = 38) and without (n = 42) PE. For the mouse study, pregnant C57Bl/6 mouse (n = 6/group) were subcutaneously administered L-NAME with or without FGL1 once daily starting on days 7–14 of pregnancy and were sacrificed on gestational day (GD) 20. Maternal body weight, blood pressure, and urinary protein were assessed during GDs 8–20. The weight and length of the placenta and fetus were assessed. The placental structure was evaluated using hematoxylin staining. In the human study, the sera of the pregnant women during the late trimester were assessed with enzyme-linked immunosorbent assays (ELISAs). FGL1 expression in human trophoblast cell lines under L-NAME stimulation was measured using Western blotting and immunofluorescence staining. The detected FGL1 protein levels in serum and placenta were both significantly upregulated in patients and mouse with PE compared with those in the non-PE groups. FGL1 treatment decreased maternal hypertension and proteinuria, decreased fetal weight in mouse with PE, downregulated proinflammatory cytokine (interleukin-1b and interleukin-6) levels, and maintained the balance between antiangiogenic (fms-like tyrosine kinase-1) and proangiogenic (placental growth factor) substances in the placenta. L-NAME-upregulated FGL1 expression was inhibited following overexpression of FoxO3a. In summary, FoxO3a reduction is a potential pathophysiological mechanism leading to upregulated placental FGL1 expression that may play a pivotal role in preventing PE progression.

Role of tumor endothelial marker 1 (Endosialin/CD248) lectin-like domain in lipopolysaccharide-induced macrophage activation and sepsis in mice

Deleterious hyper-inflammation resulting from macrophage activation may aggravate sepsis and lead to lethality. Tumor endothelial marker 1 (TEM1), a type I transmembrane glycoprotein containing six functional domains, has been implicated in cancer and chronic sterile inflammatory disorders. However, the role of TEM1 in acute sepsis remains to be determined. Herein we explored the functional significance of the TEM1 lectin-like domain (TEM1D1) in monocyte/macrophage activation and sepsis using TEM1D1-deleted (TEM1^LeD/LeD) transgenic mice and recombinant TEM1D1 (rTEM1D1) protein. Under stimulation with lipopolysaccharides (LPS) or several other toll-like receptor agonists, TEM1^LeD/LeD macrophages produced lower levels of tumor necrosis factor (TNF)-α and interleukin (IL)-6 than wild-type TEM1^wt/wt macrophages. Compared with TEM1^wt/wt macrophages, LPS-macrophage binding and intracellular mitogen-activated protein kinase (MAPK)/nuclear factor (NF)-κB activation were suppressed in TEM1^LeD/LeD macrophages. In vivo, TEM1D1 deletion improved survival in LPS-challenged mice with reduction of circulating TNF-α and IL-6 and alleviation of lung injury and pulmonary leukocyte accumulation. In contrast, rTEM1D1 could bind to LPS and markedly suppress LPS-macrophage binding, MAPK/NF-κB signaling in macrophages and proinflammatory cytokine production. Treatment with rTEM1D1 improved survival and attenuated circulating TNF-α and IL-6, lung injury and pulmonary accumulation of leukocytes in LPS-challenged mice. These findings demonstrated differential roles for the TEM1 lectin-like domain in macrophages and soluble TEM1 lectin-like domain in sepsis. TEM1 in macrophages mediates LPS-induced inflammation via its lectin-like domain, whereas rTEM1D1 interferes with LPS-induced macrophage activation and sepsis.

Different Expression of Mitochondrial and Endoplasmic Reticulum Stress Genes in Epicardial Adipose Tissue Depends on Coronary Atherosclerosis

The aim of our study was to analyze mitochondrial and endoplasmic reticulum (ER) gene expression profiles in subcutaneous (SAT) and epicardial (EAT) adipose tissue, skeletal muscle, and myocardium in patients with and without CAD undergoing elective cardiac surgery. Thirty-eight patients, 27 with (CAD group) and 11 without CAD (noCAD group), undergoing coronary artery bypass grafting and/or valvular surgery were included in the study. EAT, SAT, intercostal skeletal muscle, and right atrium tissue and blood samples were collected at the start and end of surgery; mRNA expression of selected mitochondrial and ER stress genes was assessed using qRT-PCR. The presence of CAD was associated with decreased mRNA expression of most of the investigated mitochondrial respiratory chain genes in EAT, while no such changes were seen in SAT or other tissues. In contrast, the expression of ER stress genes did not differ between the CAD and noCAD groups in almost any tissue. Cardiac surgery further augmented mitochondrial dysfunction in EAT. In our study, CAD was associated with decreased expression of mitochondrial, but not endoplasmic reticulum stress genes in EAT. These changes may contribute to the acceleration of coronary atherosclerosis.

