Cleavage of neuregulin-1 by BACE1 or ADAM10 protein produces differential effects on myelination

ADAM Metallopeptidase domain 19 promotes skin fibrosis in systemic sclerosis via neuregulin-1

BACKGROUND

ADAM19 (ADAM Metallopeptidase Domain 19) is known to be involved in extracellular matrix (ECM) remodeling, yet its specific function in systemic sclerosis (SSc) fibrosis remains unclear.

OBJECTIVES

This study sought to clarify the role and underlying mechanism of ADAM19 in SSc skin fibrosis.

METHODS

The expression of ADAM19 was assessed in skin tissues of SSc and wound healing using publicly available transcriptome datasets. This analysis was further validated through real-time PCR, western blot, and immunostaining in our SSc cohort, as well as in a mouse model of hypochlorite (HOCl)-induced fibrosis. To downregulate the expression of ADAM19, ADAM19 siRNA was employed. The influence of ADAM19 on fibroblast transcriptomics was examined using bulk RNA-seq. Data analysis and visualization were conducted using R packages, including edgeR, limma, clusterProfiler, ggplot2, gseaplot2, and complexheatmap.

RESULTS

ADAM19 exhibited a significant upregulation in skin tissues of SSc patients, as well as in wound healing and a HOCl-induced fibrosis mouse model. Additionally, there was a notable positive correlation between ADAM19 and fibrosis-related genes, local skin score, Modified Rodnan skin score, skin thickness progression rate, and the presence of ARA antibodies in SSc patients. Furthermore, ADAM19 levels were markedly elevated in SSc primary dermal fibroblasts and TGF-β-stimulated healthy controls primary dermal fibroblasts. The downregulation of ADAM19 resulted in the repression of TGF-β-induced ECM deposition and fibroblast activation. ADAM19 was identified as a mediator for the shedding of neuregulin-1 (NRG1) in fibroblasts, a pro-fibrotic cytokine that must be cleaved to exert its function.

CONCLUSION

ADAM19 plays a role in TGF-β-induced ECM deposition and fibroblast activation by mediating the shedding of NRG1, ultimately contributing to the development of skin fibrosis in SSc.

Overcoming brain-derived therapeutic resistance in HER2+ breast cancer brain metastasis

Brain metastasis of HER2+ breast cancer occurs in about 50% of all women with metastatic HER2+ breast cancer and confers poor prognosis for patients. Despite effective HER2-targeted treatments of peripheral HER2+ breast cancer with Trastuzumab +/-HER2 inhibitors, limited brain permeability renders these treatments inefficient for HER2+ breast cancer brain metastasis (BCBM). The scarcity of suitable patient-derived in-vivo models for HER2+ BCBM has compromised the study of molecular mechanisms that promote growth and therapeutic resistance in brain metastasis. We have generated and characterized new HER2+ BCBM cells (BCBM94) isolated from a patient HER2+ brain metastasis. Repeated hematogenic xenografting of BCBM94 consistently generated BCBM in mice. The clinically used receptor tyrosine kinase inhibitor (RTKi) Lapatinib blocked phosphorylation of all ErbB1-4 receptors and induced the intrinsic apoptosis pathway in BCBM94. Neuregulin-1 (NRG1), a ligand for ErbB3 and ErbB4 that is abundantly expressed in the brain, was able to rescue Lapatinib-induced apoptosis and clonogenic ability in BCBM94 and in HER2+ BT474. ErbB3 was essential to mediate the NRG1-induced survival pathway that involved PI3K-AKT signalling and the phosphorylation of BAD at serine 136 to prevent apoptosis. High throughput RTKi screening identified the brain penetrable Poziotinib as highly potent compound to reduce cell viability in HER2+ BCBM in the presence of NRG1. Successful in-vivo ablation of BCBM94- and BT474-derived HER2+ brain tumors was achieved upon two weeks of treatment with Poziotinib. MRI revealed BCBM remission upon poziotinib, but not with Lapatinib treatment. In conclusion, we have established a new patient-derived HER2+ BCBM in-vivo model and identified Poziotinib as highly efficacious RTKi with excellent brain penetrability that abrogated HER2+ BCBM brain tumors in our mouse models.

A Novel Form of Neuregulin 1 Type III Caused by N-Terminal Processing

Nrg1 (Neuregulin 1) type III, a susceptible gene of schizophrenia, exhibits a critical role in the central nervous system and is essential at each stage of Schwann's cell development. Nrg1 type III comprises double-pass transmembrane domains, with the N-terminal and C-terminal localizing inside the cells. The N-terminal transmembrane helix partially overlaps with the cysteine-rich domain (CRD). In this study, Nrg1 type III constructs with different tags were transformed into cultured cells to verify whether CRD destroyed the transmembrane helix formation. We took advantage of immunofluorescent and immunoprecipitation assays on whole cells and analyzed the N-terminal distribution. Astonishingly, we found that a novel form of Nrg1 type III, about 10% of Nrg1 type III, omitted the N-terminal transmembrane helix, with the N-terminal positioning outside the membrane. The results indicated that the novel single-pass transmembrane status was a minor form of Nrg1 type III caused by N-terminal processing, while the major form was a double-pass transmembrane status.

Type III NRG-1 plays a regulatory role in the regeneration process of nerves from the beginning of transplantation

The present study investigated the effect of type III Neuregulin-1 (NRG-1) on changes in the myelin sheath and the recovery of nerve function during the regeneration process following autologous nerve transplantation. Seventy-two Sprague-Dawley rats were divided into a Blank, Model and (antisense oligonucleotide, ASON) group. The Model and ASON groups of SD rats were subjected to autologous nerve transplantation, and the Blank group only had the sciatic nerve exposed. The Model and ASON groups were given local injections of 2 ml PBS buffer solution and 2 ml ASON of Type III NRG-1, respectively, the NRG-1 type III was inhibited by ASON. Changes in the sciatic nerve functional index (SFI) and conduction velocities were observed at different 6 time points. Regeneration of the myelin sheath was observed using transmission electron microscopy. Type III NRG-1 protein was detected using Western blotting and immunohistochemistry, and NRG-1 mRNA was detected using PCR. The SFI of the ASON group was lower than the Model group after transplantation. The conduction velocities of the ASON group on the 14th and 21st days after autologous nerve transplantation were lower than the Model group (P < 0.01). The protein and mRNA expression of type III NRG-1 in the ASON group was lower than the Model group at all 6 time points. The area of medullated nerve fibres was significantly different between the ASON group and the Model group on the 3rd day (P < 0.05), as was the number of medullated nerve fibres per unit area (P < 0.01). The diameter of axons was obviously different between the two groups (P < 0.01). Type III NRG-1 played an important regulatory role in the regeneration process of the nerve from the beginning of transplantation to the 28th day.

