Serum neuregulin 1 in relation to ventricular function and subclinical atherosclerosis in type 2 diabetes patients

BACKGROUND AND AIMS

Neuregulin 1 (NRG-1) is one of the members of the epidermal growth factors proteins. The present study provides novel insights into the relationship between serum levels of NRG-1 and insulin resistance, subclinical atherosclerosis and cardiac dysfunction that occur in type 2 diabetes (T2D).

METHODS

The study included 50 patients with T2D and 40 healthy age- and gender-matched controls. Serum NRG-1 was measured using ELISA. Glycemic parameters, lipid profile and insulin resistance were assessed. Trans-thoracic echocardiography and carotid intima media thickness (CIMT) were studied for all study subjects.

RESULTS

T2D patients had significantly lower serum NRG-1 levels than controls. Serum NRG-1 was negatively correlated with age, fasting blood glucose, HbA1c, insulin resistance, blood urea, serum creatinine and LDL-C, and positively correlated with HDL-C, eGFR and CIMT. Regarding echocardiographic variables, serum NRG-1 was found to correlate positively with left ventricular global longitudinal strain and negatively with E/Ea ratio. NRG-1 was found to predict subclinical atherosclerosis in type 2 diabetes patients at a cut-off value<108.5pg/ml with 78% sensitivity and 80% specificity.

CONCLUSIONS

A robust relationship was found between serum NRG-1 levels and hyperglycemia, insulin resistance, subclinical atherosclerosis, and cardiac dysfunction in patients with type 2 diabetes. These results shed light on a possible role of NRG-1 as a potential noninvasive biomarker for detection of cardiometabolic risk in T2D.

Pathogen effector recognition-dependent association of NRG1 with EDS1 and SAG101 in TNL receptor immunity

Plants utilise intracellular nucleotide-binding, leucine-rich repeat (NLR) immune receptors to detect pathogen effectors and activate local and systemic defence. NRG1 and ADR1 "helper" NLRs (RNLs) cooperate with enhanced disease susceptibility 1 (EDS1), senescence-associated gene 101 (SAG101) and phytoalexin-deficient 4 (PAD4) lipase-like proteins to mediate signalling from TIR domain NLR receptors (TNLs). The mechanism of RNL/EDS1 family protein cooperation is not understood. Here, we present genetic and molecular evidence for exclusive EDS1/SAG101/NRG1 and EDS1/PAD4/ADR1 co-functions in TNL immunity. Using immunoprecipitation and mass spectrometry, we show effector recognition-dependent interaction of NRG1 with EDS1 and SAG101, but not PAD4. An EDS1-SAG101 complex interacts with NRG1, and EDS1-PAD4 with ADR1, in an immune-activated state. NRG1 requires an intact nucleotide-binding P-loop motif, and EDS1 a functional EP domain and its partner SAG101, for induced association and immunity. Thus, two distinct modules (NRG1/EDS1/SAG101 and ADR1/EDS1/PAD4) mediate TNL receptor defence signalling.

Pathogen effector recognition-dependent association of NRG1 with EDS1 and SAG101 in TNL receptor immunity

Plants utilise intracellular nucleotide-binding, leucine-rich repeat (NLR) immune receptors to detect pathogen effectors and activate local and systemic defence. NRG1 and ADR1 "helper" NLRs (RNLs) cooperate with enhanced disease susceptibility 1 (EDS1), senescence-associated gene 101 (SAG101) and phytoalexin-deficient 4 (PAD4) lipase-like proteins to mediate signalling from TIR domain NLR receptors (TNLs). The mechanism of RNL/EDS1 family protein cooperation is not understood. Here, we present genetic and molecular evidence for exclusive EDS1/SAG101/NRG1 and EDS1/PAD4/ADR1 co-functions in TNL immunity. Using immunoprecipitation and mass spectrometry, we show effector recognition-dependent interaction of NRG1 with EDS1 and SAG101, but not PAD4. An EDS1-SAG101 complex interacts with NRG1, and EDS1-PAD4 with ADR1, in an immune-activated state. NRG1 requires an intact nucleotide-binding P-loop motif, and EDS1 a functional EP domain and its partner SAG101, for induced association and immunity. Thus, two distinct modules (NRG1/EDS1/SAG101 and ADR1/EDS1/PAD4) mediate TNL receptor defence signalling.

NRG1 fusions in breast cancer

BACKGROUND

NRG1 gene fusions may be clinically actionable, since cancers carrying the fusion transcripts can be sensitive to tyrosine kinase inhibitors. The NRG1 gene encodes ligands for the HER2(ERBB2)-ERBB3 heterodimeric receptor tyrosine kinase, and the gene fusions are thought to lead to autocrine stimulation of the receptor. The NRG1 fusion expressed in the breast cancer cell line MDA-MB-175 serves as a model example of such fusions, showing the proposed autocrine loop and exceptional drug sensitivity. However, its structure has not been properly characterised, its oncogenic activity has not been fully explained, and there is limited data on such fusions in breast cancer.

METHODS

We analysed genomic rearrangements and transcripts of NRG1 in MDA-MB-175 and a panel of 571 breast cancers.

RESULTS

We found that the MDA-MB-175 fusion-originally reported as a DOC4(TENM4)-NRG1 fusion, lacking the cytoplasmic tail of NRG1-is in reality a double fusion, PPP6R3-TENM4-NRG1, producing multiple transcripts, some of which include the cytoplasmic tail. We hypothesise that many NRG1 fusions may be oncogenic not for lacking the cytoplasmic domain but because they do not encode NRG1's nuclear-localised form. The fusion in MDA-MB-175 is the result of a very complex genomic rearrangement, which we partially characterised, that creates additional expressed gene fusions, RSF1-TENM4, TPCN2-RSF1, and MRPL48-GAB2. We searched for NRG1 rearrangements in 571 breast cancers subjected to genome sequencing and transcriptome sequencing and found four cases (0.7%) with fusions, WRN-NRG1, FAM91A1-NRG1, ARHGEF39-NRG1, and ZNF704-NRG1, all splicing into NRG1 at the same exon as in MDA-MB-175. However, the WRN-NRG1 and ARHGEF39-NRG1 fusions were out of frame. We identified rearrangements of NRG1 in many more (8% of) cases that seemed more likely to inactivate than to create activating fusions, or whose outcome could not be predicted because they were complex, or both. This is not surprising because NRG1 can be pro-apoptotic and is inactivated in some breast cancers.