Chromosome-level genome assembly of the Arctic fox (Vulpes lagopus) using PacBio sequencing and Hi-C technology

The Arctic fox (Vulpes lagopus) is the only fox species occurring in the Arctic and has adapted to its extreme climatic conditions. Currently, the molecular basis of its adaptation to the extreme climate has not been characterized. Here, we applied PacBio sequencing and chromosome structure capture technique to assemble the first V. lagopus genome assembly, which is assembled into chromosome fragments. The genome assembly has a total length of 2.345 Gb with a contig N50 of 31.848 Mb and a scaffold N50 of 131.537 Mb, consisting of 25 pseudochromosomal scaffolds. The V. lagopus genome had approximately 32.33% repeat sequences. In total, 21,278 protein-coding genes were predicted, of which 99.14% were functionally annotated. Compared with 12 other mammals, V. lagopus was most closely related to V. Vulpes with an estimated divergence time of ~7.1 Ma. The expanded gene families and positively selected genes potentially play roles in the adaptation of V. lagopus to Arctic extreme environment. This high-quality assembled genome will not only promote future studies of genetic diversity and evolution in foxes and other canids but also provide important resources for conservation of Arctic species.

Oxygen regulates epithelial stem cell proliferation via RhoA-actomyosin-YAP/TAZ signal in mouse incisor

Stem cells are maintained in specific niches that strictly regulate their proliferation and differentiation for proper tissue regeneration and renewal. Molecular oxygen (O₂) is an important component of the niche microenvironment, but little is known about how O₂ governs epithelial stem cell (ESC) behavior. Here, we demonstrate that O₂ plays a crucial role in regulating the proliferation of ESCs using the continuously growing mouse incisors. We have revealed that slow-cycling cells in the niche are maintained under relatively hypoxic conditions compared with actively proliferating cells, based on the blood vessel distribution and metabolic status. Mechanistically, we have demonstrated that, during hypoxia, HIF1α upregulation activates the RhoA signal, thereby promoting cortical actomyosin and stabilizing the adherens junction complex, including merlin. This leads to the cytoplasmic retention of YAP/TAZ to attenuate cell proliferation. These results shed light on the biological significance of blood-vessel geometry and the signaling mechanism through microenvironmental O₂ to orchestrate ESC behavior, providing a novel molecular basis for the microenvironmental O₂-mediated stem cell regulation during tissue development and renewal.

Recombinant thrombomodulin lectin-like domain attenuates porphyromonas gingivalis lipopolysaccharide-induced osteoclastogenesis and periodontal bone resorption

BACKGROUND

Evidence demonstrates that the thrombomodulin (TM) lectin domain (TMD1) exerts anti-inflammatory functions. Lipopolysaccharides derived from Porphyromonas gingivalis (Pg-LPS) are considered a major pathogenic factor for chronic periodontitis, promoting inflammation, osteoclastogenesis and alveolar bone resorption. Herein, we aimed to evaluate the potential therapeutic effect of recombinant TMD1 (rTMD1) in suppression of Pg-LPS-induced osteoclastogenesis and periodontal bone loss.

METHODS

In vitro, the effects of Pg-LPS, tumor necrosis factor (TNF)-α and rTMD1 on osteoclast differentiation were investigated using receptor activator of nuclear factor-κB ligand (RANKL)-stimulated RAW 264.7 macrophages. In vivo, the effects of rTMD1 treatment were evaluated in a model of experimental periodontitis induced by direct injection of Pg-LPS into the vestibular gingiva.

RESULTS

Administration of Pg-LPS to RANKL-stimulated RAW 264.7 macrophages resulted in upregulation of CD86 and osteoclast marker (eg, Dc-stamp and Trap) gene expression and increase of pro-inflammatory cytokine production (e.g., TNF-α) during osteoclast differentiation, and rTMD1 can attenuate these effects. Also, rTMD1 inhibited Pg-LPS-enhanced in vitro bone resorption in a dose-dependent manner. Moreover, TNF-α promoted phosphorylation of p38 and ERK during osteoclast differentiation, and the signal activation can be inhibited by rTMD1. Finally, treatment with rTMD1 hindered Pg-LPS-induced alveolar bone loss in experimental periodontitis in mice.

CONCLUSION

Our study demonstrated that rTMD1 attenuates Pg-LPS-enhanced M1 macrophage polarization, osteoclastogenesis and periodontal bone resorption and thus holds therapeutic promise for periodontitis.