BACE1-dependent metabolism of neuregulin 1: Bridging the gap in explaining the occurrence of schizophrenia-like symptoms in Alzheimer's disease with psychosis?

Alzheimer's disease is a neurodegenerative disease mainly characterized by cortico-neuronal atrophy, impaired memory and other cognitive declines. On the other hand, schizophrenia is a neuro-developmental disorder with an overtly active central nervous system pruning system resulting into abrupt connections with common symptoms including disorganised thoughts, hallucination and delusion. Nevertheless, the fronto-temporal anomaly presents itself as a common denominator for the two pathologies. There is even a strong presumption of increased risk of developing co-morbid dementia for schizophrenic individuals and psychosis for Alzheimer's disease patients, overall leading to a further deteriorated quality of life. However, convincing proofs of how these two disorders, although very distant from each other when considering their aetiology, develop coexisting symptoms is yet to be resolved. At the molecular level, the two primarily neuronal proteins β-amyloid precursor protein and neuregulin 1 have been considered in this relevant context, although the conclusions are for the moment only hypotheses. In order to propose a model for explaining the psychotic schizophrenia-like symptoms that sometimes accompany AD-associated dementia, this review projects out on the similar sensitivity shared by these two proteins regarding their metabolism by the β-site APP cleaving enzyme 1.

A Disintegrin and Metalloproteinase 10 (ADAM10) Is Essential for Oligodendrocyte Precursor Development and Myelination in the Mouse Brain

A disintegrin and metalloproteinase 10 (ADAM10) plays an essential role in the regulation of survival, proliferation, migration, and differentiation of various neural cells. Nevertheless, the role of ADAM10 in oligodendrocyte precursors (OPCs) and myelination in the central nervous system (CNS) of developing and adult mouse brains is still unknown. We generated ADAM10 conditional knockout (ADAM10 cKO) mice lacking the ADAM10 gene primarily in OPCs by crossing NG2-Cre mice with ADAM10 ^loxp/loxp mice. We found that OPCs expressed ADAM10 in the mouse corpus callosum and the hippocampus. ADAM10 cKO mice showed significant loss of back hair and reduction in weight and length on postnatal (30 ± 2.1) day, died at (65 ± 5) days after birth, and exhibited the "anxiety and depression-like" performances. Conditional knockout of ADAM10 in OPCs resulted in a prominent increase in myelination and a decrease in the number of OPCs in the corpus callosum at P30 owing to premyelination and lack of proliferation of OPCs. Moreover, the number of proliferating OPCs and mature oligodendrocytes (OLs) also decreased with age in the corpus callosum of ADAM10 cKO mice from P30 to P60. Western blot and RT-PCR results showed that the activation of Notch-1 and its four target genes, Hes1, Hes5, Hey1, and Hey2, was inhibited in the corpus callosum tissue of ADAM10 knockout mice. In our study, we provided experimental evidence to demonstrate that ADAM10 is essential for modulating CNS myelination and OPC development by activating Notch-1 signaling in the developing and adult mouse brain.

Myelin repair in Alzheimer's disease: a review of biological pathways and potential therapeutics

This literature review investigates the significant overlap between myelin-repair signaling pathways and pathways known to contribute to hallmark pathologies of Alzheimer's disease (AD). We discuss previously investigated therapeutic targets of amyloid, tau, and ApoE, as well as other potential therapeutic targets that have been empirically shown to contribute to both remyelination and progression of AD. Current evidence shows that there are multiple AD-relevant pathways which overlap significantly with remyelination and myelin repair through the encouragement of oligodendrocyte proliferation, maturation, and myelin production. There is a present need for a single, cohesive model of myelin homeostasis in AD. While determining a causative pathway is beyond the scope of this review, it may be possible to investigate the pathological overlap of myelin repair and AD through therapeutic approaches.

Kallikrein 8: A key sheddase to strengthen and stabilize neural plasticity

Neural networks are modified and reorganized throughout life, even in the matured brain. Synapses in the networks form, change, or disappear dynamically in the plasticity state. The pre- and postsynaptic signaling, transmission, and structural dynamics have been studied considerably well. However, not many studies have shed light on the events in the synaptic cleft and intercellular space. Neural activity-dependent protein shedding is a phenomenon in which (1) presynaptic excitation evokes secretion or activation of sheddases, (2) sheddases are involved not only in cleavage of membrane- or matrix-bound proteins but also in mechanical modulation of cell-to-cell connectivity, and (3) freed activity domains of protein factors play a role in receptor-mediated or non-mediated biological actions. Kallikrein 8/neuropsin (KLK8) is a kallikrein family serine protease rich in the mammalian limbic brain. Accumulated evidence has suggested that KLK8 is an important modulator of neural plasticity and consequently, cognition. Insufficiency, as well as excess of KLK8 may have detrimental effects on limbic functions.

Plasma neuregulin 1 as a synaptic biomarker in Alzheimer's disease: a discovery cohort study

BACKGROUND

Synaptic dysfunction is an early core feature of Alzheimer's disease (AD), closely associated with cognitive symptoms. Neuregulin 1 (NRG1) is a growth and differentiation factor with a key role in the development and maintenance of synaptic transmission. Previous reports have shown that changes in cerebrospinal fluid (CSF) NRG1 concentration are associated with cognitive status and biomarker evidence of AD pathology. Plasma biomarkers reflecting synaptic impairment would be of great clinical interest.

OBJECTIVE

To measure plasma NRG1 concentration in AD patients in comparison with other neurodegenerative disorders and neurological controls (NC) and to study its association with cerebrospinal fluid (CSF) core AD and synaptic biomarkers.

METHODS

This retrospective study enrolled 127 participants including patients with AD at mild cognitive impairment stage (AD-MCI, n = 27) and at dementia stage (n = 35), non-AD dementia (n = 26, Aβ-negative), non-AD MCI (n = 19), and neurological controls (n=20). Plasma and CSF NRG1, as well as CSF core AD biomarkers (Aβ 42/Aβ 40 ratio, phospho-tau, and total tau), were measured using ELISA. CSF synaptic markers were measured using ELISA for GAP-43 and neurogranin and through immunoprecipitation mass spectrometry for SNAP-25.

