Direct Binding of the EGF-like Domain of Neuregulin-1 to Integrins (αvβ3 and α6β4) Is Involved in Neuregulin-1/ErbB Signaling

Integrin-growth factor receptor cross-talk plays a role in growth factor signaling, but the specifics are unclear. In a current model, integrins and growth factor receptors independently bind to their ligands (extracellular matrix and growth factors, respectively). We discovered that neuregulin-1 (NRG1), either as an isolated EGF-like domain or as a native multi-domain form, binds to integrins αvβ3 (with a K_D of 1.36 × 10⁻⁷ m) and α6β4. Docking simulation predicted that three Lys residues at positions 180, 184, and 186 of the EGF-like domain are involved in integrin binding. Mutating these residues to Glu individually or in combination markedly suppressed integrin binding and ErbB3 phosphorylation. Mutating all three Lys residues to Glu (the 3KE mutation) did not affect the ability of NRG1 to bind to ErbB3 but markedly reduced the ability of NRG1 to induce ErbB3 phosphorylation and AKT and Erk1/2 activation in MCF-7 and T47D human breast cancer cells. This suggests that direct integrin binding to NRG1 is critical for NRG1/ErbB signaling. Notably, stimulation of cells with WT NRG1 induced co-precipitation of ErbB3 with α6β4 and with αvβ3 to a much lower extent. This suggests that WT NRG1 induces integrin-NRG1-ErbB3 ternary complex formation. In contrast, the 3KE mutant was much less effective in inducing ternary complex formation than WT NRG1, suggesting that this process depends on the ability of NRG1 to bind to integrins. These results suggest that direct NRG1-integrin interaction mediates integrin-ErbB cross-talk and that α6β4 plays a major role in NRG-ErbB signaling in these cancer cells.     

Pertussis toxin-sensitive modulation of glutamate transport by endothelin-1 type A receptors in glioma cells

Endothelin-1 (ET-1) is a 21 amino acids peptide that exerts several biological activities through interaction with specific G-protein coupled receptors. Increased ET-1 expression is frequently associated with pathological situations involving alterations in glutamate levels. In the present study, a brief exposure to ET-1 was found to increase aspartate uptake in C6 glioma cells, which endogenously express the neuronal glutamate transporter EAAC1 (pEC₅₀ of 9.89). The stimulatory effect of ET-1 mediated by ET_A receptors corresponds to a 62% increase in the V_max with no modification of the affinity for the substrate. While protein kinase C activity is known to participate in the regulation of EAAC1, the effect of ET-1 on the glutamate uptake was found to be independent of this kinase activation. In contrast, the inactivation of G_o/i type G-protein dependent signaling with pertussis toxin was found to impair ET-1-mediated regulation of EAAC1. An examination of the cell surface expression of EAAC1 by protein biotinylation studies or by confocal analysis of immuno-fluorescence staining demonstrated that ET-1 stimulates EAAC1 translocation to the cell surface. Hence, the disruption of the cytoskeleton with cytochalasin D prevented ET-1-stimulated aspartate uptake. Together, the data presented in the current study suggest that ET-1 participates in the acute regulation of glutamate transport in glioma cells. Considering the documented role of glutamate excitotoxicity in the development of brain tumors, endothelinergic system constitutes a putative target for the pharmacological control of glutamate transmission at the vicinity of glioma cells.     

Genetic deletion of the neuronal glutamate transporter,EAAC1, results in decreased neuronal death after pilocarpine-induced status epilepticus

