ClC-3 is required for LPA-activated Cl- current activity and fibroblast-to-myofibroblast differentiation

To determine the effects of chloride channel 3 (ClC-3) knockdown and overexpression on lysophosphatidic acid (LPA)- and volume-regulated anion channel Cl(-) currents (I(Cl,LPA) and I(Cl,VRAC), respectively), cell differentiation, and cell volume regulation, a short hairpin RNA (shRNA) expression system based on a mouse U6 promoter was used to knock down ClC-3 in human corneal keratocytes and human fetal lung fibroblasts. ClC-3 overexpression was achieved by electroporating full-length ClC-3, within a pcDNA3.1 vector, into these two cell lines. RT-PCR and Western blot analysis were used to detect ClC-3 mRNA and protein levels. Whole cell perforated patch-clamp recording was used to measure I(Cl,LPA) and I(Cl,VRAC) currents, and fluorescence-activated cell sorting analysis was used to measure cell volume regulation. ClC-3 knockdown significantly decreased I(Cl,LPA) and I(Cl,VRAC) activity in the presence of transforming growth factor-beta(1) (TGF-beta(1)) compared with controls, whereas ClC-3 overexpression resulted in increased I(Cl,LPA) activity in the absence of TGF-beta(1). ClC-3 knockdown also resulted in a reduction of alpha-smooth muscle actin (alpha-SMA) protein levels in the presence of TGF-beta(1), whereas ClC-3 overexpression increased alpha-SMA protein expression in the absence of TGF-beta(1). In addition, keratocytes transfected with ClC-3 shRNA had a significantly blunted regulatory volume decrease response following hyposmotic stimulation compared with controls. These data confirm that ClC-3 is important in VRAC function and cell volume regulation, is associated with the I(Cl,LPA) current activity, and participates in the fibroblast-to-myofibroblast transition.     

Acid-sensing ion channel 2 (ASIC2) modulates ASIC1 H+-activated currents in hippocampal neurons

Hippocampal neurons express subunits of the acid-sensing ion channel (ASIC1 and ASIC2) and exhibit large cation currents that are transiently activated by acidic extracellular solutions. Earlier work indicated that ASIC1 contributed to the current in these neurons and suggested its importance for normal behavior. However, the specific contribution of ASIC1 and ASIC2 subunits to acid-evoked currents in hippocampal neurons remained uncertain. To decipher the individual role of the ASIC subunits, we studied H(+)-gated currents in neurons from both ASIC1 and ASIC2 null mice. We found that much of the current was produced by ASIC1a/2a heteromultimeric channels, and individual subunits made distinct contributions. The ASIC1a subunit was key in establishing current amplitude. The ASIC2a subunit had little effect on amplitude but influenced desensitization, recovery from desensitization, pH sensitivity, and the response to modulatory agents. We also found heterogeneity in the contribution of ASIC2 throughout the neuronal population, with individual neurons expressing both ASIC1a homomultimeric and ASIC1a/2a heteromultimeric channels. Studies of neurons heterozygous for disrupted ASIC alleles indicated that the properties of H(+)-gated currents are dependent on the proportion of the individual subunits. These findings indicate that the absolute and relative amounts of ASIC subunits determine the amplitude and properties of hippocampal H(+)-gated currents and therefore may contribute to normal physiology and pathophysiology.     

Extracellular matrix proteins: A positive feedback loop in lung fibrosis?

Lung fibrosis is characterized by excessive deposition of extracellular matrix. This not only affects tissue architecture and function, but it also influences fibroblast behavior and thus disease progression. Here we describe the expression of elastin, type V collagen and tenascin C during the development of bleomycin-induced lung fibrosis. We further report in vitro experiments clarifying both the effect of myofibroblast differentiation on this expression and the effect of extracellular elastin on myofibroblast differentiation.Lung fibrosis was induced in female C57Bl/6 mice by bleomycin instillation. Animals were sacrificed at zero to five weeks after fibrosis induction. Collagen synthesized during the week prior to sacrifice was labeled with deuterium. After sacrifice, lung tissue was collected for determination of new collagen formation, microarray analysis, and histology. Human lung fibroblasts were grown on tissue culture plastic or BioFlex culture plates coated with type I collagen or elastin, and stimulated to undergo myofibroblast differentiation by 0–10 ng/ml transforming growth factor (TGF)β₁. mRNA expression was analyzed by quantitative real-time PCR.New collagen formation during bleomycin-induced fibrosis was highly correlated to gene expression of elastin, type V collagen and tenascin C. At the protein level, elastin, type V collagen and tenascin C were highly expressed in fibrotic areas as seen in histological sections of the lung. Type V collagen and tenascin C were transiently increased. Human lung fibroblasts stimulated with TGFβ₁ strongly increased gene expression of elastin, type V collagen and tenascin C. The extracellular presence of elastin increased gene expression of the myofibroblastic markers α smooth muscle actin and type I collagen.The extracellular matrix composition changes dramatically during the development of lung fibrosis. The increased levels of elastin, type V collagen and tenascin C are probably the result of increased expression by fibroblastic cells; reversely, elastin influences myofibroblast differentiation. This suggests a reciprocal interaction between fibroblasts and the extracellular matrix composition that could enhance the development of lung fibrosis.     