CONCLUSIONS

Our results highlight the complexity of rearrangements of NRG1 in breast cancers and confirm that some do not activate but inactivate. Careful interpretation of NRG1 rearrangements will therefore be necessary for appropriate patient management.

Synthetic NRG-1 functionalized DNA nanospindels towards HER2/neu targets for in vitro anti-cancer activity assessment against breast cancer MCF-7 cells

DNA based nano-carriers synthesized from short circular scaffolds (circular DNA nanotechnology) attains stiffer topology for ligand functionalization (neuregulin-1/NRG-1 ligand) and biological applications (targeted drug delivery). Daunorubicin (DR) is a hydrophobic chemical that requires robust vectors to efficiently encapsulate and avoid its free dispersion in water, biological media and cell culture. Here we design DNA nanospindels (DNA-NS) to efficiently load DR and target the (highly expressed) HER2/neu receptors on the plasma membrane of drug-resistant MCF-7 (breast cancer) cells. DNA-NS were synthesized by polymerizing the DNA-triangles (utilizing 84-nt short circular scaffold strand) into larger DNA nano-ribbons characterized by the native-PAGE testing. AFM results revealed the spinning of DNA nanoribbons on its (own) axis because of the intrinsic curvature of the DNA double helix resulting in the formation of the firm and twisted DNA-NS with the diameter (50-70 nm) and length (0.5-4 μm). DA loading onto DNA-NS was confirmed by the UV shift analysis. The MTT results with the blank DNA-NS evidenced its biocompatibility (remained value of 93%) compared to the decreased viability of the MCF-7 cells after treatment with DNA-NS (DR loaded). These findings were further supported by the analysis of cell proliferation/apoptosis through flow cytometry showing 64% apoptosis after treating with the DR loaded DNA-NS. Hence, through the short circular DNA nanotechnology, we have achieved a stiffer, uniform, and biocompatible DNA-NS for applications in the targeted therapy.

HER3-targeted protein chimera forms endosomolytic capsomeres and self-assembles into stealth nucleocapsids for systemic tumor homing of RNA interference in vivo

RNA interference represents a potent intervention for cancer treatment but requires a robust delivery agent for transporting gene-modulating molecules, such as small interfering RNAs (siRNAs). Although numerous molecular approaches for siRNA delivery are adequate in vitro, delivery to therapeutic targets in vivo is limited by payload integrity, cell targeting, efficient cell uptake, and membrane penetration. We constructed nonviral biomaterials to transport small nucleic acids to cell targets, including tumor cells, on the basis of the self-assembling and cell-penetrating activities of the adenovirus capsid penton base. Our recombinant penton base chimera contains polypeptide domains designed for noncovalent assembly with anionic molecules and tumor homing. Here, structural modeling, molecular dynamics simulations, and functional assays suggest that it forms pentameric units resembling viral capsomeres that assemble into larger capsid-like structures when combined with siRNA cargo. Pentamerization forms a barrel lined with charged residues mediating pH-responsive dissociation and exposing masked domains, providing insight on the endosomolytic mechanism. The therapeutic impact was examined on tumors expressing high levels of HER3/ErbB3 that are resistant to clinical inhibitors. Our findings suggest that our construct may utilize ligand mimicry to avoid host attack and target the siRNA to HER3+ tumors by forming multivalent capsid-like structures.

MiR-125a-3p regulates glioma apoptosis and invasion by regulating Nrg1

The current study was designed to examine the functional role and mechanism of miR-125a-3p in glioma development. Quantitative RT-PCR was used to evaluate miR-125a-3p expression in 60 glioma cases of different malignant grades. Then, the clinic pathologic significance of miR-125a-3p expression was determined in combination with the prognosis of the patients. In addition, the effects and mechanisms of miR-125a-3p on the proliferation, apoptosis and invasion of glioma cells were further investigated. The results showed that the expression of miR-125a-3p was decreased significantly in most malignant glioma samples relative to normal brain tissues and glioma tissues of low-malignant degree. Further kaplan-meier survival analysis showed that the lower expression of miR-125a-3p was associated with a poor prognosis of GBM patients. Functional analysis showed that the reintroduction of miR-125a-3p into glioblastoma cell lines induces markedly the apoptosis and suppresses the proliferation and migration of glioblastoma cells in vitro and in vivo. Luciferase assay and Western blot analysis revealed that Nrg1 is a direct target of miR-125a-3p. Furthermore, an increased expression of Nrg1 could reverse the effects of overexpression of miR-125a-3p on the proliferation, apoptosis and migration of glioblastoma cells. These findings suggest that miR-125a-3p performed an important role in glioma development mediated by directly regulating the expression of Nrg1. This study also provides a potential target for diagnosis and treatment of malignant glioma.

Global signaling effects of a schizophrenia-associated missense mutation in neuregulin 1: an exploratory study using whole genome and novel kinome approaches

Aberrant neuregulin 1-ErbB4 signaling has been implicated in schizophrenia. We previously identified a novel schizophrenia-associated missense mutation (valine to leucine) in the NRG1 transmembrane domain. This variant inhibits formation of the NRG1 intracellular domain (ICD) and causes decreases in dendrite formation. To assess the global effects of this mutation, we used lymphoblastoid cell lines from unaffected heterozygous carriers (Val/Leu) and non-carriers (Val/Val). Transcriptome data showed 367 genes differentially expressed between the two groups (Val/Val N = 6, Val/Leu N = 5, T test, FDR (1 %), α = 0.05, -log10 p value >1.5). Ingenuity pathway (IPA) analyses showed inflammation and NRG1 signaling as the top pathways altered. Within NRG1 signaling, protein kinase C (PKC)-eta (PRKCH) and non-receptor tyrosine kinase (SRC) were down-regulated in heterozygous carriers. Novel kinome profiling (serine/threonine) was performed after stimulating cells (V/V N = 6, V/L N = 6) with ErbB4, to induce release of the NRG1 ICD, and revealed significant effects of treatment on the phosphorylation of 35 peptides. IPA showed neurite outgrowth (six peptides) as the top annotated function. Phosphorylation of these peptides was significantly decreased in ErbB4-treated Val/Val but not in Val/Leu cells. These results show that perturbing NRG1 ICD formation has major effects on cell signaling, including inflammatory and neurite formation pathways, and may contribute significantly to schizophrenia pathophysiology.