Excitatory amino acid carrier 1 (EAAC1 also called EAAT3) is a Na⁺-dependent glutamate transporter expressed by both glutamatergic and GABAergic neurons. It provides precursors for the syntheses of glutathione and GABA and contributes to the clearance of synaptically released glutamate. Mice deleted of EAAC1 are more susceptible to neurodegeneration in models of ischemia, Parkinson’s disease, and aging. Antisense knock-down of EAAC1 causes an absence seizure-like phenotype. Additionally, EAAC1 expression increases after chemonvulsant-induced seizures in rodent models and in tissue specimens from patients with refractory epilepsy. The goal of the present study was to determine if the absence of EAAC1 affects the sensitivity of mice to seizure-induced cell death. A chemoconvulsant dose of pilocarpine was administered to EAAC1^−/− mice and to wild-type controls. Although EAAC1^−/− mice experienced increased latency to seizure onset, no significant differences in behavioral seizure severity or mortality were observed. We examined EAAC1 immunofluorescence 24 h after pilocarpine administration and confirmed that pilocarpine causes an increase in EAAC1 protein. Forty-eight hours after induction of seizures, cell death was measured in hippocampus and in cortex using Fluoro-Jade C. Surprisingly, there was ∼2-fold more cell death in area CA1 of wild-type mice than in the corresponding regions of the EAAC1^−/− mice. Together, these studies indicate that absence of EAAC1 results in either a decrease in pilocarpine-induced seizures that is not detectable by behavioral criteria (surprising, since EAAC1 provides glutamate for GABA synthesis), or that the absence of EAAC1 results in less pilocarpine/seizure-induced cell death, possible explanations as discussed.     

Regulation of neuregulin signaling by PSD-95 interacting with ErbB4 at CNS synapses

Neuregulins (NRGs) and their receptors, the ErbB protein tyrosine kinases, are essential for neuronal development, but their functions in the adult CNS are unknown. We report that ErbB4 is enriched in the postsynaptic density (PSD) and associates with PSD-95. Heterologous expression of PSD-95 enhanced NRG activation of ErbB4 and MAP kinase. Conversely, inhibiting expression of PSD-95 in neurons attenuated NRG-mediated activation of MAP kinase. PSD-95 formed a ternary complex with two molecules of ErbB4, suggesting that PSD-95 facilitates ErbB4 dimerization. Finally, NRG suppressed induction of long-term potentiation in the hippocampal CA1 region without affecting basal synaptic transmission. Thus, NRG signaling may be synaptic and regulated by PSD-95. A role of NRG signaling in the adult CNS may be modulation of synaptic plasticity.     

Glial soluble factors regulate the activity and expression of the neuronal glutamate transporter EAAC1: implication of cholesterol

A co-ordinated regulation between neurons and astrocytes is essential for the control of extracellular glutamate concentration. Here, we have investigated the influence of astrocytes and glia-derived cholesterol on the regulation of glutamate transport in primary neuronal cultures from rat embryonic cortices. Glutamate uptake rate and expression of the neuronal glutamate transporter EAAC1 were low when neurons were grown without astrocytes and neurons were unable to clear extracellular glutamate. Treatment of the neuronal cultures with glial conditioned medium (GCM) increased glutamate uptake Vmax, EAAC1 expression and restored the capacity of neurons to eliminate extracellular glutamate. Thus, astrocytes up-regulate the activity and expression of EAAC1 in neurons. We further showed that cholesterol, present in GCM, increased glutamate uptake activity when added directly to neurons and had no effect on glutamate transporter expression. Furthermore, part of the GCM-induced effect on glutamate transport activity was lost when cholesterol was removed from GCM (low cholesterol-GCM) and was restored when cholesterol was added to low cholesterol-GCM. This demonstrates that glia-derived cholesterol regulates glutamate transport activity. With these experiments, we provide new evidences for neuronal glutamate transport regulation by astrocytes and identified cholesterol as one of the factors implicated in this regulation.     