RESULTS

Plasma NRG1 concentration was higher in AD-MCI and AD dementia patients compared with neurological controls (respectively P = 0.005 and P < 0.001). Plasma NRG1 differentiated AD MCI patients from neurological controls with an area under the curve of 88.3%, and AD dementia patients from NC with an area under the curve of 87.3%. Plasma NRG1 correlated with CSF NRG1 (β = 0.372, P = 0.0056, adjusted on age and sex). Plasma NRG1 was associated with AD CSF core biomarkers in the whole cohort and in Aβ-positive patients (β = -0.197-0.423). Plasma NRG1 correlated with CSF GAP-43, neurogranin, and SNAP-25 (β = 0.278-0.355). Plasma NRG1 concentration correlated inversely with MMSE in the whole cohort and in Aβ-positive patients (all, β = -0.188, P = 0.038; Aβ+: β = -0.255, P = 0.038).

CONCLUSION

Plasma NRG1 concentration is increased in AD patients and correlates with CSF core AD and synaptic biomarkers and cognitive status. Thus, plasma NRG1 is a promising non-invasive biomarker to monitor synaptic impairment in AD.

BACE1: A Key Regulator in Alzheimer's Disease Progression and Current Development of its Inhibitors

BACKGROUND

Alzheimer's disease (AD) is a chronic neurodegenerative disease with no specific disease-modifying treatment. β-secretase (BACE1) is considered the potential and rationale target because it is involved in the rate-limiting step, which produces toxic Aβ₄₂ peptides that leads to deposits in the form of amyloid plaques extracellularly, resulting in AD.

OBJECTIVE

This study aims to discuss the role and implications of BACE1 and its inhibitors in the management of AD.

METHODS

We have searched and collected the relevant quality work from PubMed using the following keywords "BACE1", BACE2", "inhibitors", and "Alzheimer's disease". In addition, we included the work which discusses the role of BACE1 in AD and the recent work on its inhibitors.

RESULTS

In this review, we have discussed the importance of BACE1 in regulating AD progression and the current development of BACE1 inhibitors. However, the development of a BACE1 inhibitor is very challenging due to the large active site of BACE1. Nevertheless, some of the BACE1 inhibitors have managed to enter advanced phases of clinical trials, such as MK-8931 (Verubecestat), E2609 (Elenbecestat), AZD3293 (Lanabecestat), and JNJ-54861911 (Atabecestat). This review also sheds light on the prospect of BACE1 inhibitors as the most effective therapeutic approach in delaying or preventing AD progression.

CONCLUSION

BACE1 is involved in the progression of AD. The current ongoing or failed clinical trials may help understand the role of BACE1 inhibition in regulating the Aβ load and cognitive status of AD patients.

A Glance at the Molecules That Regulate Oligodendrocyte Myelination

Oligodendrocyte (OL) myelination is a critical process for the neuronal axon function in the central nervous system. After demyelination occurs because of pathophysiology, remyelination makes repairs similar to myelination. Proliferation and differentiation are the two main stages in OL myelination, and most factors commonly play converse roles in these two stages, except for a few factors and signaling pathways, such as OLIG2 (Oligodendrocyte transcription factor 2). Moreover, some OL maturation gene mutations induce hypomyelination or hypermyelination without an obvious function in proliferation and differentiation. Herein, three types of factors regulating myelination are reviewed in sequence.

PTPRD and DCC Are Novel BACE1 Substrates Differentially Expressed in Alzheimer's Disease: A Data Mining and Bioinformatics Study

The β-site Amyloid precursor protein Cleaving Enzyme 1 (BACE1) is an extensively studied therapeutic target for Alzheimer's disease (AD), owing to its role in the production of neurotoxic amyloid beta (Aβ) peptides. However, despite numerous BACE1 inhibitors entering clinical trials, none have successfully improved AD pathogenesis, despite effectively lowering Aβ concentrations. This can, in part, be attributed to an incomplete understanding of BACE1, including its physiological functions and substrate specificity. We propose that BACE1 has additional important physiological functions, mediated through substrates still to be identified. Thus, to address this, we computationally analysed a list of 533 BACE1 dependent proteins, identified from the literature, for potential BACE1 substrates, and compared them against proteins differentially expressed in AD. We identified 15 novel BACE1 substrates that were specifically altered in AD. To confirm our analysis, we validated Protein tyrosine phosphatase receptor type D (PTPRD) and Netrin receptor DCC (DCC) using Western blotting. These findings shed light on the BACE1 inhibitor failings and could enable the design of substrate-specific inhibitors to target alternative BACE1 substrates. Furthermore, it gives us a greater understanding of the roles of BACE1 and its dysfunction in AD.

Effects of microRNA-298 on APP and BACE1 translation differ according to cell type and 3'-UTR variation

Alzheimer's disease (AD) is marked by neurofibrillary tangles and senile plaques composed of amyloid β (Aβ) peptides. However, specific contributions of different cell types to Aβ deposition remain unknown. Non-coding microRNAs (miRNA) play important roles in AD by regulating translation of major associated proteins, such as Aβ precursor protein (APP) and β-site APP-cleaving enzyme (BACE1), two key proteins associated with Aβ biogenesis. MiRNAs typically silence protein expression via binding specific sites in mRNAs' 3'-untranslated regions (3'-UTR). MiRNAs regulate protein levels in a cell-type specific manner; however, mechanisms of the variation of miRNA activity remain unknown. We report that miR-298 treatment reduced native APP and BACE1 protein levels in an astrocytic but not in a neuron-like cell line. From miR-298's effects on APP-3'-UTR activity and native protein levels, we infer that differences in APP 3'-UTR length could explain differential miR-298 activity. Such varied or truncated, but natural, 3'-UTR specific to a given cell type provides an opportunity to regulate native protein levels by particular miRNA. Thus, miRNA's effect tailoring to a specific cell type, bypassing another undesired cell type with a truncated 3'-UTR would potentially advance clinically-relevant translational research.