Urokinase receptor cleavage: a crucial step in fibroblast-to-myofibroblast differentiation

Fibroblasts migrate into and repopulate connective tissue wounds. At the wound edge, fibroblasts differentiate into myofibroblasts, and they promote wound closure. Regulated fibroblast-to-myofibroblast differentiation is critical for regenerative healing. Previous studies have focused on the role in fibroblasts of urokinase plasmingen activator/urokinase plasmingen activator receptor (uPA/uPAR), an extracellular protease system that promotes matrix remodeling, growth factor activation, and cell migration. Whereas fibroblasts have substantial uPA activity and uPAR expression, we discovered that cultured myofibroblasts eventually lost cell surface uPA/uPAR. This led us to investigate the relevance of uPA/uPAR activity to myofibroblast differentiation. We found that fibroblasts expressed increased amounts of full-length cell surface uPAR (D1D2D3) compared with myofibroblasts, which had reduced expression of D1D2D3 but increased expression of the truncated form of uPAR (D2D3) on their cell surface. Retaining full-length uPAR was found to be essential for regulating myofibroblast differentiation, because 1) protease inhibitors that prevented uPAR cleavage also prevented myofibroblast differentiation, and 2) overexpression of cDNA for a noncleavable form of uPAR inhibited myofibroblast differentiation. These data support a novel hypothesis that maintaining full-length uPAR on the cell surface regulates the fibroblast to myofibroblast transition and that down-regulation of uPAR is necessary for myofibroblast differentiation.     

ROS signaling by NOX4 drives fibroblast-to-myofibroblast differentiation in the diseased prostatic stroma

Stromal remodeling, in particular fibroblast-to-myofibroblast differentiation, is a hallmark of benign prostatic hyperplasia (BPH) and solid tumors, including prostate cancer (PCa). Increased local production of TGFβ1 is considered the inducing stimulus. Given that stromal remodeling actively promotes BPH/PCa development, there is considerable interest in developing stromal-targeted therapies. Microarray and quantitative PCR analysis of primary human prostatic stromal cells induced to undergo fibroblast-to-myofibroblast differentiation with TGFβ1 revealed up-regulation of the reactive oxygen species (ROS) producer reduced nicotinamide adenine dinucleotide phosphate oxidase 4 (NOX4) and down-regulation of the selenium-containing ROS-scavenging enzymes glutathione peroxidase 3, thioredoxin reductase 1 (TXNRD1), and the selenium transporter selenoprotein P plasma 1. Consistently, NOX4 expression correlated specifically with the myofibroblast phenotype in vivo, and loss of selenoprotein P plasma 1 was observed in tumor-associated stroma of human PCa biopsies. Using lentiviral NOX4 short hairpin RNA-mediated knockdown, pharmacological inhibitors, antioxidants, and selenium, we demonstrate that TGFβ1 induction of NOX4-derived ROS is required for TGFβ1-mediated phosphorylation of c-jun N-terminal kinase, which in turn is essential for subsequent downstream cytoskeletal remodeling. Significantly, selenium supplementation inhibited differentiation by increasing ROS-scavenging selenoenzyme biosynthesis because glutathione peroxidase 3 and TXNRD1 expression and TXNRD1 enzyme activity were restored. Consistently, selenium depleted ROS levels downstream of NOX4 induction. Collectively, this work demonstrates that dysregulated redox homeostasis driven by elevated NOX4-derived ROS signaling underlies fibroblast-to-myofibroblast differentiation in the diseased prostatic stroma. Further, these data indicate the potential clinical value of selenium and/or NOX4 inhibitors in preventing the functional pathogenic changes of stromal cells in BPH and PCa.     

Local ASIC3 modulates pain and disease progression in a rat model of osteoarthritis

Background

Recent data have suggested a relationship between acute arthritic pain and acid sensing ion channel 3 (ASIC3) on primary afferent fibers innervating joints. The purpose of this study was to clarify the role of ASIC3 in a rat model of osteoarthritis (OA) which is considered a degenerative rather than an inflammatory disease.