Mitochondrial connection to ginsenosides

Mitochondria play an essential role in energy synthesis and supply, thereby maintaining cellular function, survival, and energy homeostasis via mitochondria-mediated pathways, including apoptosis and mitophagy. Ginsenosides are responsible for most immunological and pharmacological activities of ginseng, a highly beneficial herb with antioxidant, anti-inflammatory, anti-apoptotic, and neuroprotective properties. Studies have shown that ginsenosides assist in regulating mitochondrial energy metabolism, oxidative stress, biosynthesis, apoptosis, mitophagy, and the status of membrane channels, establishing mitochondria as one of their most important targets. This article reviews the regulatory effects of ginsenosides on the mitochondria and highlights their beneficial role in treating mitochondrial diseases.

The Mitochondrial Outer Membrane Protein Tom70-Mediator in Protein Traffic, Membrane Contact Sites and Innate Immunity

Tom70 is a versatile adaptor protein of 70 kDa anchored in the outer membrane of mitochondria in metazoa, fungi and amoeba. The tertiary structure was resolved for the Tom70 of yeast, showing 26 α-helices, most of them participating in the formation of 11 tetratricopeptide repeat (TPR) motifs. Tom70 serves as a docking site for cytosolic chaperone proteins and co-chaperones and is thereby involved in the uptake of newly synthesized chaperone-bound proteins in mitochondrial biogenesis. In yeast, Tom70 additionally mediates ER-mitochondria contacts via binding to sterol transporter Lam6/Ltc1. In mammalian cells, TOM70 promotes endoplasmic reticulum (ER) to mitochondria Ca2+ transfer by association with the inositol-1,4,5-triphosphate receptor type 3 (IP3R3). TOM70 is specifically targeted by the Bcl-2-related protein MCL-1 that acts as an anti-apoptotic protein in macrophages infected by intracellular pathogens, but also in many cancer cells. By participating in the recruitment of PINK1 and the E3 ubiquitin ligase Parkin, TOM70 can be implicated in the development of Parkinson's disease. TOM70 acts as receptor of the mitochondrial antiviral-signaling protein (MAVS) and thereby participates in the corresponding system of innate immunity against viral infections. The protein encoded by Orf9b in the genome of SARS-CoV-2 binds to TOM70, probably compromising the synthesis of type I interferons.

Thrombomodulin Functional Domains Support Osteoblast Differentiation and Bone Healing in Diabetes in Mice

Thrombomodulin (TM) is a transmembrane glycoprotein that contains five functional domains. Soluble TM (sTM), comprising extracellular domains TMD1 (lectin-like), TMD2 (epidermal growth factor [EGF]-like repeat containing), and TMD3 (serine-threonine rich), can be shed from cells by the intramembrane protease rhomboid-like-2 (RHBDL2). TM is expressed by osteoblasts, yet its role there has not been determined. Herein we aimed to investigate the properties of TM and its domains in osteoblast function and bone repair following injury in diabetes. In response to a scratch injury of cultured osteoblast-like MG63 cells, expression of TM and RHBDL2 was enhanced, with increased release of sTM. Conditioned media from the injured cells promoted osteoblast migration, an effect that was lacking with conditioned media from MG63 cells in which TM was silenced by shRNA. Exogenous recombinant TMD1 had no effect on osteoblast activities or on bone repair in vivo. However, TM domains 2 and 3 (TMD2/3), induced MG63 cell migration, proliferation and mineralization in vitro, and when locally administered in mice, improved in vivo healing of injured calvarium. This beneficial effect of TMD2/3, mediated via fibroblast growth factor receptor (FGFR)/ERK signaling pathways, was also observed in vitro under high glucose conditions where endogenous TM expression was reduced, and in vivo in diabetic mice following tibia fracture or calvarium injury, where the osteoblastic response and healing were otherwise dampened. Taken together, osteoblast TM participates in bone healing, and recombinant TMD2/3 holds promise as a novel therapy for diabetic bone defect healing. © 2020 American Society for Bone and Mineral Research.

ALA protects against ERS-mediated apoptosis in a cochlear cell model with low citrate synthase expression

A/J mouse is a model of age-related hearing loss (AHL). Mutation in the citrate synthase (Cs) gene of the mouse plays an important role in the hearing loss and degeneration of cochlear cells. To investigate the pathogenesis of cochlear cell damage in A/J mice resulted from Cs mutation, we downregulated the expression level of CS in HEI-OC1, a cell line of mouse cochlea, by shRNA. The results showed that low CS expression led to low ability of cell proliferation. Further study revealed an increase level of reactive oxygen species (ROS), activation of ATF6 mediated endoplasmic reticulum stress (ERS) and high expression levels of caspase12 and Bax in the cells. Moreover, the AEBSF, an ATF6 inhibitor, could reduce the expression levels of caspase-12 and Bax by inhibiting the hydrolysis of ATF6 in the cells. Finally, antioxidant alpha-lipoic acid (ALA) reduced the ROS levels and the apoptotic signals in the cell model with low CS expression. We therefore conclude that the ERS mediated apoptosis, which is triggered by ROS, may be involved in the cell degeneration in the cochleae of A/J mice.

