Neuregulin Isoforms Research Articles

Axon contact-driven Schwann cell dedifferentiation.

Mature Schwann cells (SCs) retain dedifferentiation potential throughout adulthood. Still, how dedifferentiation occurs remains uncertain. Results from a variety of cell-based assays using in vitro cultured cAMP-differentiated and myelinating SCs revealed the existence of a novel dedifferentiating activity expressed on the surface of dorsal root ganglion (DRG) axons. This activity had the capacity to prevent SC differentiation and elicit dedifferentiation through direct SC-axon contact. Evidence is provided showing that a rapid loss of myelinating SC markers concomitant to proliferation occurred even in the presence of elevated cAMP, a signal that is required to drive and maintain a differentiated state. The dedifferentiating activity was a membrane-bound protein found exclusively in DRG neurons, as judged by its subcellular partitioning, sensitivity to proteolytic degradation and cell-type specificity, and remained active even after disruption of cellular organization. It differed from the membrane-anchored neuregulin-1 isoforms that are responsible for axon contact-induced SC proliferation and exerted its action independently of mitogenic signaling emanating from receptor tyrosine kinases and mitogen-activated protein kinases such as ERK and JNK. Interestingly, dedifferentiation occurred without concomitant changes in the expression of Krox-20, a transcriptional enhancer of myelination, and c-Jun, an inhibitor of myelination. In sum, our data indicated the existence of cell surface axon-derived signals that override pro-differentiating cues, drive dedifferentiation and allow SCs to proliferate in response to axonal mitogens. This axonal signal may negatively regulate myelination at the onset or reversal of the differentiated state. GLIA 2017;65:851-863.

Type I neuregulin1\u03b1 is a novel local mediator to suppress hepatic gluconeogenesis in mice

Neuregulin1 is an epidermal growth factor (EGF)-like domain-containing protein that has multiple isoforms and functions as a local mediator in the control of various cellular functions. Here we show that type I isoform of neuregulin1 with an α-type EGF-like domain (Nrg1α) is the major isoform in mouse liver and regulates hepatic glucose production. Forced expression of Nrg1α in mouse liver enhanced systemic glucose disposal and decreased hepatic glucose production with reduced fasting blood glucose levels. Nuclear forkhead box protein O1 (FoxO1) and its downstream targets, PEPCK and G6Pase, were suppressed in liver and isolated hepatocytes by Nrg1α overexpression. In contrast, silencing of Nrg1α enhanced glucose production with increased PEPCK and G6Pase expressions in cAMP/dexamethasone-stimulated hepatocytes. Mechanistically, the recombinant α-type EGF-like domain of NRG1α (rNRG1α) stimulated the ERBB3 signalling pathway in hepatocytes, resulting in decreased nuclear FoxO1 accumulation via activation of both the AKT and ERK pathways. In addition, acute treatment with rNRG1α also suppressed elevation of blood glucose levels after both glucose and pyruvate challenge. Although a liver-specific deletion of Nrg1 gene in mice showed little effect on systemic glucose metabolism, these results suggest that NRG1α have a novel regulatory function in hepatic gluconeogenesis by regulating the ERBB3-AKT/ERK-FoxO1 cascade.

Differential expression of neuregulin 1 (NRG1) and candidate miRNA regulating NRG1 transcription in the chicken oviduct in response to hormonal changes.

Neuregulin 1 (NRG1), a signaling protein that binds to members of the erythroblastoma (ErbB) family, is known to play essential biological roles in mediating cell-cell interactions and orchestrating vital cell functions in the heart and nervous system. NRG1 is closely associated with developmental processes in various organs and expression of several NRG1 isoforms are regulated by estrogen. However, expression and its hormonal regulation during development of the female reproductive tract remain unknown. The reproductive system of chickens undergoes dynamic morphological and functional changes in response to estrogen and the molting process. Therefore, in the current study, we found differential expression of and candidate microRNA regulating chicken transcription in response to estrogen stimulation and/or the molting process. The results revealed that relative expression of mRNA increased in the oviducts of chicks treated with diesthylstilbestrol (DES; a synthetic non-steroidal estrogen). Additionally, abundant NRG1 mRNA expression was primarily in the glandular (GE) and luminal (LE) epithelia of the magnum in the developing chick oviduct in response to exogenous estrogen. Also, expression decreased during regression of the oviduct following induced molting by feeding high levels of zinc in the diet, and then gradually resurged as the oviduct underwent remodeling and recrudescence in laying hens. In addition, the present results revealed changes in relative expression of candidate chicken microRNA (, and ) targeting transcription in chicken oviducts in response to exogenous estrogen. In conclusion, these results provide the first evidence that is a novel estrogen-responsive gene closely correlated with the estrogen-dependent development of the oviduct of chicks and regeneration of the oviduct after molting. Further, regulation of expression involving at least 3 chicken miRNA is likely a prerequisite for those estrogen regulated developmental events.