Engineered axon tracts within tubular biohybrid scaffolds

Injuries to the nervous system that involve the disruption of axonal pathways are devastating to the individual and require specific tissue engineering strategies. Here we analyse a cells-biomaterials strategy to overcome the obstacles limiting axon regenerationin vivo, based on the combination of a hyaluronic acid (HA) single-channel tubular conduit filled with poly-L-lactide acid (PLA) fibres in its lumen, with pre-cultured Schwann cells (SCs) as cells supportive of axon extension. The HA conduit and PLA fibres sustain the proliferation of SC, which enhance axon growth acting as a feeder layer and growth factor pumps. The parallel unidirectional ensemble formed by PLA fibres and SC tries to recapitulate the directional features of axonal pathways in the nervous system. A dorsal root ganglion (DRG) explant is planted on one of the conduit's ends to follow axon outgrowth from the DRG. After a 21 d co-culture of the DRG + SC-seeded conduit ensemble, we analyse the axonal extension throughout the conduit by scanning, transmission electronic and confocal microscopy, in order to study the features of SC and the grown axons and their association. The separate effects of SC and PLA fibres on the axon growth are also experimentally addressed. The biohybrid thus produced may be considered a synthetic axonal pathway, and the results could be of use in strategies for the regeneration of axonal tracts.

Effect of Schwann cell transplantation combined with electroacupuncture on axonal regeneration and remyelination in rats with spinal cord injury

Axonal impairment and demyelination after compressed spinal cord injury lead to serious neurological dysfunction. Increasing studies have suggested that Schwann cells (SCs) transplantation is a reliable, effective, and promising method for treating spinal cord injury. However, single SCs transplantation is insufficient to promote the full recovery of neurological function. Additional approaches are required to support SCs transplantation as a treatment for spinal cord injury. In the study, we investigated whether the combination of electroacupuncture (EA) and SCs transplantation was a reliable intervention for spinal cord injury. We found that rats in the combination group had significantly higher functional locomotor scores than those received single treatment. By immunostaining, we found EA can not only improve survival and proliferation of transplanted SCs but also inhibit SC apoptosis and block the formation of an astrocytic scar. Additionally, EA promoted regenerated axons extending "bullet-shaped" growth cones into the lesion. Remarkably, EA can modify astrogliosis to promote axonal regeneration following SCs transplantation through inducing extension of astrocytic processes in the SCs graft interface. More importantly, the combination of SCs engraftment and EA can enhance corticospinal-tract axonal regeneration and remyelination after spinal cord injury through up-regulating neuregulin 1 type III in SCs and its downstream signaling mediators. Thus, it is concluded that SCs effectively promote axonal recovery after spinal cord injury when combined with EA stimulation. The experimental results have reinforced the theoretical basis of EA for its clinical efficacy in patients with spinal cord injury and merited further investigation for potential clinical application.

Neuregulin-1 converts reactive astrocytes toward oligodendrocyte lineage cells via upregulating the PI3K-AKT-mTOR pathway to repair spinal cord injury

Axonal demyelination is a consistent pathological characteristic of Spinal cord injury (SCI). Promoting differentiation of oligodendrocytes is of importance for remyelination. Conversion of reactive astrocytes with stem cell potential to oligodendrocytes is proposed as an innovative strategy for SCI repair. Neuregulin-1 (Nrg1) plays an essential role in the differentiation of oligodendrocytes. Therefore, it's a potential treatment for demyelination in SCI that using Nrg1 to drive reactive astrocytes toward oligodendrocyte lineage cells. In this study, tumor necrosis factor-α (TNF-α) was used to induce dedifferentiation of primary rat spinal cord astrocytes into reactive astrocytes and Nrg1 was used to induce astrocytes in vitro and in vivo. The results showed that astrocytes treated with TNF-α expressed immaturity markers CD44 and Musashi1 at mRNA and protein levels, indicating that TNF-α induced the stem cell state of astrocytes. Nrg1 induced reactive astrocytes to express oligodendrocyte markers PDGFR-α and O4 at mRNA and protein levels, indicating that Nrg1 directly converts reactive astrocytes toward oligodendrocyte lineage cells. Moreover, upregulation of PI3K-AKT-mTOR signaling activation in response to Nrg1 was observed. In rats with SCI, intrathecal treatment with Nrg1 converted reactive astrocytes to oligodendrocyte lineage cells, inhibited astrogliosis, promoted remyelination, protected axons and eventually improved BBB score. All the biological effects of Nrg1 were significantly reversed by the co-administration of Nrg1 and ErbB inhibitor, suggesting that Nrg1 functioned through the receptor ErbB. Our findings indicate that Nrg1 is sufficient to trans-differentiate reactive astrocytes to oligodendrocytes via the PI3K-AKT-mTOR signaling pathway and repair SCI. Delivery of Nrg1 for the remyelination processes could be a promising strategy for spinal cord repair.

Environmental exposures and the etiopathogenesis of Alzheimer's disease: The potential role of BACE1 as a critical neurotoxic target

Alzheimer's disease (AD) is a major public health crisis due to devastating cognitive symptoms, a lack of curative treatments, and increasing prevalence. Most cases are sporadic (>95% of cases) after the age of 65 years, implicating an important role of environmental factors in disease pathogenesis. Environmental neurotoxicants have been implicated in neurodegenerative disorders including Parkinson's Disease and AD. Animal models of AD and in vitro studies have shed light on potential neuropathological mechanisms, yet the biochemical and molecular underpinnings of AD-relevant environmental neurotoxicity remain poorly understood. Beta-site amyloid precursor protein cleaving enzyme 1 (BACE1) is a potentially critical pathogenic target of environmentally induced neurotoxicity. BACE1 clearly has a critical role in AD pathophysiology: It is required for amyloid beta production and expression and activity of BACE1 are increased in the AD brain. Though the literature on BACE1 in response to environmental insults is limited, current studies, along with extensive AD neurobiology literature suggest that BACE1 deserves attention as an important neurotoxic target. Here, we critically review research on environmental neurotoxicants such as metals, pesticides, herbicides, fungicides, polyfluoroalkyl substances, heterocyclic aromatic amines, advanced glycation end products, and acrolein that modulate BACE1 and potential mechanisms of action. Though more research is needed to clearly understand whether BACE1 is a critical mediator of AD-relevant neurotoxicity, available reports provide convincing evidence that BACE1 is altered by environmental risk factors associated with AD pathology, implying that BACE1 inhibition and its use as a biomarker should be considered in AD management and research.