Distinct neurobehavioural effects of cannabidiol in transmembrane domain neuregulin 1 mutant mice

The cannabis constituent cannabidiol (CBD) possesses anxiolytic and antipsychotic properties. We have previously shown that transmembrane domain neuregulin 1 mutant (Nrg1 TM HET) mice display altered neurobehavioural responses to the main psychoactive constituent of cannabis, Δ⁹-tetrahydrocannabinol. Here we investigated whether Nrg1 TM HET mice respond differently to CBD and whether CBD reverses schizophrenia-related phenotypes expressed by these mice. Adult male Nrg1 TM HET and wild type-like littermates (WT) received vehicle or CBD (1, 50 or 100 mg/kg i.p.) for 21 days. During treatment and 48 h after withdrawal we measured behaviour, whole blood CBD concentrations and autoradiographic receptor binding. Nrg1 HET mice displayed locomotor hyperactivity, PPI deficits and reduced 5-HT_2A receptor binding density in the substantia nigra, but these phenotypes were not reversed by CBD. However, long-term CBD (50 and 100 mg/kg) selectively enhanced social interaction in Nrg1 TM HET mice. Furthermore, acute CBD (100 mg/kg) selectively increased PPI in Nrg1 TM HET mice, although tolerance to this effect was manifest upon repeated CBD administration. Long-term CBD (50 mg/kg) also selectively increased GABA_A receptor binding in the granular retrosplenial cortex in Nrg1 TM HET mice and reduced 5-HT_2A binding in the substantia nigra in WT mice. Nrg1 appears necessary for CBD-induced anxiolysis since only WT mice developed decreased anxiety-related behaviour with repeated CBD treatment. Altered pharmacokinetics in mutant mice could not explain our findings since no genotype differences existed in CBD blood concentrations. Here we demonstrate that Nrg1 modulates acute and long-term neurobehavioural effects of CBD, which does not reverse the schizophrenia-relevant phenotypes.

Sustained high levels of neuregulin-1 in the longest-lived rodents; a key determinant of rodent longevity

Naked mole-rats (Heterocephalus glaber), the longest-lived rodents, live 7-10 times longer than similarly sized mice and exhibit normal activities for approximately 75% of their lives. Little is known about the mechanisms that allow them to delay the aging process and live so long. Neuregulin-1 (NRG-1) signaling is critical for normal brain function during both development and adulthood. We hypothesized that long-lived species will maintain higher levels of NRG-1 and that this contributes to their sustained brain function and concomitant maintenance of normal activity. We monitored the levels of NRG-1 and its receptor ErbB4 in H. glaber at different ages ranging from 1 day to 26 years and found that levels of NRG-1 and ErbB4 were sustained throughout development and adulthood. In addition, we compared seven rodent species with widely divergent (4-32 year) maximum lifespan potential (MLSP) and found that at a physiologically equivalent age, the longer-lived rodents had higher levels of NRG-1 and ErbB4. Moreover, phylogenetic independent contrast analyses revealed that this significant strong correlation between MLSP and NRG-1 levels was independent of phylogeny. These results suggest that NRG-1 is an important factor contributing to divergent species MLSP through its role in maintaining neuronal integrity.

Molecular cloning of a brain-specific, developmentally regulated neuregulin 1 (NRG1) isoform and identification of a functional promoter variant associated with schizophrenia

Neuregulin 1 (NRG1) is essential for the development and function of multiple organ systems, and its dysregulation has been linked to diseases such as cancer and schizophrenia. Recently, altered expression of a novel isoform (type IV) in the brain has been associated with schizophrenia-related genetic variants, especially rs6994992 (SNP8NRG243177). Here we have isolated and characterized full-length NRG1 type IV cDNAs from the adult and fetal human brain and identified novel splice variants of NRG1. Full-length type IV spans 1.8 kb and encodes a putative protein of 590 amino acids with a predicted molecular mass of approximately 66 kDa. The transcript consists of 11 exons with an Ig-like domain, an epidermal growth factor-like (EGF) domain, a beta-stalk, a transmembrane domain, and a cytoplasmic "a-tail," placing it in the beta1a NRG1 subclass. NRG1 type IV was not detected in any tissues except brain and a putative type IV NRG1 protein of 66 kDa was similarly brain-specific. Type IV transcripts are more abundantly expressed in the fetal brain, where, in addition to the full-length structure, two novel type IV variants were identified. In vitro luciferase-reporter assays demonstrate that the 5' promoter region upstream of type IV is functional, with differential activity associated with genetic variation at rs6994992, and that promoter competition may impact on type IV expression. Our data suggest that type IV is a unique brain-specific NRG1 that is differentially expressed and processed during early development, is translated, and its expression regulated by a schizophrenia risk-associated functional promoter or single nucleotide polymorphism (SNP).