Chronically denervated distal nerve stump inhibits peripheral nerve regeneration.

Chronically denervated distal nerve stump inhibits peripheral nerve regeneration.

IgG-Containing Isoforms of Neuregulin-1 Are Dispensable for Cardiac Trabeculation in Zebrafish.

The Neuregulin-1 (Nrg1) signaling pathway has been widely implicated in many aspects of heart development including cardiac trabeculation. Cardiac trabeculation is an important morphogenetic process where clusters of ventricular cardiomyocytes extrude and expand into the lumen of the ventricular chambers. In mouse, Nrg1 isoforms containing an immunoglobulin-like (IgG) domain are essential for cardiac trabeculation through interaction with heterodimers of the epidermal growth factor-like (EGF-like) receptors ErbB2/ErbB4. Recent reports have underscored the importance of Nrg1 signaling in cardiac homeostasis and disease, however, placental development has precluded refined evaluation of the role of this pathway in mammals. ErbB2 has been shown to have a developmentally conserved role in cardiac trabeculation in zebrafish, a vertebrate model organism with completely external development, but the requirement for Nrg1 has not been examined. We found that among the multiple Nrg1 isoforms, the IgG domain-containing, type I Nrg1 (nrg1-I) is the only isoform detectable in the heart. Then, using CRISPR/Cas9 gene editing, we targeted the IgG domain of Nrg1 to produce novel alleles, nrg1nc28 and nrg1nc29, encoding nrg1-I and nrg1-II truncations. Our results indicated that zebrafish deficient for nrg1-I developed trabeculae in an ErbB2-dependent manner. Further, these mutants survive to reproductive adulthood with no overt cardiovascular defects. We also found that additional EGF-like ligands were expressed in the zebrafish heart during development of trabeculae. Together, these results suggest that Nrg1 is not the primary effector of trabeculation and/or that other EGF-like ligand(s) activates the ErbB2/ErbB4 pathway, either through functioning as the primary ligand or acting in a redundant manner. Overall, our work provides an example of cross-species differences in EGF family member requirements for an evolutionary conserved process.

Neuregulin1 alpha activates migration of neuronal progenitors expressing ErbB4.

Deficits in neuronal migration during development in the central nervous system may contribute to psychiatric diseases. The ligand neuregulin1 (NRG1) and its receptor ErbB4 are genes conferring susceptibility to schizophrenia, playing a key role in the control of neuronal migration both during development and adulthood. Several NRG1 and ErbB4 isoforms were identified, which deeply differ in their characteristics. Here we focused on the four ErbB4 isoforms and the two NRG1 isoforms differing in their EGF-like domain, namely α and β. We hypothesized that these isoforms, which are differently regulated in schizophrenic patients, could play different roles in neuronal migration. Our hypothesis was strengthened by the observation that both NRG1α and NRG1β and the four ErbB4 isoforms are expressed in the medial and lateral ganglionic eminences and in the cortex during development in rat. We analysed in vitro the signal transduction pathways activated by the different ErbB4 isoforms following the treatment with soluble recombinant NRG1α or NRG1β and the ability to stimulate migration. Our data show that two ErbB4 isoforms, namely JMa-cyt2 and JMb-cyt1, following NRG1α and NRG1β treatment, strongly activate AKT phosphorylation, conferring high migratory activity to neuronal progenitors, thus demonstrating that both NRG1α and NRG1β can play a role in neuronal migration.