Green Tea Seed Isolated Theasaponin E1 Ameliorates AD Promoting Neurotoxic Pathogenesis by Attenuating Aβ Peptide Levels in SweAPP N2a Cells

Alzheimer’s disease (AD) is the most frequent type of dementia affecting memory, thinking and behaviour. The major hallmark of the disease is pathological neurodegeneration due to abnormal aggregation of Amyloid beta (Aβ) peptides generated by β- and γ-secretases via amyloidogenic pathway. Purpose of the current study was to evaluate the effects of theasaponin E1 on the inhibition of Aβ producing β-, γ-secretases (BACE1, PS1 and NCT) and acetylcholinesterase and activation of the non-amyloidogenic APP processing α-secretase (ADAM10). Additionally, theasaponin E1 effects on Aβ degrading and clearing proteins neprilysin and insulin degrading enzyme (IDE). The effect of theasaponin E1 on these crucial enzymes was investigated by RT-PCR, ELISA, western blotting and fluorometric assays using mouse neuroblastoma cells (SweAPP N2a). theasaponin E1 was extracted and purified from green tea seed extract via HPLC, and N2a cells were treated with different concentrations for 24 h. Gene and protein expression in the cells were measured to determine the effects of activation and/or inhibition of theasaponin E1 on β- and γ-secretases, neprilysin and IDE. Results demonstrated that theasaponin E1 significantly reduced Aβ concentration by activation of the α-secretase and neprilysin. The activities of β- and γ-secretase were reduced in a dose-dependent manner due to downregulation of BACE1, presenilin, and nicastrin. Similarly, theasaponin E1 significantly reduced the activity of acetylcholinesterase. Overall, from the results it is concluded that green tea seed extracted saponin E1 possess therapeutic significance as a neuroprotective natural product recommended for the treatment of Alzheimer’s disease.

Proteolytic Processing of Neuregulin 2

Neuregulin 2 (NRG2) belongs to the EGF family of growth factors. Most of this family members require proteolytic cleavage to liberate their ectodomains capable of binding and activating their cognate ErbB receptors. To date, most of the studies investigating proteolytic processing of neuregulins focused on NRG1, which was shown to undergo ectodomain shedding by several ADAM proteases and BACE1 and the remaining fragment was further cleaved by γ-secretase. Recently, NRG2 attracted more attention due to its role in the neurogenesis and modulation of behaviors associated with psychiatric disorders. In this study, we used genetic engineering methods to identify proteases involved in proteolytic processing of murine NRG2. Using non-neuronal cell lines as well as cultures of primary hippocampal neurons, we demonstrated that the major proteases responsible for releasing NRG2 ectodomain are ADAM10 and BACE2. Co-expression of NRG2 and BACE2 in neurons of certain brain structures including medulla oblongata and cerebellar deep nuclei was confirmed via immunohistochemical staining. The cleavage of NRG2 by ADAM10 or BACE2 generates a C-terminal fragment that serves as a substrate for γ-secretase. We also showed that murine NRG2 is subject to post-translational modifications, substantial glycosylation of its extracellular part, and phosphorylation of the cytoplasmic tail.

Functions of 'A disintegrin and metalloproteases (ADAMs)' in the mammalian nervous system

'A disintegrin and metalloproteases' (ADAMs) are a family of transmembrane proteins with diverse functions in multicellular organisms. About half of the ADAMs are active metalloproteases and cleave numerous cell surface proteins, including growth factors, receptors, cytokines and cell adhesion proteins. The other ADAMs have no catalytic activity and function as adhesion proteins or receptors. Some ADAMs are ubiquitously expressed, others are expressed tissue specifically. This review highlights functions of ADAMs in the mammalian nervous system, including their links to diseases. The non-proteolytic ADAM11, ADAM22 and ADAM23 have key functions in neural development, myelination and synaptic transmission and are linked to epilepsy. Among the proteolytic ADAMs, ADAM10 is the best characterized one due to its substrates Notch and amyloid precursor protein, where cleavage is required for nervous system development or linked to Alzheimer's disease (AD), respectively. Recent work demonstrates that ADAM10 has additional substrates and functions in the nervous system and its substrate selectivity may be regulated by tetraspanins. New roles for other proteolytic ADAMs in the nervous system are also emerging. For example, ADAM8 and ADAM17 are involved in neuroinflammation. ADAM17 additionally regulates neurite outgrowth and myelination and its activity is controlled by iRhoms. ADAM19 and ADAM21 function in regenerative processes upon neuronal injury. Several ADAMs, including ADAM9, ADAM10, ADAM15 and ADAM30, are potential drug targets for AD. Taken together, this review summarizes recent progress concerning substrates and functions of ADAMs in the nervous system and their use as drug targets for neurological and psychiatric diseases.

Elevated cleavage of neuregulin-1 by beta-secretase 1 in plasma of schizophrenia patients

Neuregulin 1 (NRG1) is a key candidate susceptibility gene for schizophrenia. It is reported that the function of NRG1 can be regulated by cleavage via the β-Secretase (BACE1), particularly during early development. While current knowledge suggested that schizophrenia might have different phenotypes, it is unknown whether BACE1-cleaved-NRG1 (BACE1-NRG1) activity is related to clinical phenotypes of schizophrenia. In the current study, we used a newly developed enzymatic assay to detect BACE1-NRG1 activity in the human plasma and investigated the levels of cleavage of NRG1 by BACE1 in the plasma from schizophrenia patients. Our results are the first to demonstrate that the level of plasma BACE1-NRG1 activity was significantly increased in subjects affected with schizophrenia compared with healthy controls. Interestingly, the elevated BACE1-NRG1 activity was correlated with the disease severity and duration of schizophrenia, such as patients suffering from shorter-term course and worse disease status expressed higher BACE1-NRG1 activity levels compared to whom with longer duration and less severity of the disease. Furthermore, this is also the first report that the alternation of BACE1-NRG1 activity was a substrate -specific event in schizophrenia. Together, our findings suggested that the plasma BACE1-NRG1 activity can be a potential biomarker for the early diagnosis of schizophrenia.

Adipocyte-secreted BMP8b mediates adrenergic-induced remodeling of the neuro-vascular network in adipose tissue

Activation of brown adipose tissue-mediated thermogenesis is a strategy for tackling obesity and promoting metabolic health. BMP8b is secreted by brown/beige adipocytes and enhances energy dissipation. Here we show that adipocyte-secreted BMP8b contributes to adrenergic-induced remodeling of the neuro-vascular network in adipose tissue (AT). Overexpression of bmp8b in AT enhances browning of the subcutaneous depot and maximal thermogenic capacity. Moreover, BMP8b-induced browning, increased sympathetic innervation and vascularization of AT were maintained at 28 °C, a condition of low adrenergic output. This reinforces the local trophic effect of BMP8b. Innervation and vascular remodeling effects required BMP8b signaling through the adipocytes to 1) secrete neuregulin-4 (NRG4), which promotes sympathetic axon growth and branching in vitro, and 2) induce a pro-angiogenic transcriptional and secretory profile that promotes vascular sprouting. Thus, BMP8b and NRG4 can be considered as interconnected regulators of neuro-vascular remodeling in AT and are potential therapeutic targets in obesity.