Otoprotective Effects of α-lipoic Acid on A/J Mice With Age-related Hearing Loss

OBJECTIVE

A/J mice are a mouse model of age-related hearing loss (AHL) with progressive degeneration of outer hair cells (OHCs), spiral ganglion neurons (SGNs), and stria vascularis. This study was carried out to observe the otoprotective effects of α-lipoic acid on A/J mice.

METHODS

A/J mouse pups at postnatal day 7 were randomly distributed into the untreated group, the dimethyl sulfoxide (DMSO) group, and the α-lipoic acid + DMSO group. α-lipoic acid was given to the mice intraperitoneally at a dosage of 50 μg/g body weight every other day. Time course auditory-evoked brainstem response (ABR) thresholds were tested. OHC loss was counted and the densities of SGNs and the width of stria vascularis were measured at 4 and 8 weeks of age.

RESULTS

Measurement of the ABR thresholds revealed that hearing loss in A/J mice was attenuated by α-lipoic acid at age from 3 to 8 weeks. Moreover, preservation effects of OHCs, SGNs, and stria vascularis by α-lipoic acid were observed in the cochleae of A/J mice at 4 and 8 weeks of age.

CONCLUSION

Hearing loss in A/J mice can be attenuated by α-lipoic acid. The otoprotective effects of α-lipoic acid on A/J mice may be obtained by preserving OHCs, SGNs, and stria vascularis in the cochleae. The oxidative damage related to gene mutations may be a potential target for AHL prevention and therapy.

Fitter Mitochondria Are Associated With Radioresistance in Human Head and Neck SQD9 Cancer Cells

The clinical management of head and neck squamous cell carcinoma (HNSCC) commonly involves chemoradiotherapy, but recurrences often occur that are associated with radioresistance. Using human SQD9 laryngeal squamous cell carcinoma cancer cells as a model, we aimed to identify metabolic changes associated with acquired radioresistance. In a top-down approach, matched radiosensitive and radioresistant SQD9 cells were generated and metabolically compared, focusing on glycolysis, oxidative phosphorylation (OXPHOS) and ROS production. The cell cycle, clonogenicity, tumor growth in mice and DNA damage-repair were assessed. Mitochondrial DNA (mtDNA) was sequenced. In a bottom-up approach, matched glycolytic and oxidative SQD9 cells were generated using FACS-sorting, and tested for their radiosensitivity/radioresistance. We found that acquired radioresistance is associated with a shift from a glycolytic to a more oxidative metabolism in SQD9 cells. The opposite was also true, as the most oxidative fraction isolated from SQD9 wild-type cells was also more radioresistant than the most glycolytic fraction. However, neither reduced hexokinase expression nor OXPHOS were directly responsible for the radioresistant phenotype. Radiosensitive and radioresistant cells had similar proliferation rates and were equally efficient for ATP production. They were equally sensitive to redox stress and had similar DNA damage repair, but radioresistant cells had an increased number of mitochondria and a higher mtDNA content. Thus, an oxidative switch is associated with but is not responsible for acquired radioresistance in human SQD9 cells. In radioresistant cells, more abundant and fitter mitochondria could help to preserve mitochondrial functions upon irradiation.

Extra-mitochondrial citrate synthase initiates calcium oscillation and suppresses age-dependent sperm dysfunction

Men and women become infertile with age, but the mechanism of declining male fertility, more specifically, the decrease in in sperm quality, is not well known. Citrate synthase (CS) is a core enzyme of the mitochondrial tricarboxylic acid (TCA) cycle, which directly controls cellular function. Extra-mitochondrial CS (eCS) is produced and abundant in the sperm head; however, its role in male fertility is unknown. We investigated the role of eCS in male fertility by producing eCs-deficient (eCs-KO) mice. The initiation of the first spike of Ca²⁺ oscillation was substantially delayed in egg fused with eCs-KO sperm, despite normal expression of sperm factor phospholipase C zeta 1. The eCs-KO male mice were initially fertile, but the fertility dropped with age. Metabolomic analysis of aged sperm revealed that the loss of eCS enhances TCA cycle in the mitochondria with age, presumably leading to depletion of extra-mitochondrial citrate. The data suggest that eCS suppresses age-dependent male infertility, providing insights into the decline of male fertility with age.