A RET-ER81-NRG1 Signaling Pathway Drives the Development of Pacinian Corpuscles.

Communication between neurons and Schwann cells is critical for development, normal function, and regeneration of the peripheral nervous system. Despite many studies about axonal communication with myelinating Schwann cells, mostly via a specific isoform of Neuregulin1, the molecular nature of axonal communication with nonmyelinating Schwann cells is poorly understood. Here, we described a RET-ER81-Neuregulin1 signaling pathway in neurons innervating Pacinian corpuscle somatosensory end organs, which is essential for communication between the innervating axon and the end organ nonmyelinating Schwann cells. We also showed that this signaling pathway uses isoforms of Neuregulin1 that are not involved in myelination, providing evidence that neurons use different isoforms of Neuregulin1 to interact with different types of Schwann cells.

Specific inhibition of secreted NRG1 types I–II by heparin enhances Schwann Cell myelination

Primary cultures of mixed neuron and Schwann cells prepared from dorsal root ganglia (DRG) are extensively used as a model to study myelination. These dissociated DRG cultures have the particular advantage of bypassing the difficulty in purifying mouse Schwann cells, which is often required when using mutant mice. However, the drawback of this experimental system is that it yields low amounts of myelin. Here we report a simple and efficient method to enhance myelination in vitro. We show that the addition of heparin or low molecular weight heparin to mixed DRG cultures markedly increases Schwann cells myelination. The myelin promoting activity of heparin results from specific inhibition of the soluble immunoglobulin (Ig)-containing isoforms of neuregulin 1 (i.e., NRG1 types I and II) that negatively regulates myelination. Heparin supplement provides a robust and reproducible method to increase myelination in a simple and commonly used culture system. GLIA 2016;64:1227-1234.

Behavioral, Neurophysiological, and Synaptic Impairment in a Transgenic Neuregulin1 (NRG1-IV) Murine Schizophrenia Model.

Schizophrenia is a chronic, disabling neuropsychiatric disorder with complex genetic origins. The development of strategies for genome manipulation in rodents provides a platform for understanding the pathogenic role of genes and for testing novel therapeutic agents. Neuregulin 1 (NRG1), a critical developmental neurotrophin, is associated with schizophrenia. The NRG1 gene undergoes extensive alternative splicing and, to date, little is known about the neurobiology of a novel NRG1 isoform, NRG1-IV, which is increased in the brains of individuals with schizophrenia and associated with genetic risk variation. Here, we developed a transgenic mouse model (NRG1-IV/NSE-tTA) in which human NRG1-IV is selectively overexpressed in a neuronal specific manner. Using a combination of molecular, biochemical, electrophysiological, and behavioral analyses, we demonstrate that NRG1-IV/NSE-tTA mice exhibit abnormal behaviors relevant to schizophrenia, including impaired sensorimotor gating, discrimination memory, and social behaviors. These neurobehavioral phenotypes are accompanied by increases in cortical expression of the NRG1 receptor, ErbB4 and the downstream signaling target, PIK3-p110δ, along with disrupted dendritic development, synaptic pathology, and altered prefrontal cortical excitatory-inhibitory balance. Pharmacological inhibition of p110δ reversed sensorimotor gating and cognitive deficits. These data demonstrate a novel role for NRG1-IV in learning, memory, and neural circuit formation and a potential neurobiological mechanism for schizophrenia risk; show that deficits are pharmacologically reversible in adulthood; and further highlight p110δ as a target for antipsychotic drug development. Schizophrenia is a disabling psychiatric disorder with neurodevelopmental origins. Genes that increase risk for schizophrenia have been identified. Understanding how these genes affect brain development and function is necessary. This work is the first report of a newly generated humanized transgenic mouse model engineered to express human NRG1-IV, an isoform of the NRG1 (Neuregulin 1) gene that is increased in the brains of patients with schizophrenia in association with genetic risk. Using behavioral neuroscience, molecular biology, electrophysiology, and pharmacology, we identify a role for NRG1-IV in learning, memory, and cognition and determine that this relates to brain excitatory-inhibitory balance and changes in ErbB4/PI3K/AKT signaling. Moreover, the study further highlights the potential of targeting the PI3K pathway for the treatment of schizophrenia.