Tissue Inhibitor of Metalloproteinase-3 Promotes Schwann Cell Myelination

Tissue inhibitor of metalloproteinase-3 (TIMP-3) inhibits the activities of various metalloproteinases including matrix metalloproteinases and ADAM family proteins. In the peripheral nervous system, ADAM17, also known as TNF-α converting enzyme (TACE), cleaves the extracellular domain of Nrg1 type III, an axonal growth factor that is essential for Schwann cell myelination. The processing by ADAM17 attenuates Nrg1 signaling and inhibits Schwann cell myelination. TIMP-3 targets ADAM17, suggesting a possibility that TIMP-3 may elicit a promyelinating function in Schwann cells by relieving ADAM17-induced myelination block. To investigate this, we used a myelinating coculture system to determine the effect of TIMP-3 on Schwann cell myelination. Treatment with TIMP-3 enhanced myelin formation in cocultures, evident by an increase in the number of myelin segments and upregulated expression of Krox20 and myelin protein. The effect of TIMP-3 was accompanied by the inhibition of ADAM17 activity and an increase in Nrg1 type III signaling in cocultures. Accordingly, the N-terminus fragment of TIMP-3, which exhibits a selective inhibitory function toward ADAM17, elicited a similar myelination-promoting effect and increased Nrg1 type III activity. TIMP-3 also enhanced laminin production in cocultures, which is likely to aid Schwann cell myelination.

Non-cell-autonomous function of DR6 in Schwann cell proliferation

Death receptor 6 (DR6) is an orphan member of the TNF receptor superfamily and controls cell death and differentiation in a cell-autonomous manner in different cell types. Here, we report an additional non-cell-autonomous function for DR6 in the peripheral nervous system (PNS). DR6-knockout (DR6 KO) mice showed precocious myelination in the PNS Using an in vitro myelination assay, we demonstrate that neuronal DR6 acts in trans on Schwann cells (SCs) and reduces SC proliferation and myelination independently of its cytoplasmic death domain. Mechanistically, DR6 was found to be cleaved in neurons by "a disintegrin and metalloprotease 10" (ADAM10), releasing the soluble DR6 ectodomain (sDR6). Notably, in the in vitro myelination assay, sDR6 was sufficient to rescue the DR6 KO phenotype. Thus, in addition to the cell-autonomous receptor function of full-length DR6, the proteolytically released sDR6 can unexpectedly also act as a paracrine signaling factor in the PNS in a non-cell-autonomous manner during SC proliferation and myelination. This new mode of DR6 signaling will be relevant in future attempts to target DR6 in disease settings.

BACE1 Function and Inhibition: Implications of Intervention in the Amyloid Pathway of Alzheimer's Disease Pathology

Alzheimer's disease (AD) is a fatal progressive neurodegenerative disorder characterized by increasing loss in memory, cognition, and function of daily living. Among the many pathologic events observed in the progression of AD, changes in amyloid β peptide (Aβ) metabolism proceed fastest, and precede clinical symptoms. BACE1 (β-secretase 1) catalyzes the initial cleavage of the amyloid precursor protein to generate Aβ. Therefore inhibition of BACE1 activity could block one of the earliest pathologic events in AD. However, therapeutic BACE1 inhibition to block Aβ production may need to be balanced with possible effects that might result from diminished physiologic functions BACE1, in particular processing of substrates involved in neuronal function of the brain and periphery. Potentials for beneficial or consequential effects resulting from pharmacologic inhibition of BACE1 are reviewed in context of ongoing clinical trials testing the effect of BACE1 candidate inhibitor drugs in AD populations.

ADAM10 as a therapeutic target for brain diseases: from developmental disorders to Alzheimer's disease

INTRODUCTION

In the central nervous system a disintegrin and metalloproteinase 10 (ADAM10) controls several functions such as neurodevelopment, synaptic plasticity and dendritic spine morphology thanks to its activity towards a high number of substrates, including the synaptic cell adhesion molecules as the Amyloid Precursor Protein, N-cadherin, Notch and Ephrins. In particular, ADAM10 plays a key role in the modulation of the molecular mechanisms responsible for dendritic spine formation, maturation and stabilization and in the regulation of the molecular organization of the glutamatergic synapse. Consequently, an alteration of ADAM10 activity is strictly correlated to the onset of different types of synaptopathies, ranging from neurodevelopmental disorders, i.e. autism spectrum disorders, to neurodegenerative diseases, i.e. Alzheimer's Disease. Areas covered: We describe the most recent discoveries in understanding of the role of ADAM10 activity at the glutamatergic excitatory synapse and its involvement in the onset of neurodevelopmental and neurodegenerative disorders. Expert opinion: A progress in the understanding of the molecular mechanisms driving ADAM10 activity at synapses and its alterations in brain disorders is the first step before designing a specific drug able to modulate ADAM10 activity.

BACE1-Dependent Neuregulin-1 Signaling: An Implication for Schizophrenia

Schizophrenia is a chronic psychiatric disorder with a lifetime prevalence of about 1% in the general population. Recent studies have shown that Neuregulin-1 (Nrg1) is a candidate gene for schizophrenia. At least 15 alternative splicing of NRG1 isoforms all contain an extracellular epidermal growth factor (EGF)-like domain, which is sufficient for Nrg1 biological activity including the formation of myelin sheaths and the regulation of synaptic plasticity. It is known that Nrg1 can be cleaved by β-secretase (BACE1) and the resulting N-terminal fragment (Nrg1-ntf) binds to receptor tyrosine kinase ErbB4, which activates Nrg1/ErbB4 signaling. While changes in Nrg1 expression levels in schizophrenia still remain controversial, understanding the BACE1-cleaved Nrg1-ntf and Nrg1/ErbB4 signaling in schizophrenia neuropathogenesis is essential and important. In this review paper, we included three major parts: (1) Nrg1 structure and cleavage pattern by BACE1; (2) BACE1-dependent Nrg1 cleavage associated with schizophrenia in human studies; and (3) Animal studies of Nrg1 and BACE1 mutations with behavioral observations. Our review will provide a better understanding of Nrg1 in schizophrenia and a potential strategy for using BACE1 cleavage of Nrg1 as a unique biomarker for diagnosis, as well as a new therapeutic target, of schizophrenia.