Cloning, expression, characterization, and immunological properties of citrate synthase from Echinococcus granulosus

The larval stages of the tapeworm Echinococcus granulosus (Cestoda: Taeniidae) are the causative agent of cystic echinococcosis, one of the most important parasitic zoonoses worldwide. E. granulosus has a complete pathway for the tricarboxylic acid cycle (TCA), in which citrate synthase (CS) is the key enzyme. Here, we cloned and expressed CS from E. granulosus (Eg-CS) and report its molecular characterization. The localization of this protein during different developmental stages and mRNA expression patterns during H₂O₂ treatment were determined. We found that Eg-CS is a highly conserved protein, consisting of 466 amino acids. In western blotting assays, recombinant Eg-CS (rEg-CS) reacted with E. granulosus-positive sheep sera and anti-rEg-CS rabbit sera, indicating that Eg-CS has good antigenicity and immunoreactivity. Localization studies, performed using immunohistochemistry, showed that Eg-CS is ubiquitously expressed in the larva, germinal layer, and adult worm sections of E. granulosus. Eg-CS mRNA expression levels increased following H₂O₂ exposure. In conclusion, citrate synthase might be involved in the metabolic process in E. granulosus. An assessment of the serodiagnostic potential of rEg-CS based on indirect ELISA showed that, although sensitivity (93.55%) and specificity (80.49%) are high, cross-reactivity with other parasites precludes its use as a diagnostic antigen.

Anticancer effect of ursolic acid via mitochondria-dependent pathways

Ursolic acid is a plant-derived pentacyclic triterpenoid found in various medicinal herbs and fruits. It has generated clinical interest due to its anti-inflammatory, antioxidative, antiapoptotic and anticarcinogenic effects. An increasing amount of evidence supports the anticancer effect of ursolic acid in various cancer cells. One of the hallmarks of malignant transformation is metabolic reprogramming that sustains macromolecule synthesis, bioenergetic demand and tumor cell survival. Mitochondria are important regulators of tumorigenes is as well as a major site of the metabolic reactions that facilitate this reprogramming and adaption to cellular and environmental changes. The current review explored the close association between the anticancer effect of ursolic acid and the activation of mitochondrial-dependent signaling pathways.

Trimetazidine combined with exercise improves exercise capacity and anti-fatal stress ability through enhancing mitochondrial quality control

AIMS

To explore the effects of trimetazidine combined with exercise on EC and anti-fatal stress ability, and illustrate the underlying mechanism.

METHODS

C57BL/6 mice were randomly assigned to four groups (n = 11 in each group): the control, exercise, trimetazidine and trimetazidine + exercise (TE) groups. Mice were accordingly given saline (ig), Aerobic exercise (AE), trimetazidine (ig), or a combination of trimetazidine (ig) and AE for five weeks. After the intervention, each group was randomly subdivided into rest and exhaustive exercise (EE) subgroups. The mice in the control-EE and TE-EE subgroups underwent fatal stress experiments. EC and anti-fatal stress ability were assessed respectively. Mitochondrial quality control (MQC) in skeletal muscle were measured at the protein level and the organelle level.

KEY FINDINGS

A significantly increased exhaustive swimming time was observed in exercise (39.10 ± 12.58 min vs 14.18 ± 4.37 min), trimetazidine (33.73 ± 8.45 min vs 14.18 ± 4.37 min) and TE groups (73.78 ± 18.95 min vs 14.18 ± 4.37 min) compared with that in the control group, and a synergistic effect was detected (P < 0.05). Fatal stress experiments successfully induced skeletal muscle damage, including increased creatine kinase activity, myofibrosis, and impaired antioxidative enzyme system, all those were significantly alleviated by trimetazidine supplementation combined with AE precondition (P < 0.05). Meanwhile, AE and trimetazidine alone or combined, significantly enhanced the MQC in normal mice by activating mitochondrial biogenesis, dynamics and mitophagy, and that in mice underwent fatal stress stimulus (P < 0.05).

SIGNIFICANCE

This study for the first time found that trimetazidine and AE have synergistic effects on improving EC. Moreover, the combination of both interventions enhances anti-fatal stress ability. Enhancing MQC may be a key mechanism of AE combined with trimetazidine that improves EC and anti-fatal stress ability.