The importance of neuregulin in the development of heart and diseases of the cardiovascular system

Neuregulin-1 belongs to the epidermal growth factors (EGF) which have a role in cell proliferation, differentiation and function maintenance of epithelial breast cells, glial cells, neurons and myocytes. It acts in a paracrine and jucstacrine manner through family ErbB tyrosine kinases receptors (ErbB2, ErbB3, and ErbB4) which contain extracellular places for ligands, transmembrane domains and tyrosin kinase domains. NRG-ErbB signaling pathway that is important in embryonic development of the heart also has its role in adult heart. Neuregulin is mainly synthesized in the endocardial endothelial cells and endothelial cells of the myocardial capillaries. ErbB3 receptors are distributed on the endothelial cells and the ErbB2/ErbB4 complex in cardiomyocytes. Soluble NRG-1 induces proliferation of cardiomyocytes in the embryo, increases survival, affects the apoptosis of the culture of cardiomyocytes and induces hypertrophic growth in neonatal and adult ventricular cardiomyocytes. In addition to the effect on the proliferation and differentiation of cardiomyocytes, NRG-1 also promotes the proper differentiation and recruitment of cells of the vascular system of the heart from contractile cardiomyocytes. Endogenous neuregulin may also affect cardiac function by acting on the nekardiomiocite promoting angiogenesis. The cardio-protective effects of neuregulin-1 through nekardiomiocita can be achieved by increasing the activity of cardiac fibroblasts, where neuregilina administration results in reduction of the size of the scar. The ischemic damage of cardiomyocytes and physical strain lead to the activation of the tissue neuregulin 1, which is a potent inducer of endothelial progenitor cell mobilization. Since neuregulin is necessary in the development of the cardiovascular system, regulation of the cardio-protective mechanism of signaling, it is possible that the application of recombinant neuregulin isoforms may have potential application in the treatment of certain cardiac diseases. However, the use of a recombinant NRG-1 can stimulate the proliferation non-cardiac cells as well. The question is whether the use of neuregulin in the cardioprotection will trigger only the receptors intended for that because if NRG-1 is given systemically it can activate the receptors in other tissues and promote unwanted reactions.

Neuregulin1 and ErbB expression in the uninjured and regenerating olfactory mucosa

Neuregulin1, a protein involved in signaling through the ErbB receptors, is required for the proper development of multiple organ systems. A complete understanding of the expression profile of Neuregulin1 is complicated by the presence of multiple isoform variants that result from extensive alternative splicing. Remarkably, these numerous protein products display a wide range of divergent functional roles, making the characterization of tissue-specific isoforms critical to understanding signaling. Recent evidence suggests an important role for Neuregulin1 signaling during olfactory epithelium development and regeneration. In order to understand the physiological consequences of this signaling, we sought to identify the isoform-specific and cell type-specific expression pattern of Neuregulin1 in the adult olfactory mucosa using a combination of RT-qPCR, FACS, and immunohistochemistry. To complement this information, we also analyzed the cell-type specific expression patterns of the ErbB receptors using immunohistochemistry. We found that multiple Neuregulin1 isoforms, containing predominantly the Type I and Type III N-termini, are expressed in the uninjured olfactory mucosa. Specifically, we found that Type III Neuregulin1 is highly expressed in mature olfactory sensory neurons and Type I Neuregulin1 is highly expressed in duct gland cells. Surprisingly, the divergent localization of these Neuregulin isoforms and their corresponding ErbB receptors does not support a role for active signaling during normal turnover and maintenance of the olfactory mucosa. Conversely, we found that injury to the olfactory epithelium specifically upregulates the Neuregulin1 Type I isoform bringing the expression pattern adjacent to cells expressing both ErbB2 and ErbB3 which is compatible with active signaling, supporting a functional role for Neuregulin1 specifically during regeneration.

The Neuregulin1/ErbB system is selectively regulated during peripheral nerve degeneration and regeneration.