Heat Shock Factor Hsf1 Cooperates with ErbB2 (Her2/Neu) Protein to Promote Mammary Tumorigenesis and Metastasis

ErbB2/Neu oncogene is overexpressed in 25% of invasive/metastatic breast cancers. We have found that deletion of heat shock factor Hsf1 in mice overexpressing ErbB2/Neu significantly reduces mammary tumorigenesis and metastasis. Hsf1^+/−ErbB2/Neu⁺ tumors exhibit reduced cellular proliferative and invasive properties associated with reduced activated ERK1/2 and reduced epithelial-mesenchymal transition (EMT). Hsf1^+/+Neu⁺ mammary epithelial cells exposed to TGFβ show high levels of ERK1/2 activity and EMT; this is associated with reduced expression of E-cadherin and increased expression of Slug and vimentin, a mesenchymal marker. In contrast, Hsf1^−/−Neu⁺ or Hsf1^+/+Neu⁺ cells do not exhibit activated ERK1/2 and show reduced EMT in the presence of TGFβ. The ineffective activation of the RAS/RAF/MEK/ERK1/2 signaling pathway in cells with reduced levels of HSF1 is due to the low levels of HSP90 in complex with RAF1 that are required for RAF1 stability and maturation. These results indicate a powerful inhibitory effect conferred by HSF1 downstream target genes in the inhibition of ErbB2-induced breast cancers in the absence of the Hsf1 gene.     

Kinin B1 Receptor in Adipocytes Regulates Glucose Tolerance and Predisposition to Obesity

Background

Kinins participate in the pathophysiology of obesity and type 2 diabetes by mechanisms which are not fully understood. Kinin B₁ receptor knockout mice (B₁ ^−/−) are leaner and exhibit improved insulin sensitivity.

Methodology/Principal Findings

Here we show that kinin B₁ receptors in adipocytes play a role in controlling whole body insulin action and glucose homeostasis. Adipocytes isolated from mouse white adipose tissue (WAT) constitutively express kinin B₁ receptors. In these cells, treatment with the B₁ receptor agonist des-Arg⁹-bradykinin improved insulin signaling, GLUT4 translocation, and glucose uptake. Adipocytes from B₁ ^−/− mice showed reduced GLUT4 expression and impaired glucose uptake at both basal and insulin-stimulated states. To investigate the consequences of these phenomena to whole body metabolism, we generated mice where the expression of the kinin B₁ receptor was limited to cells of the adipose tissue (aP2-B₁/B₁ ^−/−). Similarly to B₁ ^−/− mice, aP2-B₁/B₁ ^−/− mice were leaner than wild type controls. However, exclusive expression of the kinin B₁ receptor in adipose tissue completely rescued the improved systemic insulin sensitivity phenotype of B₁ ^−/− mice. Adipose tissue gene expression analysis also revealed that genes involved in insulin signaling were significantly affected by the presence of the kinin B₁ receptor in adipose tissue. In agreement, GLUT4 expression and glucose uptake were increased in fat tissue of aP2-B₁/B₁ ^−/− when compared to B₁ ^−/− mice. When subjected to high fat diet, aP2-B₁/B₁ ^−/− mice gained more weight than B₁ ^−/− littermates, becoming as obese as the wild types.

Conclusions/Significance

Thus, kinin B₁ receptor participates in the modulation of insulin action in adipocytes, contributing to systemic insulin sensitivity and predisposition to obesity.     

Spine impairment in mice high-expressing neuregulin 1 due to LIMK1 activation

The genes encoding for neuregulin1 (NRG1), a growth factor, and its receptor ErbB4 are both risk factors of major depression disorder and schizophrenia (SZ). They have been implicated in neural development and synaptic plasticity. However, exactly how NRG1 variations lead to SZ remains unclear. Indeed, NRG1 levels are increased in postmortem brain tissues of patients with brain disorders. Here, we studied the effects of high-level NRG1 on dendritic spine development and function. We showed that spine density in the prefrontal cortex and hippocampus was reduced in mice (ctoNrg1) that overexpressed NRG1 in neurons. The frequency of miniature excitatory postsynaptic currents (mEPSCs) was reduced in both brain regions of ctoNrg1 mice. High expression of NRG1 activated LIMK1 and increased cofilin phosphorylation in postsynaptic densities. Spine reduction was attenuated by inhibiting LIMK1 or blocking the NRG1–LIMK1 interaction, or by restoring NRG1 protein level. These results indicate that a normal NRG1 protein level is necessary for spine homeostasis and suggest a pathophysiological mechanism of abnormal spines in relevant brain disorders.Subject terms: Molecular neuroscience, Schizophrenia     