A mitochondrial FUNDC1/HSC70 interaction organizes the proteostatic stress response at the risk of cell morbidity

Both protein quality and mitochondrial quality are vital for the cellular activity, and impaired proteostasis and mitochondrial dysfunction are common etiologies of aging and age-related disorders. Here, we report that the mitochondrial outer membrane protein FUNDC1 interacts with the chaperone HSC70 to promote the mitochondrial translocation of unfolded cytosolic proteins for degradation by LONP1 or for formation of non-aggresomal mitochondrion-associated protein aggregates (MAPAs) upon proteasome inhibition in cultured human cells. Integrative approaches including csCLEM, Apex, and biochemical analysis reveal that MAPAs contain ubiquitinated cytosolic proteins, autophagy receptor p62, and mitochondrial proteins. MAPAs are segregated from mitochondria in a FIS1-dependent manner and can subsequently be degraded via autophagy. Although the FUNDC1/HSC70 pathway promotes the degradation of unfolded cytosolic proteins, excessive accumulation of unfolded proteins on the mitochondria prior to MAPA formation impairs mitochondrial integrity and activates AMPK, leading to cellular senescence. We suggest that human mitochondria organize cellular proteostatic response at the risk of their own malfunction and cell lethality.

Plasminogen/thrombomodulin signaling enhances VEGF expression to promote cutaneous wound healing

Plasminogen (Plg) and thrombomodulin (TM) are glycoproteins well known for fibrinolytic and anticoagulant functions, respectively. Both Plg and TM are essential for wound healing. However, their significance during the reparative process was separately demonstrated in previous studies. Here, we investigate the interaction between Plg and epithelial TM and its effect on wound healing. Characterization of the wound margin revealed that Plg and TM were simultaneously upregulated at the early stage of wound healing and the two molecules were bound together. In vitro, TM silencing or knockout in keratinocytes inhibited Plg activation. Plg treatment enhanced keratinocyte proliferation and migration, and these actions were abolished by TM antibody. Keratinocyte-expressed vascular endothelial growth factor (VEGF), which presented a dose-response relationship with Plg treatment, can be suppressed by TM silencing. Moreover, treatment with VEGF antibody inhibited Plg-enhanced keratinocyte proliferation and wound recovery. In vivo, TM antibody treatment and keratinocyte-specific TM knockout can impede Plg-enhanced wound healing in mice. In high-glucose environments, Plg-enhanced VEGF expression and wound healing were suppressed due at least in part to downregulation of keratinocyte-expressed TM. Taken together, our findings suggest that activation of Plg/TM signaling may hold therapeutic potential for chronic wounds in diabetic or non-diabetic individuals. KEY MESSAGES: Plg binds to TM in cutaneous wound healing. TM facilitates the activation of Plg to Plm in keratinocytes. Epithelial TM regulates Plg-enhanced wound healing through VEGF expression.

Exercise Combined with Rhodiola sacra Supplementation Improves Exercise Capacity and Ameliorates Exhaustive Exercise-Induced Muscle Damage through Enhancement of Mitochondrial Quality Control

Mounting evidence has firmly established that increased exercise capacity (EC) is associated with considerable improvements in the survival of patients with cardiovascular disease (CVD) and that antistress capacity is a prognostic predictor of adverse cardiovascular events in patients with CVD. Previous studies have indicated that aerobic exercise (AE) and supplementation with Rhodiola sacra (RS), a natural plant pharmaceutical, improve EC and enable resistance to stress; however, the underlying mechanism remains unclear. This study explored the ability of AE and RS, alone or combined, to improve EC and ameliorate exhaustive exercise- (EE-) induced stress and elucidate the mechanism involved. We found that AE and RS significantly increased EC in mice and ameliorated EE-induced stress damage in skeletal and cardiac muscles (SCM); furthermore, a synergistic effect was detected for the first time. To our knowledge, the present work is the first to report that AE and RS activate mitophagy, mitochondrial dynamics, and biogenesis in SCM, both in the resting state and after EE. These data indicate that AE and RS synergistically improve EC in mice and protect SCM from EE-induced stress by enhancing mitochondrial quality control, including the activation of mitophagy, mitochondrial dynamics, and biogenesis, both at rest and after EE.