The peripheral nervous system has an intrinsic capability to regenerate, crucially related to the ability of Schwann cells (SC) to create a permissive environment, for example, through production of regeneration-promoting neurotrophic factors. Survival, proliferation, migration and differentiation of SC into a myelinating phenotype during development and after injury is regulated by different Neuregulin1 (NRG1) isoforms. This study investigates the expression of different NRG1 isoforms and of their ErbB receptors in distal rat median nerve samples under regenerating conditions after a mild (crush) and more severe (end-to-end repair) injury and under degenerating condition. The expression of the NRG1/ErbB system was evaluated at mRNA and protein level, and demonstrated to be specific for distinct and consecutive phases following nerve injury and regeneration or the progress in degeneration. For the first time a detailed analysis of expression profiles not only of soluble and transmembrane NRG1 isoforms, but also of alpha and beta as well as type a, b and c isoforms is presented. The results of mRNA and protein expression pattern analyses were related to nerve ultrastructure changes evaluated by electron microscopy. In particular, transmembrane NRG1 isoforms are differentially regulated and proteolytically processed under regeneration and degeneration conditions. Soluble NRG1 isoforms alpha and beta, as well as type a and b, are strongly upregulated during axonal regrowth, while type c NRG1 isoform is downregulated. This is accompanied by an upregulation of ErbB receptors. This accurate regulation suggests that each molecule plays a specific role that could be clinically exploited to improve nerve regeneration.

Activation of ERBB3 Receptor Prevents TNFα Production from Human Non‐Classical Monocytes

Immune cells amplify the progression of chronic heart failure (CHF). Recombinant neuregulin‐1 (NRG1) represents a potential therapy for the treatment of various forms of heart failure. We hypothesized that NRG1, acting via ERBB receptors, inhibits pro‐inflammatory cell activation in CHF. Using flow cytometry, we found that peripheral blood monocytes (PBMs) represent a major subpopulation of mononuclear cells expressing both ERBB2 and ERBB3 receptors. A strong inverse correlation was found between expression of ERBB3 and TNFa mRNA in PBMs from patients with heart failure (rs=‐0.682, p=0.001) but not in control subjects (rs=‐0.042, p=0.886). To determine direct effects of ERBB3 signaling on activation of PBMs we used a model of LPS‐induced TNFa production in ex vivo short‐term culture of PBMs. We found that glial growth factor 2(GGF2; USAN ‐ cimaglermin alfa), NRG1 isoform type II, decreased LPS‐induced TNFa production in CD14lowCD16+ non‐classical monocytes. The inhibitory effect of GGF2 on LPS‐induced TNFa production in CD14lowCD16+ monocytes strongly correlated with the expression of ERBB3 (rs=0.769, p=0.013). In contrast to non‐classical PBMs, we found no effect of GGF2 in CD14highCD16‐ subset of classical monocytes irrespective of their expression of ERBB3. Taken together, our results suggest that ERBB3 signaling plays an important role in inhibition of TNFa production from CD14lowCD16+ cells. Given that non‐classical monocytes play a central role in initiation of vascular inflammation, the stimulation of ERBB3 signaling in these cells may potentially slow progression of CHF.

Differential effects of short- and long-term antipsychotic treatment on the expression of neuregulin-1 and ErbB4 receptors in the rat brain

Neuregulin-1 (NRG1) and ErbB4 genes have been identified as candidate genes for schizophrenia. Post-mortem studies indicated that NRG1-ErbB4 signalling is impaired in schizophrenia subjects. This study investigated whether short- or long-term antipsychotic treatment has different effects on the expression of NRG1 and ErbB4 receptors. Female Sprague-Dawley rats were treated orally with either aripiprazole (0.75mg/kg), haloperidol (0.1mg/kg), olanzapine (0.5mg/kg), or vehicle, 3 times/day for 1 or 12 weeks. Western blotting was performed to examine the expression of NRG1 isoforms (135kDa, 70kDa and 40kDa) and ErbB4 receptors. Both 1-week haloperidol and olanzapine treatment increased NRG1-70kDa expression in the hippocampus; haloperidol also up-regulated ErbB4 levels in the prefrontal cortex (PFC). In the 12-week group, aripiprazole decreased the expression of all three NRG1 isoforms and ErbB4 receptors in the PFC, NRG1-70kDa and -40kDa in the cingulate cortex (Cg), and NRG1-135kDa, -70kDa and ErbB4 receptors in the hippocampus; haloperidol reduced NRG1-135kDa in the PFC, NRG1-40kDa in all three brain regions, and ErbB4 receptor levels in the PFC and hippocampus; NRG1-40kDa in the PFC and Cg was also down-regulated by olanzapine. These results suggest that the time-dependent and region-specific effects of antipsychotics on NRG1-ErbB4 signalling may contribute to the efficacy of antipsychotics to treat schizophrenia.