Identification of Neuregulin as a Factor Required for Formation of Aligned Spermatogonia

In the absence of somatic cells, medium conditioned by the SNL fibroblast line (SNL-CM) is able to stimulate primary cultures of rat type-A single spermatogonia to develop into chains of aligned spermatogonia at the 8-, 16-, and 32-cell stages. By comparison, medium conditioned by an MSC-1 Sertoli cell line is ineffective. Glial cell line-derived neurotrophic factor (GDNF)-like molecules were identified in SNL-CM and recombinant forms of GDNF, neurturin, and artemin were shown to stimulate formation of aligned spermatogonia, but principally to only the 4- and 8-cell stages. Because SNL-CM and GDNF-like molecules stimulated the formation of spermatogonial chain length differently, we purified components of SNL-CM to identify the additional contributing factor(s). A fraction was isolated that was dependent on GDNF, but required for effective formation of 16- and 32-cell chain lengths. Sequence analysis identified the factor as mouse neuregulin-1. At picomolar concentrations, recombinant neuregulin-1 in combination with GDNF effectively stimulated formation of aligned spermatogonia up to the 32-cell stage. Neuregulin in the absence of GDNF was relatively ineffective. Soluble receptors for neuregulins blocked the effects of GDNF and SNL-CM, suggesting that both neuregulin and GDNF are required for effective formation of long spermatogonial chains. Addition of neuregulin-1 to cultures on MSC-1 feeder layers resulted in spermatogonial behavior similar to that seen on feeder layers of SNL fibroblasts. In fact, SNL cells were found to express 100-fold higher levels of neuregulin-1 transcripts than MSC-1 cells. Thus, we identify neuregulin as a factor required for spermatogonial amplification and differentiation in culture.

The immunoglobulin-like domain is involved in interaction of Neuregulin1 with ErbB

Neuregulin1 (NRG1) is a growth factor that signals through the interaction of the epidermal growth factor (EGF)-like domain with ErbB receptors. An immunoglobulin (Ig)-like domain is contained together with EGF-like domain in the ectodomain of some isoforms generated by alternative splicing, but its role in NRG1 signaling remained unclear. In the present study, we identified a novel isoform of NRG1 containing an Ig-like domain conserved among species from adult Xenopus laevis, which is predominantly expressed in the testis and brain. We generated recombinant proteins for the whole ectodomain and EGF-like domain alone of the isoform to compare their effects on cell proliferation, and phosphorylation of and their association with ErbB receptor, demonstrating that the ectodomain had approximately 10(3)-fold higher abilities than the EGF-like domain. Therefore, the Ig-like domain is probably essential for efficient interaction of an EGF-like domain with ErbB receptors.

Control of Peripheral Nerve Myelination by the -Secretase BACE1

Although BACE1 (beta-site amyloid precursor protein-cleaving enzyme 1) is essential for the generation of amyloid-b peptide in Alzheimer's disease, its physiological function is unclear. We found that very high levels of BACE1 were expressed at time points when peripheral nerves become myelinated. Deficiency of BACE1 resulted in the accumulation of unprocessed neuregulin 1 (NRG1), an axonally expressed factor required for glial cell development and myelination. BACE1-/- mice displayed hypomyelination of peripheral nerves and aberrant axonal segregation of small-diameter afferent fibers, very similar to that seen in mice with mutations in type III NRG1 or Schwann cell-specific ErbB2 knockouts. Thus, BACE1 is required for myelination and correct bundling of axons by Schwann cells, probably through processing of type III NRG1.

Inactivation of neuregulin-1 by nitration

Nitration is a posttranslational modification that can compromise protein function. We hypothesized that nitration of growth factors secreted in the lung may alter their interaction with their respective receptors and modulate the normal growth and differentiation program induced by ligand-receptor interaction. We tested this hypothesis in vitro by nitration of neuregulin-1's (NRG-1) EGF-like domain and studying the effect on NRG-1's activity. Nitration of NRG-1's (nNRG-1) EGF-like domain resulted in an inability to activate its receptor, the human epidermal growth factor receptors 2 and 3 (HER2/HER3) heterodimer, as defined by loss of HER2 tyrosine phosphorylation induced by nNRG-1 in MCF-7 cells. Receptor activation was not restored with increasing nNRG-1 concentration or exposure times. nNRG-1 did not compete with NRG-1 for HER2/HER3 binding in competition assays. In addition, nNRG-1 no longer induced proliferation of the MCF-7 cell line, as MCF-7 cells exposed to nNRG-1 and NRG-1 concurrently had the same proliferation rate as that induced by NRG-1 alone. Thus nitration of NRG-1's EGF-like domain caused it to lose its ability to bind and activate its receptor with loss of ligand-induced proliferation. Posttranslational nitration of growth factors in states where reactive nitrogen species are increased may be an important means of regulating growth factor receptor effects in the lung.

A novel missense mutation in the transmembrane domain of neuregulin 1 is associated with schizophrenia

BACKGROUND

Although genetic factors are known to play an important role in schizophrenia, the identification of genes involved in this disorder has remained elusive. The neuregulin 1 gene is among the few candidate genes to have been implicated in schizophrenia susceptibility in several populations. However, no causal mutations within this gene have been identified.

METHODS

In attempts to identify polymorphisms within the neuregulin 1 gene, we performed DNA sequencing using 12 subjects with a history of psychosis from the Central Valley of Costa Rica. DNA genotyping and association studies were then performed in an extended cohort of 142 affected individuals and their relatives from the same population.

RESULTS

We identified a novel missense mutation (Val to Leu) in exon 11, which codes for the transmembrane region of the neuregulin 1 protein. Association analysis by the Family Based Association Test (FBAT) revealed that this mutation is associated with psychosis (p = .0049) and schizophrenia (p = .0191) in this population.

CONCLUSIONS

We report the finding of a missense mutation in the neuregulin 1 gene associated with schizophrenia. Additional analyses of an independent sample as well as detailed functional studies should be performed to determine the relevance of this novel polymorphism to the pathophysiology of schizophrenia.

The N-terminal domains of neuregulin 1 confer signal attenuation

Degradation of activated ERBB receptors is an important mechanism for signal attenuation. However, compared with epidermal growth factor (EGF) receptor, the ERBB2/ERBB3 signaling pair is considered to be attenuation-deficient. The ERBB2/ERBB3 ligands of the neuregulin family rely on an EGF-like domain for signaling and are generated from larger membrane-bound precursors. In contrast to EGF, which is processed to yield a 6-kDa peptide ligand, mature neuregulins retain a variety of segments N-terminal to the EGF-like domain. Here we evaluate the role of the N-terminal domain of neuregulin 1 in signaling and turnover of ERBB2/ERBB3. Our data suggest that whereas the EGF-like domain of neuregulin 1 is required and sufficient for the formation of active receptor heterodimers, the presence of the N-terminal Ig-like domain is required for efficient signal attenuation. This manifests itself for both ERBB2 and ERBB3 but is more pronounced and coupled directly to degradation for ERBB3. When stimulated with only the EGF-like domain, ERBB3 shows degradation rates comparable with constitutive turnover, but stimulation with full-length neuregulin 1 resulted in receptor degradation at rates that are comparable with activated EGF receptor. Most of the enhancement in down-regulation was maintained after replacing the Ig-like domain with a thioredoxin protein of comparable size but different amino acid composition, suggesting that the physical presence but not specific properties of the Ig-like domain are needed. This sequence-independent effect of the N-terminal domain correlates with an enhanced ability of full-size neuregulin 1 to disrupt higher order oligomers of the ERBB3 extracellular domains in vitro.