Sub-chronic Antipsychotic Drug Administration Reverses the Expression of Neuregulin 1 and ErbB4 in a Cultured MK801-Induced Mouse Primary Hippocampal Neuron or a Neurodevelopmental Schizophrenia Model

It has been reported that specific environmental influences during the postpartum period might contribute to the development of schizophrenia (SZ). Administration of MK801 during early development led to persistent brain pathology. Glutamate decarboxylase 1 (GAD67) and parvalbumin (PV), and neuregulin 1 (NRG1)/ErbB4 signaling were closely associated with SZ pathology. We postulated therefore that NMDA receptor antagonists exposure during the postpartum period may be associated with expression dysregulation of some of the SZ candidate proteins. To test this, we used mouse primary hippocampal neurons and neonatal male mice treated with the NMDA receptor antagonist, MK801 at postnatal day 4 (P4) or P7, followed by the treatments of antipsychotic drugs (i.e., olanzapine, risperidone, and haloperidol). The expressions of GAD67, PV, NRG1, and ErbB4 in in vitro and in vivo SZ models were detected with Western blot analysis and immunohistochemistry, respectively. Behavioral tests (locomotion activity, social interaction, novel object recognition and prepulse inhibition) were measured. We found MK801 decreased the expression of GAD67, PV, NRG1 and ErbB4, and induced obvious behavioral alterations, while antipsychotics reversed these alterations. These results suggest that exposure to the NMDA receptor antagonist in early development may lead to long-lasting influence on the expression of specific proteins, such as GAD67, PV, NRG1, and ErbB4. Moreover, our results suggest that rescue of the activation of the NRG1/ErbB4 signaling pathway may be one of the mechanisms by which antipsychotic drugs have an antipsychotic effect.     

Schizophrenia susceptibility pathway neuregulin 1-ErbB4 suppresses Src upregulation of NMDA receptors

Hypofunction of the N-methyl D-aspartate subtype of glutamate receptor (NMDAR) is hypothesized to be a mechanism underlying cognitive dysfunction in individuals with schizophrenia. For the schizophrenia-linked genes NRG1 and ERBB4, NMDAR hypofunction is thus considered a key detrimental consequence of the excessive NRG1-ErbB4 signaling found in people with schizophrenia. However, we show here that neuregulin 1β-ErbB4 (NRG1β-ErbB4) signaling does not cause general hypofunction of NMDARs. Rather, we find that, in the hippocampus and prefrontal cortex, NRG1β-ErbB4 signaling suppresses the enhancement of synaptic NMDAR currents by the nonreceptor tyrosine kinase Src. NRG1β-ErbB4 signaling prevented induction of long-term potentiation at hippocampal Schaffer collateral-CA1 synapses and suppressed Src-dependent enhancement of NMDAR responses during theta-burst stimulation. Moreover, NRG1β-ErbB4 signaling prevented theta burst-induced phosphorylation of GluN2B by inhibiting Src kinase activity. We propose that NRG1-ErbB4 signaling participates in cognitive dysfunction in schizophrenia by aberrantly suppressing Src-mediated enhancement of synaptic NMDAR function.     

Glutamate transporter expression and function in a striatal neuronal model of Huntington’s disease