Reduced expression of citrate synthase leads to excessive superoxide formation and cell apoptosis

A/J mice are a mouse model of age-related hearing loss. It has been demonstrated that a mutation in gene of citrate synthase (CS) contributes to the early onset of hearing loss occurring at about one month of age. To understand the effects of a decreased CS activity that results from the mutation in Cs gene on hearing loss in A/J mice, human kidney cell line (293T) was transiently transfected with short hairpin RNA for Cs (shRNA-Cs) to reduce expression of CS. In comparison with those of cells transfected with a scrambled sequence (shRNA-NC), the oxygen consumption rate and adenosine trisphosphate (ATP) production level were decreased in 293T cells transfected with shRNA-Cs. Meanwhile, excessive superoxide production was induced as determined by mitochondrial superoxide formation assay (MitoSOX) and superoxide dismutase 2 (SOD2) detection. Moreover, the expression levels of BIP (binding immunoglobulin protein) and CHOP (CCAAT/enhancer-binding protein-homologous protein), markers of endoplasmic reticulum stress, were upregulated. Furthermore, apoptosis related molecule caspase-3 and the mitochondrial membrane potential were reduced. It is therefore concluded that downregulation of Cs expression in 293T cells leads to low level of ATP production, excessive superoxide formation and cell apoptosis, which implies a possible mechanism for hearing loss in A/J mice.

Heterologous mitochondrial targeting sequences can deliver functional proteins into mitochondria

Mitochondrial Targeting Sequences (MTSs) are responsible for trafficking nuclear-encoded proteins into mitochondria. Once entering the mitochondria, the MTS is recognized and cleaved off. Some MTSs are long and undergo two-step processing, as in the case of the human frataxin (FXN) protein (80aa), implicated in Friedreich's ataxia (FA). Therefore, we chose the FXN protein to examine whether nuclear-encoded mitochondrial proteins can efficiently be targeted via a heterologous MTS (hMTS) and deliver a functional protein into mitochondria. We examined three hMTSs; that of citrate synthase (cs), lipoamide deydrogenase (LAD) and C6ORF66 (ORF), as classically MTS sequences, known to be removed by one-step processing, to deliver FXN into mitochondria, in the form of fusion proteins. We demonstrate that using hMTSs for delivering FXN results in the production of 4-5-fold larger amounts of the fusion proteins, and at 4-5-fold higher concentrations. Moreover, hMTSs delivered a functional FXN protein into the mitochondria even more efficiently than the native MTSfxn, as evidenced by the rescue of FA patients' cells from oxidative stress; demonstrating a 18%-54% increase in cell survival; and a 13%-33% increase in ATP levels, as compared to the fusion protein carrying the native MTS. One fusion protein with MTScs increased aconitase activity within patients' cells, by 400-fold. The implications form our studies are of vast importance for both basic and translational research of mitochondrial proteins as any mitochondrial protein can be delivered efficiently by an hMTS. Moreover, effective targeting of functional proteins is important for restoration of mitochondrial function and treatment of related disorders.

Caspase-mediated apoptosis in the cochleae contributes to the early onset of hearing loss in A/J mice

A/J and C57BL/6 J (B6) mice share a mutation in Cdh23 (ahl allele) and are characterized by age-related hearing loss. However, hearing loss occurs much earlier in A/J mice at about four weeks of age. Recent study has revealed that a mutation in citrate synthase (Cs) is one of the main contributors, but the mechanism is largely unknown. In the present study, we showed that A/J mice displayed more severe degeneration of hair cells, spiral ganglion neurons, and stria vascularis in the cochleae compared with B6 mice. Moreover, messenger RNA accumulation levels of caspase-3 and caspase-9 in the inner ears of A/J mice were significantly higher than those in B6 mice at 2 and 8 weeks of age. Immunohistochemistry localized caspase-3 expression mainly to the hair cells, spiral ganglion neurons, and stria vascularis in cochleae. In vitro transfection with Cs short hairpin RNA (shRNA) alone or cotransfection with Cs shRNA and Cdh23 shRNA significantly increased the levels of caspase-3 in an inner ear cell line (HEI-OC1). Finally, a pan-caspase inhibitor Z-VAD-FMK could preserve the hearing of A/J mice by lowering about 15 decibels of the sound pressure level for the auditory-evoked brainstem response thresholds. In conclusion, our results suggest that caspase-mediated apoptosis in the cochleae, which may be related to a Cs mutation, contributes to the early onset of hearing loss in A/J mice.

Promotion of mitochondrial energy metabolism during hepatocyte apoptosis in a rat model of acute liver failure