Computational analysis of molecular basis of 1:1 interactions of NRG-1beta wild-type and variants with ErbB3 and ErbB4

The neuregulin/ErbB system is a growth factor/receptor cascade that has been proven to be essential in the development of the heart and the sympathetic nervous system. However, the basis of the specificity of ligand-receptor recognition remains to be elucidated. In this study, the structures of NRG-1beta/ErbB3 and NRG-1beta/ErbB4 complexes were modeled based on the available structures of the homologous proteins. The binding free energies of NRG-1beta to ErbB3 and ErbB4 were calculated using the molecular mechanics Poisson-Boltzmann surface area (MM-PBSA) computational method. In addition, computational alanine-scanning mutagenesis was performed in the binding site of NRG-1beta and the difference in the binding free energies between NRG-1beta mutants and the receptors was calculated. The results specify the contribution of each residue at the interaction interfaces to the binding affinity of NRG-1beta with ErbB3 and ErbB4, identifying several important interaction residue pairs that are in agreement with previously acquired experimental data. This indicates that the presented structural models of NRG-1beta/ErbB3 and NRG-1beta/ErbB4 complexes are reliable and could be used to guide future studies, such as performing desirable mutations on NRG-1beta to increase the binding affinity and selectivity to the receptor and discovering new therapeutic agents for the treatment of heart failure.

Schizophrenia: do the genetics and neurobiology of neuregulin provide a pathogenesis model?

The sequencing of the human genome and an emerging dense map of markers across the human genome have spawned new approaches to search for risk genes for human diseases with complex genetics. These approaches are particularly relevant to the search for risk genes for bipolar disorder and schizophrenia. A gene called neuregulin 1 has been reported to be a risk gene for schizophrenia. This article reviews aspects of the genetics, cellular neurobiology, and biochemistry of neuregulin 1 and attempts to integrate several observations from disparate fields into a model for the pathogenesis of schizophrenia. The model outlines experimental approaches that may, in the future, shed more light on its validity.

Heregulins Implicated in Cellular Functions Other Than Receptor Activation

Heregulins (HRG) are known as soluble secreted growth factors that, on binding and activating ErbB3 and ErbB4 cell surface receptors, are involved in cell proliferation, metastasis, survival, and differentiation in normal and malignant tissues. Previous studies have shown that some HRG1 splice variants are translocated to the nucleus. By investigating the subcellular localization of HRGalpha(1-241), nuclear translocation and accumulation in nuclear dot-like structures was shown in breast cancer cells. This subcellular distribution pattern depends on the presence of at least one of two nuclear localization sequences and on two domains on the HRG construct that were found to be necessary for nuclear dot formation. Focusing on the nuclear function of HRG, a mammary gland cDNA library was screened with the mature form of HRGalpha in a yeast two-hybrid system, and coimmunoprecipitation of endogenous HRG was done. The data reveal positive interactions of HRGalpha(1-241) with nuclear factors implicated in different biological functions, including transcriptional control as exemplified by interaction with the transcriptional repressor histone deacetylase 2. In addition, HRGalpha(1-241) showed transcriptional repression activity in a reporter gene assay. Furthermore, a potential of HRG proteins to form homodimers was reported and the HRG sequence responsible for dimerization was identified. These observations strongly support the notion that HRG1 splice variants have multifunctional properties, including previously unknown regulatory functions within the nucleus that are different from the activation of ErbB receptor signaling.

MUC1 oncoprotein is targeted to mitochondria by heregulin-induced activation of c-Src and the molecular chaperone HSP90

The MUC1 heterodimeric transmembrane glycoprotein is aberrantly overexpressed by most human carcinomas. The MUC1 C-terminal subunit localizes to mitochondria and blocks stress-induced activation of the intrinsic apoptotic pathway. How MUC1 is delivered to mitochondria is not known. The present studies demonstrate that MUC1 forms intracellular complexes with HSP70 and HSP90. We show that the MUC1 cytoplasmic domain binds directly to HSP70 in vitro. By contrast, binding of MUC1 to HSP90 in vitro is induced by c-Src-mediated phosphorylation of the MUC1 cytoplasmic domain. c-Src also increases binding of MUC1 to HSP90 in cells. In concert with these results, we show that heregulin (HRG), a ligand for ErbB receptors, activates c-Src and, in turn, stimulates binding of MUC1 to HSP90. We also show that inhibitors of c-Src or HSP90 block HRG-induced targeting of MUC1 to mitochondria and integration of MUC1 into the mitochondrial outer membrane. These findings indicate that MUC1 is delivered to mitochondria by a mechanism involving activation of the ErbB receptor-->c-Src pathway and transport by the molecular chaperone HSP70/HSP90 complex.

Neuregulin-1 immunoglobulin-like domain mutant mice: clozapine sensitivity and impaired latent inhibition

Genetic and behavioral studies in humans and mouse mutants have implicated the gene encoding neuregulin-1 (Nrg-1) as a candidate susceptibility gene for schizophrenia. We examined the behavior of mice heterozygous for a mutation in neuregulin-1's immunoglobulin (Ig)-like domain (Ig-nrg-1 mice). We found that these animals displayed behaviors related to a schizophrenia-like phenotype, such as clozapine suppression of open-field and running wheel activity and impaired latent inhibition. Contrary to findings with other nrg-1 mutants, Ig-nrg-1 mice did not exhibit significantly elevated locomotion relative to littermate controls. These results suggest that Ig-Nrg-1's contribute to some - but not all - aspects of the schizophrenia-like phenotype of nrg-1 mutants, and further support nrg-1 as a candidate gene for schizophrenia.