Abstract 607: Cardiac-specific Deletion Of ErbB4 In The Adult Mouse Increases Cardiac Cell Proliferation And Causes Physiological Cardiac Hypertrophy

The epidermal growth factor receptor type 4 (ErbB4) is a receptor tyrosine kinase that is activated by the growth factor neuregulin 1 (NRG1). ErbB4 is essential for cardiac development and promotes cardiomyocyte survival in culture. To investigate the physiological importance of ErbB4 in the adult heart, we generated a mouse model with conditional deletion of ErbB4 in cardiomyocytes using a tamoxifen-inducible Cre recombinase (MHC-MerCreMer). Adult MHC-MerCreMer/ErbB4 fl/fl (ErbB4 conditional knockout, cKO, n=6) and MHC-MerCreMer/ErbB4 WT/WT (control, n=8) animals were injected with tamoxifen (20 mg/kg/day ip for 10 days). Ten days after tamoxifen treatment, cardiac specific deletion of ErbB4 was confirmed by qPCR. At 3 months after ErbB4 deletion, echocardiography showed no differences in cardiac function (fractional shortening), however the heart weight:tibia length ratio was increased by 45% in the cKO animals (5.1±0.2 vs 7.4±0.6 mg/mm, P <0.05 vs controls). Cardiac-specific ErbB4 deletion did not increase expression of genes associated with pathological hypertrophy (BNP, α-MHC:β-MHC ratio), suggesting that the hypertrophy seen in the cKO animals may be physiological rather than pathological. Similarly, cardiac-specific ErbB4 deletion did not cause cardiac fibrosis (Masson’s Trichrome) or alter fibrotic gene expression (Col1A1, Col3A1 and PAI1). Cardiomyocyte cross sectional area was measured using wheat germ agglutinin staining, and showed no differences between controls and cKO. Interestingly, cardiac mRNA levels of both isoforms of the endogenous ErbB4 agonist NRG1 were strongly increased in cKO animals (fold change compared to controls: 5.3±1.7 for NRG1α; 4.3±1.2 for NRG1β, P <0.01) and this correlated with a significant increase in the total number of cells positive for a marker of proliferation (phosphorylated-histone H3; 1.8±0.4 vs 8.2±2.8 cells/section, P <0.05). Conclusion: Cardiac-specific deletion of ErbB4 in the adult increases heart weight without altering cardiac function, cardiomyocyte size or fibrosis. ErbB4 deletion also increases NRG1 expression, which may contribute to the maintenance of cardiac function and the development of cardiac hypertrophy via cell proliferation.

Association of the 3′ region of the neuregulin 1 gene with bipolar I disorder in the Chinese Han population

BackgroundBased on the function of neuregulin 1 (NRG1) in neurodevelopment, susceptibility to bipolar disorder presumably involves this gene. The 3′ region of NRG1 contains the majority of the coding exons, and transcripts from this region encode 8 of the 9 known NRG1 isoforms; therefore, this region is likely to be predominant versus the 5′ region in terms of their relative contributions to NRG1 function. We investigated the association between the 3′ region of the NRG1 gene and bipolar I disorder (BPI) in the Chinese Han population and performed further analyses depending on the presence or absence of psychotic features. MethodsA total of 385 BPI patients and 475 healthy controls were recruited for this study. Thirty tag single nucleotide polymorphisms (SNPs) across the 3′ region of the NRG1 gene were genotyped for allelic and haplotypic associations with BPI and subgroups with psychotic features (BPI-P) or without psychotic features (BPI-NP). ResultsIndividual marker analysis showed that 2 SNPs (rs12547858 and rs6468121) in this region were significantly associated with BPI. Moreover, subgroup analyses showed significant but marginal associations of rs6468121 with BPI-P and rs3757933 with BPI-NP. Haplotype analyses showed that 6 haplotypes were associated with BPI only. LimitationsThe sample size was relatively small. The investigated tag SNPs only represented 83% of the information on the targeted region. There might be a retrospective bias in the subgroup analyses. ConclusionThe results suggest that the 3′ region of the NRG1 gene plays a role in BPI susceptibility in the Chinese Han population. In addition, the preliminary results show that BPI with psychotic features and BPI without psychotic features may constitute different sub-phenotypes; however, this finding should be confirmed in a larger population sample.