Hepatocyte apoptosis and energy metabolism in mitochondria have an important role in the mechanism of acute liver failure (ALF). However, data on the association between apoptosis and the energy metabolism of hepatocytes are lacking. The current study assessed the activity of several key enzymes in mitochondria during ALF, including citrate synthase (CS), carnitine palmitoyltransferase-1 (CPT-1) and cytochrome c oxidase (COX), which are involved in hepatocyte energy metabolism. A total of 40 male Sprague-Dawley rats were divided into five groups and administered D-galactosamine and lipopolysaccharide to induce ALF. Hepatic pathology and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling examinations indicated that hepatocyte apoptosis was observed at 4 h and increased 8 h after ALF. Hepatocyte necrosis appeared at 12 h and was significantly higher at 24 h with inflammatory cell invasion. The results measured by electron microscopy indicated that ultrastructural changes in mitochondria began at 4 h and the mitochondrial outer membrane was completely disrupted at 24 h resulting in mitochondrial collapse. The expression of CS, CPT-1 and COX was measured and analyzed using assay kits. The activity and protein expression of CS, CPT-1 and COX began to increase at 4 h, reached a peak at 8 h and decreased at 12 h during ALF. The activities of CS, CPT-1 and COX were enhanced during hepatocyte apoptosis suggesting that these enzymes are involved in the initiation and development of ALF. Therefore, these results demonstrated that energy metabolism is important in hepatocyte apoptosis during ALF and hepatocyte apoptosis is an active and energy-consuming procedure. The current study on how hepatocyte energy metabolism affects the transmission of death signals may provide a basis for the early diagnosis and development of an improved therapeutic strategy for ALF.

Function of alternative splicing

Almost all polymerase II transcripts undergo alternative pre-mRNA splicing. Here, we review the functions of alternative splicing events that have been experimentally determined. The overall function of alternative splicing is to increase the diversity of mRNAs expressed from the genome. Alternative splicing changes proteins encoded by mRNAs, which has profound functional effects. Experimental analysis of these protein isoforms showed that alternative splicing regulates binding between proteins, between proteins and nucleic acids as well as between proteins and membranes. Alternative splicing regulates the localization of proteins, their enzymatic properties and their interaction with ligands. In most cases, changes caused by individual splicing isoforms are small. However, cells typically coordinate numerous changes in 'splicing programs', which can have strong effects on cell proliferation, cell survival and properties of the nervous system. Due to its widespread usage and molecular versatility, alternative splicing emerges as a central element in gene regulation that interferes with almost every biological function analyzed.

Impaired TCA cycle flux in mitochondria in skeletal muscle from type 2 diabetic subjects: marker or maker of the diabetic phenotype?

The diabetic phenotype is complex, requiring elucidation of key initiating defects. Recent research has shown that diabetic myotubes express a primary reduced tricarboxylic acid (TCA) cycle flux. A reduced TCA cycle flux has also been shown both in insulin resistant offspring of T2D patients and exercising T2D patients in vivo. This review will discuss the latest advances in the understanding of the molecular mechanisms regulating the TCA cycle with focus on possible underlying mechanism which could explain the impaired TCA flux in insulin resistant human skeletal muscle in type 2 diabetes. A reduced TCA is both a marker and a maker of the diabetic phenotype.

Association of a citrate synthase missense mutation with age-related hearing loss in A/J mice

We previously mapped a locus (ahl4) on distal Chromosome 10 that contributes to the age-related hearing loss of A/J strain mice. Here, we report on a refined genetic map position for ahl4 and its association with a mutation in the citrate synthase gene (Cs). We mapped ahl4 to the distal-most 7 megabases (Mb) of chromosome 10 by analysis of a new linkage backcross and then further narrowed the interval to 5.5 Mb by analysis of 8 C57BL/6J congenic lines with different A/J-derived segments of chromosome 10. A nucleotide variant in exon 3 of Cs is the only known DNA difference within the ahl4 candidate gene interval that is unique to the A/J strain and that causes a nonsynonymous codon change. Multiple lines of evidence implicate this missense mutation (H55N) as the underlying cause of ahl4-related hearing loss, likely through its effects on mitochondrial adenosine trisphosphate (ATP) and free radical production in cochlear hair cells. The A/J mouse thus provides a new model system for in vivo studies of mitochondrial function and hearing loss.

Blood-feeding and immunogenic Aedes aegypti saliva proteins

Mosquito-transmitted pathogens pass through the insect's midgut (MG) and salivary gland (SG). What occurs in these organs in response to a blood meal is poorly understood, but identifying the physiological differences between sugar-fed and blood-fed (BF) mosquitoes could shed light on factors important in pathogens transmission. We compared differential protein expression in the MGs and SGs of female Aedes aegypti mosquitoes after a sugar- or blood-based diet. No difference was observed in the MG protein expression levels but certain SG proteins were highly expressed only in BF mosquitoes. In sugar-fed mosquitoes, housekeeping proteins were highly expressed (especially those related to energy metabolism) and actin was up-regulated. The immunofluorescence assay shows that there is no disruption of the SG cytoskeletal after the blood meal. We have generated for the first time the 2-DE profiles of immunogenic Ae. aegypti SG BF-related proteins. These new data could contribute to the understanding of the physiological processes that appear during the blood meal.