Requirement of plasminogen binding to its cell-surface receptor α-enolase for efficient regeneration of normal and dystrophic skeletal muscle

Liver-derived plasminogen mediates muscle stem cell expansion during caloric restriction through the plasminogen receptor Plg-RKT

An intriguing effect of short-term caloric restriction (CR) is the expansion of certain stem cell populations, including muscle stem cells (satellite cells), which facilitate an accelerated regenerative program after injury. Here, we utilized the MetRSL274G (MetRS) transgenic mouse to identify liver-secreted plasminogen as a candidate for regulating satellite cell expansion during short-term CR. Knockdown of circulating plasminogen prevents satellite cell expansion during short-term CR. Furthermore, loss of the plasminogen receptor KT (Plg-RKT) is also sufficient to prevent CR-related satellite cell expansion, consistent with direct signaling of plasminogen through the plasminogen receptor Plg-RKT/ERK kinase to promote proliferation of satellite cells. Importantly, we are able to replicate many of these findings in human participants from the CALERIE trial. Our results demonstrate that CR enhances liver protein secretion of plasminogen, which signals directly to the muscle satellite cell through Plg-RKT to promote proliferation and subsequent muscle resilience during CR.

Genome-wide epigenetic dynamics during postnatal skeletal muscle growth in Hu sheep

Hypertrophy and fiber transformation are two prominent features of postnatal skeletal muscle development. However, the role of epigenetic modifications is less understood. ATAC-seq, whole genome bisulfite sequencing, and RNA-seq were applied to investigate the epigenetic dynamics of muscle in Hu sheep at 3 days, 3 months, 6 months, and 12 months after birth. All 6865 differentially expressed genes were assigned into three distinct tendencies, highlighting the balanced protein synthesis, accumulated immune activities, and restrained cell division in postnatal development. We identified 3742 differentially accessible regions and 11799 differentially methylated regions that were associated with muscle-development-related pathways in certain stages, like D3-M6. Transcription factor network analysis, based on genomic loci with high chromatin accessibility and low methylation, showed that ARID5B, MYOG, and ENO1 were associated with muscle hypertrophy, while NR1D1, FADS1, ZFP36L2, and SLC25A1 were associated with muscle fiber transformation. Taken together, these results suggest that DNA methylation and chromatin accessibility contributed toward regulating the growth and fiber transformation of postnatal skeletal muscle in Hu sheep.

Regulation of enolase activation to promote neural protection and regeneration in spinal cord injury

Spinal cord injury (SCI) is a debilitating condition characterized by damage to the spinal cord resulting in loss of function, mobility, and sensation with no U.S. Food and Drug Administration-approved cure. Enolase, a multifunctional glycolytic enzyme upregulated after SCI, promotes pro- and anti-inflammatory events and regulates functional recovery in SCI. Enolase is normally expressed in the cytosol, but the expression is upregulated at the cell surface following cellular injury, promoting glial cell activation and signal transduction pathway activation. SCI-induced microglia activation triggers pro-inflammatory mediators at the injury site, activating other immune cells and metabolic events, i.e., Rho-associated kinase, contributing to the neuroinflammation found in SCI. Enolase surface expression also activates cathepsin X, resulting in cleavage of the C-terminal end of neuron-specific enolase (NSE) and non-neuronal enolase (NNE). Fully functional enolase is necessary as NSE/NNE C-terminal proteins activate many neurotrophic processes, i.e., the plasminogen activation system, phosphatidylinositol-4,5-bisphosphate 3-kinase/protein kinase B, and mitogen-activated protein kinase/extracellular signal-regulated kinase. Studies here suggest an enolase inhibitor, ENOblock, attenuates the activation of Rho-associated kinase, which may decrease glial cell activation and promote functional recovery following SCI. Also, ENOblock inhibits cathepsin X, which may help prevent the cleavage of the neurotrophic C-terminal protein allowing full plasminogen activation and phosphatidylinositol-4,5-bisphosphate 3-kinase/mitogen-activated protein kinase activity. The combined NSE/cathepsin X inhibition may serve as a potential therapeutic strategy for preventing neuroinflammation/degeneration and promoting neural cell regeneration and recovery following SCI. The role of cell membrane-expressed enolase and associated metabolic events should be investigated to determine if the same strategies can be applied to other neurodegenerative diseases. Hence, this review discusses the importance of enolase activation and inhibition as a potential therapeutic target following SCI to promote neuronal survival and regeneration.