Methods

We induced OA via intra-articular mono-iodoacetate (MIA) injection, and evaluated pain-related behaviors including weight bearing measured with an incapacitance tester and paw withdrawal threshold in a von Frey hair test, histology of affected knee joint, and immunohistochemistry of knee joint afferents. We also assessed the effect of ASIC3 selective peptide blocker (APETx2) on pain behavior, disease progression, and ASIC3 expression in knee joint afferents.

Results

OA rats showed not only weight-bearing pain but also mechanical hyperalgesia outside the knee joint (secondary hyperalgesia). ASIC3 expression in knee joint afferents was significantly upregulated approximately twofold at Day 14. Continuous intra-articular injections of APETx2 inhibited weight distribution asymmetry and secondary hyperalgesia by attenuating ASIC3 upregulation in knee joint afferents. Histology of ipsilateral knee joint showed APETx2 worked chondroprotectively if administered in the early, but not late phase.

Conclusions

Local ASIC3 immunoreactive nerve is strongly associated with weight-bearing pain and secondary hyperalgesia in MIA-induced OA model. APETx2 inhibited ASIC3 upregulation in knee joint afferents regardless of the time-point of administration. Furthermore, early administration of APETx2 prevented cartilage damage. APETx2 is a novel, promising drug for OA by relieving pain and inhibiting disease progression.     

Cdkn1c drives muscle differentiation through a positive feedback loop with Myod

Differentiation often requires conversion of analogue signals to a stable binary output through positive feedback. Hedgehog (Hh) signalling promotes myogenesis in the vertebrate somite, in part by raising the activity of muscle regulatory factors (MRFs) of the Myod family above a threshold. Hh is known to enhance MRF expression. Here we show that Hh is also essential at a second step that increases Myod protein activity, permitting it to promote Myogenin expression. Hh acts by inducing expression of cdkn1c (p57^Kip2) in slow muscle precursor cells, but neither Hh nor Cdkn1c is required for their cell cycle exit. Cdkn1c co-operates with Myod to drive differentiation of several early zebrafish muscle fibre types. Myod in turn up-regulates cdkn1c, thereby providing a positive feedback loop that switches myogenic cells to terminal differentiation.     

TRB3 modulates C2C12 differentiation by interfering with Akt activation

TRB3 is a member of TRB protein family characterized by containing a variant kinase domain without enzymatic activity. Interacting with Ser/Thr protein kinases Akt, TRB3 impairs Akt activation induced by growth factors and insulin. In this study we have examined the potential role of TRB3 in muscle differentiation. Our data indicated that the expression of TRB3 is downregulated during C2C12 cells undergoing muscle differentiation and that overexpression of TRB3 inhibits Akt activation during differentiation. Correspondingly, overexpression of TRB3 inhibits, while knockdown TRB3 enhances C2C12 differentiation. Thus, our studies indicated that TRB3 plays a critical role in muscle differentiation.     

ROS-triggered caspase 2 activation and feedback amplification loop in beta-carotene-induced apoptosis

Reactive oxygen species (ROS) and caspases 8, 9, and 3 are reported to be crucial players in apoptosis induced by various stimuli. Recently, caspase 2 has been implicated in stress-induced apoptosis but the exact mechanism remains unclear. In this study, we report that ROS generation led to activation of caspase 2 during beta-carotene-induced apoptosis in the human leukemic T cell line Molt 4. The apoptosis progressed by simultaneous activation of caspases 8 and 9, and a cross talk between these initiator caspases was mediated by the proapoptotic protein Bid. Inhibition of caspases 2, 8, 9, and 3 independently suppressed the caspase cascade. The kinetics and function of caspase 2 were similar to those of caspase 3, suggesting its role as an effector caspase. Interestingly, beta-carotene-induced apoptosis was caspase 2 dependent but caspase 3 independent. The study also revealed cleavage of the antiapoptotic protein BclXL as an important event during apoptosis, which was regulated by ROS. The mechanistic studies identify a functional link between ROS and the caspase cascade involving caspase 2 and cleavage of BclXL. The interdependence of caspases 8, 9, 2, and 3 in the cascade provides evidence for the presence of an extensive feedback amplification loop in beta-carotene-induced apoptosis in Molt 4 cells.     

