Activation of microglial neuregulin1 signaling in the corticospinal tracts of ALS patients with upper motor neuron signs

Multiple Roles for Neuregulins and Their ERBB Receptors in Neurodegenerative Disease Pathogenesis and Therapy

The role that neurotrophins, such as nerve growth factor, play in the pathogenesis of neurodegenerative diseases has long been appreciated. However, the neuregulin (NRG) family of growth factors and/or their v-erb-B2 avian erythroblastic leukemia viral oncogene homolog (ERBB) receptors have also been implicated in the pathogenesis of conditions, such as Alzheimer disease (AD), frontotemporal lobar degeneration (FTLD), and amyotrophic lateral sclerosis (ALS). In this review, we consider i) the structural variability of NRG isoforms generated by alternative RNA splicing, the use of multiple promoters and proteolysis, and the impact that this structural variability has on neuronal and glial physiology during development and adulthood. We discuss ii) the NRG receptors ERBB2, ERBB3, and ERBB4, how activation of each of these receptors further diversifies NRG actions in the central nervous system, and how dementia-related proteins, such as γ-secretase modulate the action of NRGs and their ERBB receptors. We then iii) turn to the abnormalities in NRG and ERBB expression and function evident in human AD and mouse AD models, how these abnormalities affect brain function, and attempts to use NRGs to treat AD. Finally, iv) we discuss NRG effects on the survival and function of neurons relevant to FTLD and ALS, alterations in NRG/ERBB signaling identified in these conditions, and the recent discovery of multiple human pedigrees in which autosomal dominant FTLD/ALS potentially results from point mutations in ERBB4.

Prevention of Alzheimer Pathology by Blocking Neuregulin Signaling on Microglia

Plaque formation, microglial activation, and synaptic loss are pathologic hallmarks of Alzheimer's disease; however, removing plaques has had little clinical benefit. Here, we show that neuregulin-1, a glial growth factor, induces inflammatory cytokines and promotes phagocytic activity in vitro and augments microglial activation and plaque formation in 5XFAD Alzheimer's mice. Brain-specific targeting of neuregulin-1 by intraventricular delivery of a novel neuregulin-1 fusion protein antagonist, GlyB4, significantly alters microglial morphology and function to a nonpathogenic morphology in early-stage 5XFAD mice and prevents plaques from forming. Once plaques have already formed, GlyB4 reduces new plaque formation and prevents synaptic loss. Selective, targeted disruption of neuregulin-1 signaling on brain microglia with GlyB4 could be a novel "upstream" approach to slow or stop disease progression in Alzheimer's disease.

Disease propagation in amyotrophic lateral sclerosis (ALS): an interplay between genetics and environment

Amyotrophic lateral sclerosis (ALS) is a progressive, fatal disease affecting the neuromuscular system. While there have been a number of important genetic discoveries, there are no therapeutics capable of stopping its insidious progression. Lessons from clinical histories reveal that ALS can start focally at a single limb, but then segmentally spread up and down the spinal cord as well as in the motor cortex and cortex of frontal and temporal lobes until respiratory muscles fail. With or without a clear genetic etiology, often there is no explanation as to why it starts in one region of the body versus another. Similarly, once the disease starts the mechanisms by which the neurodegenerative process spreads are not known. Here, we summarize recent work in animal models that support the hypothesis that critical environmental contributions, such as a nerve injury, can initiate the disease process. We also propose that pathological axoglial signaling by the glial growth factor neuregulin-1 leads to the slow propagation of neuroinflammation resulting in neurodegeneration up and down the spinal cord and that locally applied drugs that block neuregulin-1 signaling could slow or halt the spread of disease.

Predicting disease specific spinal motor neurons and glia in sporadic ALS

Increasing evidence shows that neuroinflammation mediated by activated glia and infiltrated immune cells is involved in the pathogenesis of sporadic amyotrophic lateral sclerosis (sALS). However, the mechanisms of interaction between activated glia and motor neuron degeneration are unclear. To determine the relationship between motor neurons and glial activation in the central nervous system of sALS patients, we applied new cellular interactome bioinformatics tools to transcriptome profiles established from laser captured motor neurons in regions remote from site of onset. We found a disease specific subtype of motor neuron that inversely correlated with survival of sALS patients. Interestingly, two subtypes of motor neurons (motorneuron2 and 3) and two subtypes of microglia/macrophages (microglia/macrophage1 and 2) were unique to sALS patients compared to controls. Increased microglia/macrophage1 correlated with decreased motorneuron2 and increased microglia/macrophage2 correlated with decreased motor neuron3. Increased MHC class II genes correlated with microglia/macrophage1-2. Tissue staining using immunofluorescence confirmed a significant increase of microglia/macrophage expressing MHC class II, suggesting that they were activated. Identified gene pathways and biological changes included apoptosis and protein phosphorylation in motorneuron3 and antigen processing/presentation and immune cell activation in microglia/macrophages in sALS patients. Our findings support the hypothesis that neuro-glia physical interactions are important in pathogenesis, and targeting disease-specific motor neurons and/or glia could be a useful therapy to slow disease progression.