Neuregulin 1 isoforms could be an effective therapeutic candidate to promote peripheral nerve regeneration.

Traumatic injuries of peripheral nerves represent common casualties and their social impact is considerably high. Although peripheral nerves retain a good regeneration potential, the clinical outcome after nerve lesion is far from being satisfactory and functional recovery is almost never complete, especially in the case of large nerve defects, that result in loss or diminished sensitivity and/or motor activity of the innervated target organs. Therefore, to improve the outcome after nerve damage, or in peripheral neuropathies, there is a need for further research in nerve repair and regeneration to identify factors that promote axonal regrowth, remyelination and target reinnervation. Among the different factors involved in these processes (Taveggia et al., 2010; Pereira et al., 2012), stands out neuregulin 1 (NRG1), a factor which plays a role both in the myelination occurring during development (Lemke, 2006) and in the response to peripheral nerve injury (Syed and Kim, 2010; Fricker and Bennett, 2011). NRG1 is a pleiotropic factor characterized by the existence of numerous isoforms arising from alternative splicing of exons that confer to the protein with deeply different characteristics (Falls, 2003; Mei and Xiong, 2008). NRG1 can be produced as a secreted or as a transmembrane protein ready to interact with its receptor, or as a transmembrane pro-protein that needs a proteolytic cleavage to release a soluble fragment or to protrude its receptor binding domain in the extracellular environment (Figure 1). According to its structure, NRG1 signals in a paracrine, autocrine or juxtacrine manner; moreover, juxtacrine interactions can signal both in a forward and reverse manner due to the production of a fragment containing the intracellular domain (ICD, Figure 1B) that can translocate into the nucleus and influence gene transcription (Bao et al., 2003; Bao et al., 2004; Chen et al., 2010). NRG1 interacts directly with two of the four members of the tyrosine kinase receptor family ErbB: ErbB4, that signals as homo or heterodimer, and ErbB3, that forms a heterodimer with ErbB2. Figure 1 Structure of soluble or transmembrane neuregulin 1 (NRG1) isoforms. In the peripheral nervous system, NRG1 soluble isoforms are mainly released by Schwann cells, while transmembrane isoforms are mainly expressed by the axon and both interact with the heterodimer receptor ErbB2-ErbB3, generally expressed by Schwann cells. NRG1 plays an important role both in the myelination occurring during development and in the different phases occurring after injury in the peripheral nerve: axon degeneration, axon regrowth, remyelination and target reinnervation (Taveggia et al., 2010; Fricker and Bennett, 2011; Pereira et al., 2012; Salzer, 2012; Gambarotta et al., 2013; Heermann and Schwab, 2013). These processes respond to different cues, as can be inferred from the analysis of transgenic mice models summarized in Figure 2. The difference between the myelination process occurring during development and the regeneration process occurring after nerve injury is underlined by the fact that soluble NRG1 isoforms play an important role after nerve injury, while their lack seems irrelevant during development. Figure 2 The role played by soluble and transmembrane neuregulin 1 (NRG1) isoforms in the myelination occurring during development and in the different phases occurring after nerve injury (axon degeneration, axon regeneration, remyelination and target reinnervation) ...