Oligomers of ERBB3 have two distinct interfaces that differ in their sensitivity to disruption by heregulin

ErbB receptors associate in a ligand-dependent or -independent manner, and overexpression of epidermal growth factor receptor (ErbB1) or ErbB2 results in ligand-independent activation. Ligand-independent activation is poorly understood, and dimerization alone is not sufficient for activation. ErbB receptors also form higher order oligomers, but the mechanism of oligomer formation and their contribution to signaling are not known. The kinase-deficient ErbB3 as well as its extracellular domains are particularly prone to ligand-independent oligomerization, and oligomers are destabilized by binding of the ligand heregulin. In contrast, ligand binding facilitates heterodimerization with ErbB2 and is expected to stabilize an extended conformation of the ErbB3 extracellular domain (ECD) in which the dimerization interface is exposed. In the absence of ligand, ErbB3 can adopt a closed conformation that is held together by an intramolecular tether. We used a constitutively extended form of the ErbB3-ECD to analyze the conformation of the ECD in oligomers and the mechanism of oligomer disruption by heregulin. The extended conformation of the ECD forms oligomers more readily, suggesting the crystallographically defined dimer interface is one of the interfaces involved in oligomerization. Heregulin destabilizes oligomeric complexes but not dimers, which are neither stabilized nor disrupted by ligand binding, indicating a distinct second interface in oligomers of ErbB3. Cross-linking and activation studies on membrane-embedded ErbB3/ErbB2 chimeras confirm this dual effect of heregulin. Most of the ErbB3-ECD on the cell surface is apparently kept in an open conformation through oligomerization, and the resulting oligomers adopt a conformation representing a state of reduced activity.

Extracellular matrix enhances heregulin-dependent BRCA1 phosphorylation and suppresses BRCA1 expression through its C terminus

Germ line mutations in the breast cancer susceptibility gene BRCA1 account for the increased risk of early onset of familial breast cancer, whereas overexpression of the ErbB family of receptor tyrosine kinases has been linked to the development of nonfamilial or sporadic breast cancer. To analyze whether there is a link between these two regulatory molecules, we studied the effects of ErbB-2 activation by heregulin (HRG) on BRCA1 function. It was previously demonstrated that HRG induced the phosphorylation of BRCA1, which was mediated by the phosphatidylinositol 3-kinase (PI3K)/Akt pathway. Since altered interaction between cells and the surrounding extracellular matrix (ECM) is a common feature in a variety of tumors and since ECM modulates intracellular signaling, we hypothesized that ECM may affect the expression and HRG-dependent phosphorylation of BRCA1. Following stimulation by HRG, a strong increase in [(3)H]thymidine incorporation was observed in human T47D breast cancer cells seeded on plastic (PL). When T47D cells were seeded on laminin (LAM) or Matrigel, HRG induced a significantly higher proliferation than it did in cells seeded on PL. T47D cells seeded on poly-L-lysine had an abrogated mitogenic response, indicating the involvement of integrins in this process. HRG treatment induced a transient phosphorylation of BRCA1 that was enhanced in T47D cells grown on LAM. LAM-enhanced BRCA1 phosphorylation was mediated through alpha(6) integrin upon HRG stimulation. Accordingly, T47D cells grown on LAM had the greatest increase in ErbB-2 activation, PI3K activity, and phosphorylation of Akt. A similar pattern of BRCA1 mRNA expression was observed when T47D cells were seeded on PL, LAM, or COL4. There was a significant decrease in the steady state of the BRCA1 mRNA level on both the LAM and COL4 matrices compared to that for cells seeded on PL. In addition, HRG stimulation caused a significant decrease in BRCA1 mRNA expression that was dependent on protein synthesis. Pretreatment with both the calpain inhibitor ALLN (N-acetyl-Leu-Leu-norleucinal) and the proteosome inhibitor lactacystin inhibited the HRG-induced down-regulation of BRCA1 mRNA expression. Likewise, there was a strong decrease in the protein level of BRCA1 in T47D cells 4 h after treatment with HRG compared to its level in control nontreated T47D cells. Pretreatment with the proteosome inhibitors ALLN, lactacystin, and PSI [N-benzyloxycarbonyl-Ile-Glu-(O-t-butyl)-Ala-leucinal] inhibited also the HRG-induced down-regulation of BRCA1 protein in breast cancer cells. Interestingly, BRCA1 mRNA expression in HCC-1937 breast cancer cells, which express C-terminally truncated BRCA1, was not affected by either LAM or CL4. No phosphorylation of BRCA1 from HCC-1937 cells was observed in response to HRG. While Cdk4 phosphorylated wild-type BRCA1 in response to HRG in T47D cells, Cdk4 failed to phosphorylate the truncated form of BRCA1 in HCC-1937 cells. Furthermore, overexpression of wild-type BRCA1 in HCC-1937 cells resulted in the phosphorylation of BRCA1 and decreased BRCA1 expression upon HRG stimulation while overexpression of truncated BRCA1 in T47D cells resulted in a lack of BRCA1 phosphorylation and restoration of BRCA1 expression. These findings suggest that ECM enhances HRG-dependent BRCA1 phosphorylation and that ECM and HRG down-regulate BRCA1 expression in breast cancer cells. Furthermore, ECM suppresses BRCA1 expression through the C terminus of BRCA1.