Neuregulin-1/ErbB network: An emerging modulator of nervous system injury and repair

Neuregulin-1 (Nrg-1) is a member of the Neuregulin family of growth factors with essential roles in the developing and adult nervous system. Six different types of Nrg-1 (Nrg-1 type I-VI) and over 30 isoforms have been discovered; however, their specific roles are not fully determined. Nrg-1 signals through a complex network of protein-tyrosine kinase receptors, ErbB2, ErbB3, ErbB4 and multiple intracellular pathways. Genetic and pharmacological studies of Nrg-1 and ErbB receptors have identified a critical role for Nrg-1/ErbB network in neurodevelopment including neuronal migration, neural differentiation, myelination as well as formation of synapses and neuromuscular junctions. Nrg-1 signaling is best known for its characterized role in development and repair of the peripheral nervous system (PNS) due to its essential role in Schwann cell development, survival and myelination. However, our knowledge of the impact of Nrg-1/ErbB on the central nervous system (CNS) has emerged in recent years. Ongoing efforts have uncovered a multi-faceted role for Nrg-1 in regulating CNS injury and repair processes. In this review, we provide a timely overview of the most recent updates on Nrg-1 signaling and its role in nervous system injury and diseases. We will specifically highlight the emerging role of Nrg-1 in modulating the glial and immune responses and its capacity to foster neuroprotection and remyelination in CNS injury. Nrg-1/ErbB network is a key regulatory pathway in the developing nervous system; therefore, unraveling its role in neuropathology and repair can aid in development of new therapeutic approaches for nervous system injuries and associated disorders.

Localization and dynamic changes of neuregulin-1 at C-type synaptic boutons in association with motor neuron injury and repair

C-type synaptic boutons (C-boutons) provide cholinergic afferent input to spinal cord motor neurons (MNs), which display an endoplasmic reticulum (ER)-related subsurface cistern (SSC) adjacent to their postsynaptic membrane. A constellation of postsynaptic proteins is clustered at C-boutons, including M2 muscarinic receptors, potassium channels, and σ-1 receptors. In addition, we previously found that neuregulin (NRG)1 is associated with C-boutons at postsynaptic SSCs, whereas its ErbB receptors are located in the presynaptic compartment. C-bouton-mediated regulation of MN excitability has been implicated in MN disease, but NRG1-mediated functions and the impact of various pathologic conditions on C-bouton integrity have not been studied in detail. Here, we investigated changes in C-boutons after electrical stimulation, pharmacological treatment, and peripheral nerve axotomy. SSC-linked NRG1 clusters were severely disrupted in acutely stressed MNs and after tunicamycin-induced ER stress. In axotomized MNs, C-bouton loss occurred in concomitance with microglial recruitment and was prevented by the ER stress inhibitor salubrinal. Activated microglia displayed a positive chemotaxis to C-boutons. Analysis of transgenic mice overexpressing NRG1 type I and type III isoforms in MNs indicated that NRG1 type III acts as an organizer of SSC-like structures, whereas NRG1 type I promotes synaptogenesis of presynaptic cholinergic terminals. Moreover, MN-derived NRG1 signals may regulate the activity of perineuronal microglial cells. Together, these data provide new insights into the molecular and cellular pathology of C-boutons in MN injury and suggest that distinct NRG1 isoform-mediated signaling functions regulate the complex matching between pre- and postsynaptic C-bouton elements.-Salvany, S., Casanovas, A., Tarabal, O., Piedrafita, L., Hernández, S., Santafé, M., Soto-Bernardini, M. C., Calderó, J., Schwab, M. H., Esquerda, J. E. Localization and dynamic changes of neuregulin-1 at C-type synaptic boutons in association with motor neuron injury and repair.

CNS-derived extracellular vesicles from superoxide dismutase 1 (SOD1)G93A ALS mice originate from astrocytes and neurons and carry misfolded SOD1

Extracellular vesicles (EVs) are secreted by myriad cells in culture and also by unicellular organisms, and their identification in mammalian fluids suggests that EV release also occurs at the organism level. However, although it is clearly important to better understand EVs' roles in organismal biology, EVs in solid tissues have received little attention. Here, we modified a protocol for EV isolation from primary neural cell culture to collect EVs from frozen whole murine and human neural tissues by serial centrifugation and purification on a sucrose gradient. Quantitative proteomics comparing brain-derived EVs from nontransgenic (NTg) and a transgenic amyotrophic lateral sclerosis (ALS) mouse model, superoxide dismutase 1 (SOD1)^G93A, revealed that these EVs contain canonical exosomal markers and are enriched in synaptic and RNA-binding proteins. The compiled brain EV proteome contained numerous proteins implicated in ALS, and EVs from SOD1^G93A mice were significantly depleted in myelin-oligodendrocyte glycoprotein compared with those from NTg animals. We observed that brain- and spinal cord-derived EVs, from NTg and SOD1^G93A mice, are positive for the astrocyte marker GLAST and the synaptic marker SNAP25, whereas CD11b, a microglial marker, was largely absent. EVs from brains and spinal cords of the SOD1^G93A ALS mouse model, as well as from human SOD1 familial ALS patient spinal cord, contained abundant misfolded and nonnative disulfide-cross-linked aggregated SOD1. Our results indicate that CNS-derived EVs from an ALS animal model contain pathogenic disease-causing proteins and suggest that brain astrocytes and neurons, but not microglia, are the main EV source.