Excitotoxicity may contribute to the pathogenesis of Huntington’s disease. High affinity Na⁺ dependent glutamate transporters, residing in the plasma membrane, clear glutamate from the extracellular space and are the primary means of protection against excitotoxicity. Many reports suggest that Huntington’s disease is associated with a decrease in the expression and function of glutamate transporters. We studied the expression and function of these transporters in a cellular model of Huntington’s disease, STHdh^Q111/Q111 and STHdh^Q7/Q7 cells. We found that only GLT-1b and EAAC1 were expressed in these cell lines and only EAAC1 significantly contributed to the glutamate uptake. Surprisingly, there was an increase in Na⁺-dependent glutamate uptake in STHdh^Q111/Q111 cells accompanied by an increase in surface expression of EAAC1. We studied the influence of the Akt pathway on EAAC1 mediated uptake, since EAAC1 surface expression is influenced by Akt and previous studies have shown increased Akt expression in STHdh^Q111/Q111 cells. Glutamate uptake was inhibited by Akt pathway inhibitors in both the STHdh^Q7/Q7 and the STHdh^Q111/Q111 cell lines. We found no difference in Akt activation between the two cell lines under our conditions of culture. Therefore a difference in Akt activation does not seem to explain the increase in EAAC1 mediated uptake in the STHdh^Q111/Q111 cells.     

EAAC1 gene deletion alters zinc homeostasis and enhances cortical neuronal injury after transient cerebral ischemia in mice

The excitatory amino acids glutamate and cysteine are actively transported into neurons from the extracellular space by the high affinity glutamate transporter EAAC1. The astrocyte glutamate transporters, GLT1 and GLAST, are the primary mediators of glutamate clearance. EAAC1 has a limited role in this function. However, uptake of cysteine into neurons via EAAC1 contributes to neuronal antioxidant function by providing cysteine substrate for glutathione synthesis. Mice in which the EAAC1 gene has been deleted were seen to have enhanced susceptibility to neuronal oxidative stress and developed brain atrophy and cognitive function decline with aging. The aim of the current study was to evaluate if EAAC1 confers protection against ischemic events. Young adult CD-1 wild-type or EAAC1(-/-) mice were subjected to 30 min of bilateral common carotid artery occlusion and evaluated for neuronal death and zinc translocation. The intensity of TSQ fluorescence in the cytoplasm of cortical neurons in the EAAC1(-/-) mice was significantly higher than wild-type mice, indicating that the cortical neurons of EAAC1(-/-) mice contain higher cytoplasmic concentrations of labile (or free) zinc. Zinc translocation into cortical neurons was also enhanced in EAAC1(-/-) mice. Three days after ischemia, Fluoro-Jade B staining revealed that EAAC1(-/-) mice had more than twice as many degenerating neurons as wild-type mice. N-acetylcysteine, a membrane-permeant cysteine pro-drug, normalized basal zinc levels, reduced TSQ (+) neurons and reduced ischemic neuronal death in the EAAC1(-/-) mice when delivered in a pre-treatment fashion. Taken together, this study implicates EAAC1-dependent cysteine uptake as an endogenous source of enhancing antioxidant function and zinc homeostasis in neurons in the ischemic brain.     

Caveolin‐1 negatively regulates inflammation and fibrosis in silicosis

Inhalation of crystalline silica causes silicosis, the most common and serious occupational disease, which is characterized by progressive lung inflammation and fibrosis. Recent studies revealed the anti‐inflammatory and anti‐fibrosis role of Caveolin‐1 (Cav‐1) in lung, but this role in silicosis has not been investigated. Thus, this study evaluated Cav‐1 regulatory effects in silicosis. It was found that Cav‐1 levels were significantly reduced in the lung from silicosis patients and silicotic mice. The silicosis models were established in C57BL/6 (wild‐type) and Cav‐1 deficiency (Cav‐1 ^−/−) mice, and Cav‐1 ^−/− mice displayed wider alveolar septa, increased collagen deposition and more silicotic nodules. The mice peritoneal‐derived macrophages were used to explore the role of Cav‐1 in silica‐induced inflammation, which plays a central role in mechanism of silicosis. Cav‐1 inhibited silica‐induced infiltration of inflammatory cells and secretion of inflammatory factors in vitro and in vivo, partly by downregulating NF‐κB pathway. Additionally, silica uptake and expression of 4‐hydroxynonenal in silicotic mice were observed, and it was found that Cav‐1 absence triggered excessive silica deposition, causing a stronger oxidative stress response. These findings demonstrate the protective effects of Cav‐1 in silica‐induced lung injury, suggesting its potential therapeutic value in silicosis.     