Replacement of N-terminal portions of TGF-alpha with corresponding heregulin sequences affects ligand-induced receptor signaling and intoxication of tumor cells by chimeric growth-factor toxins

ErbB receptor tyrosine kinases play an important role in developmental processes and tumor formation. Their activity is regulated by a family of structurally related ligands that bind to distinct ErbB receptor subsets, with transforming growth factor (TGF)-alpha preferentially interacting with epidermal growth-factor receptor (EGFR) and heregulin (HRG)-beta1 recognizing ErbB3 and ErbB4. To investigate the contribution of N-terminal ligand sequences to binding specificity, we have constructed 2 chimeric growth factors termed H181T8 and H194T20, which contain N-terminal HRG-beta1 sequences linked to complementary fragments of TGF-alpha. For bacterial expression and analysis of cell binding, the chimeric ligands were genetically fused to truncated Pseudomonas exotoxin A (ETA). H181T8-ETA and H194T20-ETA toxins both were cytotoxic for human tumor cell lines overexpressing EGFR but did not significantly affect the growth of cells that express ErbB receptors other than EGFR. Binding of H181T8, which contains HRG-beta1 residues 177-181, induced rapid autophosphorylation of EGFR, but in contrast to a previously described chimeric ligand based on EGF was unable to activate other ErbB receptors. H194T20, which contains HRG-beta1 residues 177-194, despite specific binding to EGFR was unable to induce autophosphorylation of any of the ErbB family members. However, H194T20 enhanced and modified the activity of parental TGF-alpha and HRG-beta1 when these ligands were simultaneously present. Our results show that modification of the N-terminal TGF-alpha sequence can have a significant effect on the signaling properties of the ligand and suggest that different EGF-like ligands can synergize in the activation of ErbB receptors.

Neuregulin-1 proteins in rat brain and transfected cells are localized to lipid rafts

Neuregulin-1 proteins and their receptors, which are members of the ErbB subfamily of receptor tyrosine kinases, play essential roles in the development of the nervous system and heart. Most neuregulin-1 isoforms are synthesized as transmembrane proproteins that are proteolytically processed to yield an N-terminal fragment containing the bioactive EGF-like domain. In this study we investigated whether neuregulins are found in lipid rafts, membrane microdomains hypothesized to have important roles in signal transduction, protein trafficking, and proteolytic processing. We found that 45% of a 140-kDa neuregulin protein in rat brain synaptosomal plasma membrane fractions was insoluble in 1% Triton X-100. Flotation gradient analysis demonstrated the presence of the brain 140 kDa neuregulin protein in low-density fractions enriched in PSD-95, a known lipid raft protein. In transfected cells expressing the neuregulin I-beta 1a or the III-beta 1a isoform, most of the neuregulin proprotein was insoluble in 1% Triton X-100, and neuregulin proproteins and C-terminal fragments were detected in lipid raft fractions. In contrast, the III-beta 1a N-terminal fragment was detected only in the detergent-soluble fraction. These results suggest that localization of neuregulins to lipid rafts may play a role in neuregulin signaling within the nervous system.

Neuregulin, a factor with many functions in the life of a schwann cell

The signalling system comprising the ligand Neuregulin-1, and its receptors, ErbB2 and ErbB3, plays multiple and important roles in glial development. These include functions in early development of neural crest cells, in expansion of the Schwann cell precursor pool and in myelination. Neuregulin is one of the crucial axon-derived signals that influence development of Schwann cells. These are specialized cells that ensheath peripheral axons and provide electrical insulation. Schwann cells have also long been implicated in providing more than a simple ensheathing function. Compelling evidence for this has emerged from the analysis of mice lacking these cells, resulting from a non-functional or compromised Neuregulin signalling system. They serve as a model to study glia-nerve interactions in vivo and indicate that Schwann cells provide important neurotrophic signals, and also cues that regulate perineurium development and nerve fasciculation.

Heregulin reverses the oligomerization of HER3

We analyzed the propensity of the HER3 receptor and its extracellular domain (ECD) to undergo ligand-independent self-association. The HER3-ECD, purified from Drosophila S2 cells, binds the EGF-like domain of heregulin (hrg) with a K(d) of 1.9 nM as measured by surface plasmon resonance (SPR) studies. In a gel shift assay, the HER3-ECD self-associates into a uniform, slowly migrating species in a concentration-dependent manner, starting at concentrations of <10 nM. In contrast to the HER3-ECD, the ECD from the related HER2 receptor does not oligomerize under the same conditions. The direct interaction of HER3-ECDs was also demonstrated by pull-down assays and SPR measurements under physiological salt conditions. This self-association of the HER3-ECD was reversed by the addition of hrg but not by EGF. The apparent equilibrium dissociation constant for the HER3-ECD self-association is 15 nM, based on SPR measurements. In this analysis, hrg blocks HER3-ECD self-association, and the addition of hrg during the dissociation phase resulted in an accelerated off rate. This finding suggests that hrg can bind to and disrupt preexisting HER3-ECD oligomers. Full-length HER3 likewise exhibited self-association. Under conditions where co-immunoprecipitation and cross-linking of HER2 and HER3 were stimulated by hrg, HER3 self-association and cross-linking were disrupted by hrg. The implication is that the self-association of HER3-ECD favors the formation of catalytically inactive complexes of the HER3 receptor. Binding of hrg releases HER3 which may then form signaling-competent HER3-HER2 heterodimers.

The extracellular region of heregulin is sufficient to promote mammary gland proliferation and tumorigenesis but not apoptosis

Heregulin (HRG) is a member of the neuregulin family of ligands that have been shown to interact with and activate erbB receptors. A transgenic mouse model in which full-length HRG is overexpressed has proven that this protein can induce carcinomas in the murine mammary gland. These tumors display a high level of apoptosis, which appears to be mediated by the cytoplasmic domain of HRG. Because both proliferation and apoptosis play a role in tumor formation, we wished to separately view those perturbations by removing the suspected apoptosis-inducing cytoplasmic domain of HRG. We thereby sought to determine whether overexpression of the extracellular region of HRG would be sufficient to induce mammary gland carcinomas. A HRG construct lacking the cytoplasmic domain was targeted to the mammary gland using the murine mammary tumor virus promoter. Multiple lines of transgenic mice carrying the transgene developed mammary gland tumors at approximately 15 months of age. These tumors did not display high levels of apoptosis as compared with tumors from murine mammary tumor virus/full-length HRG transgenic animals. In addition, virgin transgenic mice show a persistence of terminal end bud structures, which normally disappear at the onset of puberty in wild-type mice. To examine the signal transduction pathway activated by extracellular HRG in tumors, we investigated the phosphorylation status of the epidermal growth factor receptor family members. Western blot analysis showed activation of ErbB2 and ErbB3, suggesting a possible mode of action of extracellular HRG in mammary gland carcinomas. We conclude that the extracellular and transmembrane domains of HRG are sufficient for the induction of tumorigenesis but that induction of apoptosis requires the cytoplasmic tail.