Neuregulin‐1/glial growth factor stimulates Schwann cell migration by inducing α5 β1 integrin–ErbB2–focal adhesion kinase complex formation

After peripheral nerve injury, Schwann cells gain a migratory phenotype and remodel their extracellular matrix to provide a supportive environment for axonal regeneration. The soluble neuregulin-1 isoform, that is, glial growth factor (GGF), is expressed in regenerating axons of injured peripheral nerves and regulates Schwann cell motility by activating the ErbB family of tyrosine kinase receptors, but how GGF/ErbB signaling contributes to Schwann cell motility remains unclear. Here, we show that GGF stimulates Schwann cell migration by inducing the formation of a protein complex containing the fibronectin receptor α5β1 integrin, ErbB2, and focal adhesion kinase (FAK). ErbB2 co-localizes and co-immunoprecipitates with the focal complex members including α5β1 integrin and FAK after GGF treatment. These effects of GGF appear to involve FAK activation, which occurs downstream of ErbB2 stimulation. RNAi-mediated down-regulation of α5 integrin expression in primary cultured Schwann cells resulted in significantly decreased interaction between FAK and ErbB2, as well as decreased GGF-induced migration. An increase in the α5β1 integrin-ErbB2-FAK complex formation was observed in injured nerve Schwann cells, but not uninjured control. Taken together, these data suggest that GGF plays an important modulatory role in Schwann cell migration after nerve crush by inducing α5β1 integrin-ErbB2-FAK complex formation.

Neuregulin as a Heart Failure Therapy and Mediator of Reverse Remodeling

The beta isoform of Neuregulin-1 (NRG-1β), along with its receptors (ErbB2-4), is required for cardiac development. NRG-1β, as well as the ErbB2 and ErbB4 receptors, is also essential for maintenance of adult heart function. These observations have led to its evaluation as a therapeutic for heart failure. Animal studies and ongoing clinical trials have demonstrated beneficial effects of two forms of recombinant NRG-1β on cardiac function. In addition to the possible role for recombinant NRG-1βs as heart failure therapies, endogenous NRG-1β/ErbB signaling appears to play a role in restoring cardiac function after injury. The potential mechanisms by which NRG-1β may act as both a therapy and a mediator of reverse remodeling remain incompletely understood. In addition to direct effects on cardiac myocytes NRG-1β acts on the vasculature, interstitium, cardiac fibroblasts, and hematopoietic and immune cells, which, collectively, may contribute to NRG-1β's role in maintaining cardiac structure and function, as well as mediating reverse remodeling.

Differential expression of neuregulin-1 isoforms and downregulation of erbin are associated with Erb B2 receptor activation in diabetic peripheral neuropathy

BackgroundAberrant neuron/glia interactions can contribute to a variety of neurodegenerative diseases and we have previously demonstrated that enhanced activation of Erb B2, which is a member of the epidermal growth factor receptor (EGFR) family, can contribute to the development of diabetic peripheral neuropathy (DPN). In peripheral nerves, Erb B receptors are activated by various members of the neuregulin-1 (NRG1) family including NRG1 Type I, NRG1 Type II and NRG1 Type III to regulate Schwann cell (SC) growth, migration, differentiation and dedifferentiation. Alternatively, Erb B2 activity can be negatively regulated by association with the Erb B2-interacting protein, erbin. Since the effect of diabetes on the expression of NRG1 isoforms and erbin in peripheral nerve are unknown, the current study determined whether changes in NRG1 isoforms and erbin may be associated with altered Erb B2 signaling in DPN.ResultsSwiss Webster mice were rendered diabetic with streptozotocin (STZ) and after 12 weeks of diabetes, treated with erlotinib, an inhibitor of Erb B2 activation. Inhibition of Erb B2 signaling partially reversed several pathophysiologic aspects of DPN including a pronounced sensory hypoalgesia, nerve conduction velocity deficits and the decrease in epidermal nerve fiber innervation. We also observed a decrease of NRG1 Type III but an increase of NRG1 Type I level in diabetic sural nerves at early stage of diabetes. With disease progression, we detected reduced erbin expression and enhanced MAPK pathway activity in diabetic mice. Inhibition of Erb B2 receptor suppressed MAPK pathway activity in treated-diabetic sural nerves.ConclusionsThese results support that hyperglycemia may impair NRG1/Erb B2 signaling by disrupting the balance between NRG1 isoforms, decreasing the expression of erbin and correspondingly activating the MAPK pathway. Together, imbalanced NRG1 isoforms and downregulated erbin may contribute to the dysregulation of Erb B2 signaling in the development of DPN.