Giardia duodenalis enolase is secreted as monomer during trophozoite-epithelial cell interactions, activates plasminogen and induces necroptotic damage

Enolase, a multifunctional protein expressed by multiple pathogens activates plasminogen to promote proteolysis on components of the extracellular matrix, an important event in early host-pathogen interactions. A secreted form of enolase that is released upon the interaction of trophozoites with epithelial cells has been detected in the secretome of G. duodenalis. However, the role of enolase in the host-pathogen interactions remains largely unknown. In this work, the effects of G. duodenalis enolase (Gd-eno) on the epithelial cell model (IEC-6) were analyzed. Firstly, the coding sequence of Giardia enolase was cloned and the recombinant protein used to raise antibodies that were then used to define the localization and role of enolase in epithelial cell-trophozoite interactions. Gd-eno was detected in small cytoplasmic vesicles as well as at the surface and is enriched in the region of the ventral disk of Giardia trophozoites. Moreover, the blocking of the soluble monomeric form of the enzyme, which is secreted upon interaction with IEC-6 cells by the anti-rGd-eno antibodies, significantly inhibited trophozoite attachment to intestinal IEC-6 cell monolayers. Further, rGd-eno was able to bind human plasminogen (HsPlg) and enhanced plasmin activity in vitro when the trophozoites were incubated with the intrinsic plasminogen activators of epithelial cells. In IEC-6 cells, rGd-eno treatment induced a profuse cell damage characterized by copious vacuolization, intercellular separation and detachment from the substrate; this effect was inhibited by either anti-Gd-eno Abs or the plasmin inhibitor ϵ- aminocaproic acid. Lastly, we established that in epithelial cells rGd-eno treatment induced a necroptotic-like process mediated by tumor necrosis factor α (TNF-α) and the apoptosis inducing factor (AIF), but independent of caspase-3. All together, these results suggest that Giardia enolase is a secreted moonlighting protein that stimulates a necroptotic-like process in IEC-6 epithelial cells via plasminogen activation along to TNFα and AIF activities and must be considered as a virulence factor.

A critical role for plasminogen in inflammation

Plasminogen and its active form, plasmin, have diverse functions related to the inflammatory response in mammals. Due to these roles in inflammation, plasminogen has been implicated in the progression of a wide range of diseases with an inflammatory component. In this review, we discuss the functions of plasminogen in inflammatory regulation and how this system plays a role in the pathogenesis of diseases spanning organ systems throughout the body.

Novel LRAP-binding partner revealing the plasminogen activation system as a regulator of cementoblast differentiation and mineral nodule formation in vitro

Amelogenin isoforms, including full-length amelogenin (AMEL) and leucine-rich amelogenin peptide (LRAP), are major components of the enamel matrix, and are considered as signaling molecules in epithelial-mesenchymal interactions regulating tooth development and periodontal regeneration. Nevertheless, the molecular mechanisms involved are still poorly understood. The aim of the present study was to identify novel binding partners for amelogenin isoforms in the cementoblast (OCCM-30), using an affinity purification assay (GST pull-down) followed by mass spectrometry and immunoblotting. Protein-protein interaction analysis for AMEL and LRAP evidenced the plasminogen activation system (PAS) as a potential player regulating OCCM-30 response to amelogenin isoforms. For functional assays, PAS was either activated (plasmin) or inhibited (ε-aminocaproic acid [aminocaproic]) in OCCM-30 cells and the cell morphology, mineral nodule formation, and gene expression were assessed. PAS inhibition (EACA 100 mM) dramatically decreased mineral nodule formation and expression of OCCM-30 differentiation markers, including osteocalcin (Bglap), bone sialoprotein (Ibsp), osteopontin (Spp1), tissue-nonspecific alkaline phosphatase (Alpl) and collagen type I (Col1a1), and had no effect on runt-related transcription factor 2 (Runx2) and Osterix (Osx) mRNA levels. PAS activation (plasmin 5 µg/µl) significantly increased Col1a1 and decreased Bglap mRNA levels (p < .05). Together, our findings shed new light on the potential role of plasminogen signaling pathway in the control of the amelogenin isoform-mediated response in cementoblasts and provide new insights into the development of targeted therapies.

Co-purification of arrestin like proteins with alpha-enolase from bovine myocardial tissues and the possible role in heart diseases as an autoantigen

AIM

Previously, we reported that visual arrestin co-purified with glycolytic enzymes. The aim of this study was to analyze the co-purification of arrestin like proteins (ALP) in bovine cardiac tissues with enolases.