Presenilin-1 processing of ErbB4 in fetal type II cells is necessary for control of fetal lung maturation

Maturation of pulmonary fetal type II cells to initiate adequate surfactant production is crucial for postnatal respiratory function. Little is known about specific mechanisms of signal transduction controlling type II cell maturation. The ErbB4 receptor and its ligand neuregulin (NRG) are critical for lung development. ErbB4 is cleaved at the cell membrane by the γ-secretase enzyme complex whose active component is either presenilin-1 (PSEN-1) or presenilin-2. ErbB4 cleavage releases the 80 kDa intracellular domain (4ICD), which associates with chaperone proteins such as YAP (Yes-associated protein) and translocates to the nucleus to regulate gene expression. We hypothesized that PSEN-1 and YAP have a development-specific expression in fetal type II cells and are important for ErbB4 signaling in surfactant production. In primary fetal mouse E16, E17, and E18 type II cells, PSEN-1 and YAP expression increased at E17 and E18 over E16. Subcellular fractionation showed a strong cytosolic and a weaker membrane location of both PSEN-1 and YAP. This was enhanced by NRG stimulation. Co-immunoprecipitations showed ErbB4 associated separately with PSEN-1 and with YAP. Their association, phosphorylation, and co-localization were induced by NRG. Confocal immunofluorescence and nuclear fractionation confirmed these associations in a time-dependent manner after NRG stimulation. Primary ErbB4-deleted E17 type II cells were transfected with a mutant ErbB4 lacking the γ-secretase binding site. When compared to transfection with wild-type ErbB4, the stimulatory effect of NRG on surfactant protein mRNA expression was lost. We conclude that PSEN-1 and YAP have crucial roles in ErbB4 signal transduction during type II cell maturation.     

Stimulated ErbB4 internalization is necessary for neuregulin signaling in neurons

Neuregulin-1 (NRG1) plays an important role in neural development, synapse formation, and synaptic plasticity by activating ErbB receptor tyrosine kinases. Although ligand-induced endocytosis has been shown to be important for many receptor tyrosine kinases, whether NRG1 signaling depends on ErbB endocytosis remains controversial. Here, we provide evidence that ErbB4, a prominent ErbB protein in the brain, becomes internalized in NRG1-stimulated neurons. The induced ErbB4 endocytosis requires its kinase activity. Remarkably, inhibition of ErbB endocytosis attenuates NRG1-induced activation of Erk and Akt in neurons. These observations indicate a role of ErbB endocytosis in NRG1 signaling in neurons.     

PTPN21 exerts pro-neuronal survival and neuritic elongation via ErbB4/NRG3 signaling

Although expression quantitative trait locus, eQTL, serves as an explicit indicator of gene–gene associations, challenges remain to disentangle the mechanisms by which genetic variations alter gene expression. Here we combined eQTL and molecular analyses to identify an association between two seemingly non-associated genes in brain expression data from BXD inbred mice, namely Ptpn21 and Nrg3. Using biotinylated receptor tracking and immunoprecipitation analyses, we determined that PTPN21 de-phosphorylates the upstream receptor tyrosine kinase ErbB4 leading to the up-regulation of its downstream signaling. Conversely, kinase-dead ErbB4 (K751R) or phosphatase-dead PTPN21 (C1108S) mutants impede PTPN21-dependent signaling. Furthermore, PTPN21 also induced Elk-1 activation in embryonic cortical neurons and a novel Elk-1 binding motif was identified in a region located 1919 bp upstream of the NRG3 initiation codon. This enables PTPN21 to promote NRG3 expression through Elk-1, which provides a biochemical mechanism for the PTPN21–NRG3 association identified by eQTL. Biologically, PTPN21 positively influences cortical neuronal survival and, similar to Elk-1, it also enhances neuritic length. Our combined approaches show for the first time, a link between NRG3 and PTPN21 within a signaling cascade. This may explain why these two seemingly unrelated genes have previously been identified as risk genes for schizophrenia.