BACE1 Deficiency Causes Abnormal Neuronal Clustering in the Dentate Gyrus

BACE1 is validated as Alzheimer's β-secretase and a therapeutic target for Alzheimer's disease. In examining BACE1-null mice, we discovered that BACE1 deficiency develops abnormal clusters of immature neurons, forming doublecortin-positive neuroblasts, in the developing dentate gyrus, mainly in the subpial zone (SPZ). Such clusters were rarely observed in wild-type SPZ and not reported in other mouse models. To understand their origins and fates, we examined how neuroblasts in BACE1-null SPZ mature and migrate during early postnatal development. We show that such neuroblasts are destined to form Prox1-positive granule cells in the dentate granule cell layer, and mainly mature to form excitatory neurons, but not inhibitory neurons. Mechanistically, higher levels of reelin potentially contribute to abnormal neurogenesis and timely migration in BACE1-null SPZ. Altogether, we demonstrate that BACE1 is a critical regulator in forming the dentate granule cell layer through timely maturation and migration of SPZ neuroblasts.

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BACE1 deficiency causes abnormal neuronal clusters retained in the mouse SPZ

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Mis-migrated neural progenitor cells in the SPZ are destined to form granule cells

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Such neural progenitor cells form excitatory neurons but not inhibitor neurons

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Elevated levels of reelin contribute to abnormal neuronal maturation and migration

Dysregulation of intracellular trafficking and endosomal sorting in Alzheimer's disease: controversies and unanswered questions

Alzheimer's disease (AD) is characterized by the accumulation of amyloid plaques in the brain consisting of an aggregated form of amyloid β-peptide (Aβ) derived from sequential amyloidogenic processing of the amyloid precursor protein (APP) by membrane-bound proteases β-site APP-cleaving enzyme 1 (BACE1) and γ-secretase. The initial processing of APP by BACE1 is re-gulated by intracellular sorting events of the enzyme, which is a prime target for therapeutic intervention. GWAS (genome-wide sequencing studies) have identified several AD-susceptibility genes that are associated with the regulation of membrane trafficking, and substantial evidence now indicates that AD is likely to arise from defective membrane trafficking in either or both of the secretory and endocytic pathways. Considerable progress has been made in defining the intracellular trafficking pathways of BACE1 and APP and the sorting signals of these membrane proteins that define their itineraries. In this review we highlight recent advances in understanding the regulation of the intracellular sorting of BACE1 and APP, discuss how dysregulation of these trafficking events may lead to enhanced generation of the neurotoxic Aβ products in AD and highlight the unresolved questions in the field.

BACE1 regulates the proliferation and cellular functions of Schwann cells

BACE1 is an indispensable enzyme for generating β-amyloid peptides, which are excessively accumulated in brains of Alzheimer's patients. However, BACE1 is also required for proper myelination of peripheral nerves, as BACE1-null mice display hypomyelination. To determine the precise effects of BACE1 on myelination, here we have uncovered a role of BACE1 in the control of Schwann cell proliferation during development. We demonstrate that BACE1 regulates the cleavage of Jagged-1 and Delta-1, two membrane-bound ligands of Notch. BACE1 deficiency induces elevated Jag-Notch signaling activity, which in turn facilitates proliferation of Schwann cells. This increase in proliferation leads to shortened internodes and decreased Schmidt-Lanterman incisures. Functionally, evoked compound action potentials in BACE1-null nerves were significantly smaller and slower, with a clear decrease in excitability. BACE1-null nerves failed to effectively use lactate as an alternative energy source under conditions of increased physiological activity. Correlatively, BACE1-null mice showed reduced performance on rotarod tests. Collectively, our data suggest that BACE1 deficiency enhances proliferation of Schwann cell due to the elevated Jag1/Delta1-Notch signaling, but fails to myelinate axons efficiently due to impaired the neuregulin1-ErbB signaling, which has been documented.

Physiological Functions of the β-Site Amyloid Precursor Protein Cleaving Enzyme 1 and 2

BACE1 was discovered as the β-secretase for initiating the cleavage of amyloid precursor protein (APP) at the β-secretase site, while its close homology BACE2 cleaves APP within the β-amyloid (Aβ) domain region and shows distinct cleavage preferences in vivo. Inhibition of BACE1 proteolytic activity has been confirmed to decrease Aβ generation and amyloid deposition, and thus specific inhibition of BACE1 by small molecules is a current focus for Alzheimer’s disease therapy. While BACE1 inhibitors are being tested in advanced clinical trials, knowledge regarding the properties and physiological functions of BACE is highly important and this review summarizes advancements in BACE1 research over the past several years. We and others have shown that BACE1 is not only a critical enzyme for testing the “Amyloid Hypothesis” associated with Alzheimer’s pathogenesis, but also important for various functions such as axon growth and pathfinding, astrogenesis, neurogenesis, hyperexcitation, and synaptic plasticity. BACE2 appears to play different roles such as glucose homeostasis and pigmentation. This knowledge regarding BACE1 functions is critical for monitoring the safe use of BACE1 inhibitors in humans.

Glutamate-dependent ectodomain shedding of neuregulin-1 type II precursors in rat forebrain neurons

The neurotrophic factor neuregulin 1 (NRG1) regulates neuronal development, glial differentiation, and excitatory synapse maturation. NRG1 is synthesized as a membrane-anchored precursor and is then liberated by proteolytic processing or exocytosis. Mature NRG1 then binds to its receptors expressed by neighboring neurons or glial cells. However, the molecular mechanisms that govern this process in the nervous system are not defined in detail. Here we prepared neuron-enriched and glia-enriched cultures from embryonic rat neocortex to investigate the role of neurotransmitters that regulate the liberation/release of NRG1 from the membrane of neurons or glial cells. Using a two-site enzyme immunoassay to detect soluble NRG1, we show that, of various neurotransmitters, glutamate was the most potent inducer of NRG1 release in neuron-enriched cultures. NRG1 release in glia-enriched cultures was relatively limited. Furthermore, among glutamate receptor agonists, N-Methyl-D-Aspartate (NMDA) and kainate (KA), but not AMPA or tACPD, mimicked the effects of glutamate. Similar findings were acquired from analysis of the hippocampus of rats with KA-induced seizures. To evaluate the contribution of members of a disintegrin and metalloproteinase (ADAM) families to NRG1 release, we transfected primary cultures of neurons with cDNA vectors encoding NRG1 types I, II, or III precursors, each tagged with the alkaline phosphatase reporter. Analysis of alkaline phosphatase activity revealed that the NRG1 type II precursor was subjected to tumor necrosis factor-α-converting enzyme (TACE) / a Disintegrin And Metalloproteinase 17 (ADAM17) -dependent ectodomain shedding in a protein kinase C-dependent manner. These results suggest that glutamatergic neurotransmission positively regulates the ectodomain shedding of NRG1 type II precursors and liberates the active NRG1 domain in an activity-dependent manner.