Modified genetic algorithm resolves ambiguous NOE restraints and reduces unsightly NOE violations

In an ideal world, every NOE cross peak would have a unique assignment. However, the interpretation of NOE peaks is frequently complicated by overlapping resonances. In theory, ambiguous assignments could be resolved by performing separate structure calculations with each possible interpretation. Unfortunately, this would require an astronomical amount of computing time. A modified genetic algorithm has been developed that efficiently resolves hundreds of ambiguous restraints in parallel. Each NOE assignment becomes a gene that can be passed on to a new generation. New individuals are constructed by making a constraint lists from a subset of the genes. The constraint lists are then tested for self-consistency by using molecular dynamics to generate new structures for each list. To a first-degree approximation, there is enough information retained in each list to determine the global fold of the protein. Self-consistent constraint lists receive higher scores and their genes (or NOEs) stand a better chance of surviving into the next generation. The process selects NOEs that are consistent with the global fold. Under normal conditions, the program converges in 3 to 8 generations using 70 structures per generation. The final constraints are self-consistent and contain almost no residual NOE violations.

A comparison of folding techniques in the chemical synthesis of the epidermal growth factor-like domain in neu differentiation factor alpha/beta

The 52-residue alpha/beta chimera of the epidermal growth factor-like domain in neu differentiation factor (NDFealpha/beta) has been synthesized and folded to form a three disulfide bridge (Cys182-Cys196, Cys190-Cys210, Cys212-Cys221) containing peptide. We investigated two general strategies for the formation of the intramolecular disulfide bridges including, the single-step approach, which used fully deprotected and reduced peptide, and a sequential approach that relied on orthogonal cysteine protection in which specific pairs are excluded from the first oxidation step. Because there are 15 possible disulfide bridge arrangements in a peptide with six cysteines, the one-step approach may not always provide the desired disulfide pairing. Here, we compare the single-step approach with a systematic evaluation of the sequential approach. We employed the acetamidomethyl group to protect each pair of cysteines involved in disulfide bridges, i.e. Cys182 to Cys196, Cys190 to Cys210 and Cys212 to Cys221. This reduced the number of possible disulfide patterns from 15 to three in the first folding step. We compared the efficiencies of folding for each protected pair using RP-HPLC, mapped the disulfide connectivity of the predominant product and then formed the final disulfide from the partially folded intermediate via 12 oxidation. Only the peptide having the Cys182-Cys196 pair blocked with acetamidomethyl forms the desired disulfide isomer (Cys190-Cys210/Cys212-Cys221) as a single homogeneous product. By optimizing both approaches, as well as other steps in the synthesis, we can now rapidly provide large-scale syntheses of NDFealpha/beta and other novel EGF-like peptides.

Peptide map procedure using immobilized protease cartridges in tandem for disulfide linkage identification of neu differentiation factor epidermal growth factor domain

Immobilized proteolytic enzyme cartridges were used to rapidly digest neu differentiation factor EGF domain in order to obtain improved peptide maps useful for assignment of disulfide linkages. The procedure described here involves an on-line digestion of proteins using immobilized trypsin and endoproteinase Glu-C cartridges connected in series, followed by on-line RP-HPLC separation of the peptides. The entire process can be automated using a commercially available workstation; and the total time required for both proteolytic digestion and the HPLC separation can be shortened to within 1 h. Using these immobilized columns, we demonstrated that disulfide structure assignment of the EGF domains of recombinant human neu differentiation factor can be performed by isolation of individual disulfide-containing peptides followed by assignment of disulfide linkages with prompt fragmentation of peptides using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. The use of immobilized protease cartridges in tandem eliminates undesirable digestion artifacts associated with longer digestion time and higher protease-to-substrate ratio and results in the development of a reproducible and high quality peptide map.

Cysteine-rich domain isoforms of the neuregulin-1 gene are required for maintenance of peripheral synapses

Neuregulin-1 (NRG-1) signaling has been implicated in inductive interactions between pre- and postsynaptic partners during synaptogenesis. We used gene targeting to selectively disrupt cysteine-rich domain-(CRD-) containing NRG-1 isoforms. In CRD-NRG-1-/-mice, peripheral projections defasciculated and displayed aberrant branching patterns within their targets. Motor nerve terminals were transiently associated with broad bands of postsynaptic ACh receptor (AChR) clusters. Initially, Schwann cell precursors accompanied peripheral projections, but later, Schwann cells were absent from axons in the periphery. Following initial stages of synapse formation, sensory and motor nerves withdrew and degenerated. Our data demonstrate the essential role of CRD-NRG-1-mediated signaling for coordinating nerve, target, and Schwann cell interactions in the normal maintenance of peripheral synapses, and ultimately in the survival of CRD-NRG-1-expressing neurons.

Analysis of heregulin symmetry by weighted evolutionary tracing

Heregulins are members of the protein family of EGF-like growth and differentiation factors. The primary cell-surface targets of heregulins are heterodimers of the EGF-receptor homolog HER2 with either HER3 or HER4. We used a weighted evolutionary trace analysis to identify structural features that distinguish the EGF-like domain (hrg) of heregulins from other members of the EGF family. In this analysis, each amino acid sequence is weighted according to its uniqueness and the variability in each position is assigned by an amino acid substitution matrix. Conserved residues in heregulin that are variable in other EGF-like domains are considered possible specificity-conferring residues. This analysis identifies two clusters of residues at the foot of the boot-shaped hrg domain. The residues in one cluster are recruited from the N-terminus; those in the other are from the ohm-loop region and show a weak sequence similarity to the N-terminal residues at the opposite side of the boot. The remaining residues with high conservation scores distribute themselves into these two distinct surfaces on hrg. This pseudo-twofold symmetry and the presence of two distinct interfaces may reflect the preference of hrg for heterodimeric versus homodimeric HER complexes.