METHODS

The soluble extract of bovine myocardial tissues from different regions such as left and right atriums and ventricles of the bovine heart (n = 3) was analyzed by ACA-34 gel filtration, immuno-affinity column, SDS-PAGE, ELISA, western blot and a sandwich immune assay for quantification of ALP and sequence analysis.

RESULTS

We observed that; 1) The cardiac muscle contained a 50 kDa ALP at a concentration of 751 pg/mg of soluble protein extract, 2) ALP purified, by immunoaffinity, contained alpha-enolase of 48 kDa confirmed by protein sequence analysis; 3) Cardiomyocyte cells exposed to anti arrestin and anti enolase monoclonal antibodies showed decreased proliferation in vitro, 4) High level of autoantibodies were detected by ELISA (3.57% for arrestin and 9.12% for α-enolase) in serum of patients with infarcted heart disease.

CONCLUSION

We suggest a possible interaction between ALP and alpha-enolases yielding a complex that may be involved in the induction of cardiac autoimmune diseases.

Multiplex Screen of Serum Biomarkers in Facioscapulohumeral Muscular Dystrophy

BACKGROUND

Recent studies have proposed a unified genetic model for Facioscapulohumeral muscular dystrophy (FSHD), identifying potential therapeutic targets for future clinical trials. Serum biomarkers related to disease activity will be important for proof of concept or early phase clinical studies.

OBJECTIVE

To identify potential serum biomarkers in FSHD for possible use in future clinical trials.

METHODS

We performed a prospective cross-sectional study of serum biomarkers in 22 FSHD patients (19 FSHD1, 3 FSHD2) compared to 23 age and gender-matched healthy controls using a commercial multiplex, microsphere-based immune-fluorescent assay of 243 markers (Myriad, Human Discovery MAP 250, v2.0).

RESULTS

169 markers had values sufficient for analysis. Correction for multiple testing identified 7 biomarkers below a 5% false discovery rate: creatine kinase MB fraction (CKMB, 6.52 fold change, P<0.0001), tissue-type plasminogen activator (PLAT, 1.64 fold change, P<0.0001), myoglobin (2.23 fold change, P=0.0001), epidermal growth factor (EGF, 2.33 fold change, P=0.0004), chemokine (C-C motif) ligand 2 (1.48 fold change, P=0.0004), CD 40 ligand (1.89 fold change, P=0.001), and vitronectin (VTN, 1.28 fold change, P=0.001). Moderate correlations to measures of FSHD disease were seen for CKMB, PLAT, and EGF. Markers in the plasminogen pathway (PLAT, serpin peptidase inhibitor, and VTN) were correlated with each other in FSHD but not healthy controls.

CONCLUSIONS

Commercial multiplex immune-fluorescent screening is a potentially powerful tool for identifying biomarkers for future FSHD therapeutic trials. Biomarkers identified in this study warrant further study in a larger prospective validation study.

An evolutionary perspective on protein moonlighting

Moonlighting proteins serve one or more novel functions in addition to their canonical roles. Moonlighting functions arise when an adventitious interaction between a protein and a new partner improves fitness of the organism. Selective pressure for improvement in the new function can result in two alternative outcomes. The gene encoding the newly bifunctional protein may duplicate and diverge so as to encode two proteins, each of which serves only one function. Alternatively, genetic changes that minimize adaptive conflict between the two functions and/or improve control over the time and place at which each function is served can lead to a moonlighting protein. Importantly, genetic changes that enhance a moonlighting function can occur in the gene encoding the moonlighting protein itself, in a gene that affects the structure of its new partner or in a gene encoding a transcription factor that controls expression of either partner. The evolutionary history of each moonlighting protein is complex, depending on the stochastic occurrence of genetic changes such as gene duplication and point mutations, and the effects of those changes on fitness. Population effects, particularly loss of promising individuals due to random genetic drift, also play a role in the emergence of a moonlighting protein. The ultimate outcome is not necessarily the 'optimal' solution to the problem of serving two functions, but may be 'good enough' so that fitness becomes limited by some other function.