Neuregulin1 modulation of experimental autoimmune encephalomyelitis (EAE)

Neuregulin1 (NRG1) is a differentiation factor that regulates glial development, survival, synaptogenesis, axoglial interactions, and microglial activation. We previously reported that a targeted NRG1 antagonist (HBD-S-H4) given intrathecally, reduces inflammatory microglial activation in a spinal cord pain model and a neurodegenerative disease mouse model in vivo, suggesting that it may have effects in neuroninflammatory and neuronal disorders. We hypothesized that expression of HBD-S-H4 in the central nervous system (CNS) could reduce disease severity in experimental autoimmune encephalomyelitis (EAE), a widely used animal model for multiple sclerosis (MS). In the present study, we generated tetO-HBD-S-H4, a single transgenic (Tg) mouse line in, which the fusion protein in expressed in the brain, resulting in reduction of disease severity in both male and female mice when compared to sex- and age-matched wild type littermates. We also generated GFAP-tTA:tetO-HBD-S-H4 double Tg mice, which express this fusion protein in the brain and the spinal cord, they displayed sex differences in the reduction of disease severity. In healthy mice, expression of HBD-S-H4 in the CNS does not result in any significant neurological or other overt phenotypes. In myelin oligodendrocyte glycoprotein (MOG)-induced EAE, female double Tg mice show delayed disease onset and reduced disease severity compared to male double Tg as well as wild type littermates. In male double Tg mice, the levels of HBD-S-H4 gene expression negatively correlates with disease severity and increased microglia associated genes' expression. In conclusion, expression of neuregulin antagonist in the brain and spinal cord protects females but not males, suggesting a complex interplay between NRG1 and sex difference in EAE that may be associated with microglia-mediated inflammation. This study provides important information for understanding the heterogeneity of disease pathology and the therapeutic potential of targeting microglial activation in male and female MS patients.

Slowing disease progression in the SOD1 mouse model of ALS by blocking neuregulin-induced microglial activation

There are no effective treatments to slow disease progression in ALS. We previously reported that neuregulin (NRG) receptors are constitutively activated on microglia in the ventral horns in both ALS patients and SOD1 mice and in the corticospinal tracts of ALS patients, and that NRG receptor activation occurs prior to significant clinical disease onset in SOD1 mice. Here, we hypothesize that blocking NRG signaling on microglia would slow disease progression in SOD1 mice using a targeted NRG antagonist (HBD-S-H4). Recombinant HBD-S-H4 directly delivered into the central nervous system (CNS) through implanted intracerebroventricular cannulas showed no signs of toxicity and significantly inhibited NRG receptor activation on microglia resulting in reduced microglial activation and motor neuron loss. The treatment also resulted in a delay in disease onset and an increase in survival. The therapeutic effect was dose-dependent that varied as a function of genetic background in two different strains of SOD1 mice. As a complementary drug delivery approach, transgenic mice expressing HBD-S-H4 driven by an astrocytic promoter (GFAP) had slower disease progression in a dose dependent manner, based on the level of HBD-S-H4 expression. These studies provide mechanistic insights into how NRG signaling on microglia may lead to disease progression and demonstrate the utility of a humanized fusion protein that blocks NRG as a novel therapeutic for human ALS.

Neuregulin1 fine-tunes pre-, post-, and perisynaptic neuromuscular junction development

BACKGROUND

Neuromuscular junction (NMJ) development is a multistep process mediated by coordinated interactions between the nerve terminal, target muscle, and perisynaptic Schwann cell that require constant back-and-forth communication. Retrograde and anterograde growth and differentiation factors have been postulated to participate in this communication. While neuregulin1 (NRG1) has been shown to be potent anterograde signal that activates acetylcholine receptor (AChR) transcription and clustering in vitro, its roles in NMJ development in vivo remain elusive.

RESULTS

Using the model of chicken embryo, we measured the effects of NRG1 signaling during NMJ development in ovo using quantitative, sequential measures of AChR cluster size and density, pre- and postsynaptic apposition, and the alignment of perisynaptic Schwann cells. Using in ovo electroporation at early stages and a targeted soluble neuregulin antagonist through all developmental stages, we found soluble NRG1 regulates AChR cluster density and size at the earliest stage prior to nerve-AChR cluster contact. Once the nerve contacts with muscle AChRs, NRG1 has pronounced effects on presynaptic specialization and on the alignment of perisynaptic Schwann cells at endplates.

CONCLUSION

These findings suggest that, while NRG1 may not be critical for overall development, it appears to be important in fine-tuning pre-, post-, and perisynaptic development of the NMJ. Developmental Dynamics 246:368-380, 2017. © 2017 Wiley Periodicals, Inc.