RELA/NEAT1/miR-302a-3p/RELA feedback loop modulates pancreatic ductal adenocarcinoma cell proliferation and migration

Pancreatic ductal adenocarcinoma (PDAC) remains a challenging malignancy due to distant metastasis. RELA, a major component of the NF-κB pathway, could serve as an oncogene through activating proliferation or migration-related gene expression, including NEAT1, a well-known oncogenic long noncoding RNA. In the current study, the expression and function of RELA and NEAT1 in PDAC were examined. The potential upstream regulatory microRNAs of RELA were screened and verified for their correlation with RELA and NEAT1. The expression and function of the selected miR-302a-3p were evaluated. RELA and NEAT1 expression were upregulated in PDAC tissues, particularly in PDAC tissues with lymph node metastasis, and their expression correlated with clinical parameters. RELA overexpression promoted PDAC cell proliferation and migration, which could be partially attenuated by the NEAT1 knockdown. By binding to RELA, miR-302a-3p inhibited RELA expression, as well as PDAC cell proliferation and migration. RELA downstream NEAT1 expression was negatively regulated by miR-302a-3p; the suppressive effect of NEAT1 knockdown on PDAC cell proliferation and migration was partially attenuated by miR-302a-3p inhibition. Moreover, through direct binding, the expression of miR-302a-3p was also negatively regulated by NEAT1. The expression of miR-302a-3p was downregulated and negatively correlated with RELA or NEAT1 in tissue samples, indicating that rescuing miR-302a-3p expression may inhibit PDAC cell proliferation and migration through RELA/NEAT1. In summary, RELA, NEAT1, and miR-302a-3p form a feedback loop in PDAC to modulate PDAC cell proliferation and migration.     

Pde1 phosphodiesterase modulates cyclic AMP levels through a protein kinase A-mediated negative feedback loop in Cryptococcus neoformans

The virulence of the human pathogenic fungus Cryptococcus neoformans is regulated by a cyclic AMP (cAMP)-dependent protein kinase A (PKA) signaling cascade that promotes mating and the production of melanin and capsule. In this study, genes encoding homologs of the Saccharomyces cerevisiae low- and high-affinity phosphodiesterases, PDE1 and PDE2, respectively, were deleted in serotype A strains of C. neoformans. The resulting mutants exhibited moderately elevated levels of melanin and capsule production relative to the wild type. Epistasis experiments indicate that Pde1 functions downstream of the Galpha subunit Gpa1, which initiates cAMP-dependent signaling in response to an extracellular signal. Previous work has shown that the PKA catalytic subunit Pka1 governs cAMP levels via a negative feedback loop. Here we show that a pde1Delta pka1Delta mutant strain exhibits cAMP levels that are dramatically increased ( approximately 15-fold) relative to those in a pka1Delta single mutant strain and that a site-directed mutation in a consensus PKA phosphorylation site reduces Pde1 function. These data provide evidence that fluctuations in cAMP levels are modulated by both Pka1-dependent regulation of Pde1 and another target that comprise a robust negative feedback loop to tightly constrain intracellular cAMP levels.     

The Per2 negative feedback loop sets the period in the mammalian circadian clock mechanism

Processes that repeat in time, such as the cell cycle, the circadian rhythm, and seasonal variations, are prevalent in biology. Mathematical models can represent our knowledge of the underlying mechanisms, and numerical methods can then facilitate analysis, which forms the foundation for a more integrated understanding as well as for design and intervention. Here, the intracellular molecular network responsible for the mammalian circadian clock system was studied. A new formulation of detailed sensitivity analysis is introduced and applied to elucidate the influence of individual rate processes, represented through their parameters, on network functional characteristics. One of four negative feedback loops in the model, the Per2 loop, was uniquely identified as most responsible for setting the period of oscillation; none of the other feedback loops were found to play as substantial a role. The analysis further suggested that the activity of the kinases CK1δ and CK1 were well placed within the network such that they could be instrumental in implementing short-term adjustments to the period in the circadian clock system. The numerical results reported here are supported by previously published experimental data.     

The cell substratum modulates skeletal muscle differentiation

During chick embryogenesis, massive alterations occur in the migrating cell's substratum, or extracellular matrix. The possibility that some of the components of this milieu play a regulatory role in cell differentiation was explored in a cell-culture system derived from embryonic chick skeletal muscle tissue. In particular, the effects of collagen and the glycosaminoglycans were studied. Collagen is required for muscle cell attachment and spreading onto plastic and glass tissue-culture dishes. A major constituent of the early embryonic extracellular space, hyaluronate (HA), while having no significant effect on collagen-stimulated cell attachment and spreading, was found to inhibit myogenesis. The muscle-specific M subunit of creatine kinase was preferentially inhibited. Control experiments indicated that the inhibition was specifically caused by HA and not by other glycosaminoglycans. A general metabolic inhibition of the cultures was not observed. Muscle cells could bind to HA-coated beads at all stages of differentiation but were inhibited only when HA was added within the first 24 h of culture. Endogenous GAG in the culture is normally degraded during the first 24 h after plating as well; this may parallel the massive degradation of HA that occurs in the early embryo in vivo. These findings suggest a regulatory role for HA in modulating skeletal muscle differentiation, with degradation of an inhibitory component of the cell substratum a requirement for myogenesis.