Modulation of α-enolase post-translational modifications by dengue virus: increased secretion of the basic isoforms in infected hepatic cells

Hepatic cells are major sites of dengue virus (DENV) replication and liver injury constitutes a characteristic of severe forms of dengue. The role of hepatic cells in dengue pathogenesis is not well established, but since hepatocytes are the major source of plasma proteins, changes in protein secretion by these cells during infection might contribute to disease progression. Previously, we showed that DENV infection alters the secretion pattern of hepatic HepG2 cells, with α-enolase appearing as one of the major proteins secreted in higher levels by infected cells. ELISA analysis demonstrated that DENV infection modulates α-enolase secretion in HepG2 cells in a dose-dependent manner, but has no effect on its gene expression and on the intracellular content of the protein as assessed by PCR and western blot analyses, respectively. Two-dimensional western blots showed that both intracellular and secreted forms of α-enolase appear as five spots, revealing α-enolase isoforms with similar molecular weights but distinct isoeletric points. Remarkably, quantification of each spot content revealed that DENV infection shifts the isoform distribution pattern of secreted α-enolase towards the basic isoforms, whereas the intracellular protein remains unaltered, suggesting that post-translational modifications might be involved in α-enolase secretion by infected cells. These findings provide new insights into the mechanisms underlying α-enolase secretion by hepatic cells and its relationship with the role of liver in dengue pathogenesis. In addition, preliminary results obtained with plasma samples from DENV-infected patients suggest an association between plasma levels of α-enolase and disease severity. Since α-enolase binds plasminogen and modulates its activation, it is plausible to speculate the association of the increase in α-enolase secretion by infected hepatic cells with the haemostatic dysfunction observed in dengue patients including the promotion of fibrinolysis and vascular permeability alterations.

Formation of elongated fascicle-inspired 3D tissues consisting of high-density, aligned cells using sacrificial outer molding

The majority of muscles, nerves, and tendons are composed of fiber-like fascicle morphology. Each fascicle has a) elongated cells highly aligned with the length of the construct, b) a high volumetric cell density, and c) a high length-to-width ratio with a diameter small enough to facilitate perfusion. Fiber-like fascicles are important building blocks for forming tissues of various sizes and cross-sectional shapes, yet no effective technology is currently available for producing long and thin fascicle-like constructs with aligned, high-density cells. Here we present a method for molding cell-laden hydrogels that generate cylindrical tissue structures that are ~100 μm in diameter with an extremely high length to diameter ratio (>100 : 1). Using this method we have successfully created skeletal muscle tissue with a high volumetric density (~50%) and perfect cell alignment along the axis. A new molding technique, sacrificial outer molding, allows us to i) create a long and thin cylindrical cavity of the desired size in a sacrificial mold that is solid at a low temperature, ii) release gelling agents from the sacrificial mold material after the cell-laden hydrogel is injected into fiber cavities, iii) generate a uniform axial tension between anchor points at both ends that promotes cell alignment and maturation, and iv) perfuse the tissue effectively by exposing it to media after melting the sacrificial outer mold at 37 °C. The effects of key parameters and conditions, including initial cavity diameter, axial tension, and concentrations of the hydrogel and gelling agent upon tissue compaction, volumetric cell density, and cell alignment are presented.

Proteomic and bioinformatic analysis of differentially expressed proteins in denervated skeletal muscle

The aim of this study was to improve our understanding and the current treatment of denervation-induced skeletal muscle atrophy. We used isobaric tags for relative and absolute quantification (iTRAQ) coupled with two-dimensional liquid chromatography-tandem mass spectrometry (2D LC-MS/MS) to identify the differentially expressed proteins in the tibialis anterior (TA) muscle of rats at 1 and 4 weeks following sciatic nerve transection. A total of 110 proteins was differentially expressed and was further classified using terms from the Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases to unravel their molecular functions. Among the differentially expressed metabolic enzymes involved in glycolysis, Krebs cycle and oxidative phosphorylation, α- and β-enolase displayed an increased and decreased expression, respectively, which was further validated by western blot analysis and immunohistochemistry. These findings suggest that the enolase isozymic switch during denervation-induced muscle atrophy is the reverse of that occurring during muscle maturation. Notably, protein‑protein interaction analysis using the STRING database indicated that the protein expression of tumor necrosis factor receptor-associated factor-6 (TRAF6), muscle ring-finger protein 1 (MuRF1) and muscle atrophy F-box (MAFBx) was also upregulated during denervation‑induced skeletal muscle atrophy, which was confirmed by western blot analysis. TRAF6 knockdown experiments in L6 myotubes suggested that the decreased expression of TRAF6 attenuated glucocorticoid‑induced myotube atrophy. Therefore, we hypothesized that the upregulation of TRAF6 may be involved in the development of denervation‑induced muscle atrophy, at least in part, by regulating the expression of MAFBx and MuRF1 proteins. The data from the present study provide valuable insight into the molecular mechanisms regulating denervation-induced muscle atrophy.