ADAM10-mediated release of heregulin confers resistance to trastuzumab by activating HER3

Receptor tyrosine kinases of the HER-family are involved in the development and progression of multiple epithelial tumors, and have consequently become widely used targets for new anti-cancer therapies. Trastuzumab, an antibody against HER2, has shown potent growth inhibitory effects on HER2 overexpressing tumors, including gastro-esophageal cancer, however, resistance to this therapy is inevitable. Unfortunately, a paucity of data on the cellular mechanisms of resistance to targeted therapeutic agents exists in esophageal adenocarcinoma. Using primary established HER2-overexpressing cultures and patient-derived xenograft models, we now reveal a novel resistance mechanism to trastuzumab in esophageal cancer: In response to trastuzumab, both HER3 and the metalloprotease ADAM10 are simultaneously upregulated. The proteolytic activity of the latter then releases the HER3 ligand heregulin from the cell surface to activate HER3 and confer resistance to trastuzumab by inducing compensatory growth factor receptor signaling. Blocking either HER3 or ADAM10 effectively reverts the acquired resistance to trastuzumab. Our data thus provide strategies to inhibit this signaling and circumvent resistance to trastuzumab.

Presumed LRP1-targeting transport peptide delivers β-secretase inhibitor to neurons in vitro with limited efficiency

Interfering with the activity of β-secretase to reduce the production of Aβ peptides is a conceivable therapeutic strategy for Alzheimer’s disease. However, the development of efficient yet safe inhibitors is hampered by secondary effects, usually linked to the indiscriminate inhibition of other substrates’ processing by the targeted enzyme. Based on the spatial compartmentalization of the cleavage of the amyloid precursor protein by β-secretase, we hypothesized that by exploiting the endocytosis receptor low-density lipoprotein receptor-related protein it would be possible to direct an otherwise cell-impermeable inhibitor to the endosomes of neurons, boosting the drug’s efficacy and importantly, sparing the off-target effects. We used the transport peptide Angiopep to build an endocytosis-competent conjugate and found that although the peptide facilitated the inhibitor’s internalization into neurons and delivered it to the endosomes, the delivery was not efficient enough to potently reduce β-secretase activity at the cellular level. This is likely connected to the finding that in the cell lines we used, Angiopep’s internalization was not mediated by its presumed receptor to a significant extent. Additionally, Angiopep exploited different internalization mechanisms when applied alone or when conjugated to the inhibitor, highlighting the impact that drug conjugation can have on transport peptides.

Visualization of Neuregulin 1 ectodomain shedding reveals its local processing in vitro and in vivo

Neuregulin1 (NRG1) plays diverse developmental roles and is likely involved in several neurological disorders including schizophrenia. The transmembrane NRG1 protein is proteolytically cleaved and released as a soluble ligand for ErbB receptors. Such post-translational processing, referred to as 'ectodomain shedding', is thought to be crucial for NRG1 function. However, little is known regarding the regulatory mechanism of NRG1 cleavage in vivo. Here, we developed a fluorescent probe, NRG1 Cleavage Indicating SenSOR (N-CISSOR), by fusing mCherry and GFP to the extracellular and intracellular domains of NRG1, respectively. N-CISSOR mimicked the subcellular localization and biochemical properties of NRG1 including cleavage dynamics and ErbB phosphorylation in cultured cells. mCherry/GFP ratio imaging of phorbol-12-myristate-13-acetate-stimulated N-CISSOR-expressing HEK293T cells enabled to monitor rapid ectodomain shedding of NRG1 at the subcellular level. Utilizing N-CISSOR in zebrafish embryos revealed preferential axonal NRG1 ectodomain shedding in developing motor neurons, demonstrating that NRG1 ectodomain shedding is spatially regulated at the subcellular level. Thus, N-CISSOR will be a valuable tool for elucidating the spatiotemporal regulation of NRG1 ectodomain shedding, both in vitro and in vivo.

Regulating Axonal Responses to Injury: The Intersection between Signaling Pathways Involved in Axon Myelination and The Inhibition of Axon Regeneration

Following spinal cord injury (SCI), a multitude of intrinsic and extrinsic factors adversely affect the gene programs that govern the expression of regeneration-associated genes (RAGs) and the production of a diversity of extracellular matrix molecules (ECM). Insufficient RAG expression in the injured neuron and the presence of inhibitory ECM at the lesion, leads to structural alterations in the axon that perturb the growth machinery, or form an extraneous barrier to axonal regeneration, respectively. Here, the role of myelin, both intact and debris, in antagonizing axon regeneration has been the focus of numerous investigations. These studies have employed antagonizing antibodies and knockout animals to examine how the growth cone of the re-growing axon responds to the presence of myelin and myelin-associated inhibitors (MAIs) within the lesion environment and caudal spinal cord. However, less attention has been placed on how the myelination of the axon after SCI, whether by endogenous glia or exogenously implanted glia, may alter axon regeneration. Here, we examine the intersection between intracellular signaling pathways in neurons and glia that are involved in axon myelination and axon growth, to provide greater insight into how interrogating this complex network of molecular interactions may lead to new therapeutics targeting SCI.

Inhibiting BACE1 to reverse synaptic dysfunctions in Alzheimer's disease

Over the past two decades, many studies have identified significant contributions of toxic β-amyloid peptides (Aβ) to the etiology of Alzheimer's disease (AD), which is the most common age-dependent neurodegenerative disease. AD is also recognized as a disease of synaptic failure. Aβ, generated by sequential proteolytic cleavages of amyloid precursor protein (APP) by BACE1 and γ-secretase, is one of major culprits that cause this failure. In this review, we summarize current findings on how BACE1-cleaved APP products impact learning and memory through proteins localized on glutamatergic, GABAergic, and dopaminergic synapses. Considering the broad effects of Aβ on all three types of synapses, BACE1 inhibition emerges as a practical approach for ameliorating Aβ-mediated synaptic dysfunctions. Since BACE1 inhibitory drugs are currently in clinical trials, this review also discusses potential complications arising from BACE1 inhibition. We emphasize that the benefits of BACE1 inhibitory drugs will outweigh the concerns.

Amyloid Proteins and Their Role in Multiple Sclerosis. Considerations in the Use of Amyloid-PET Imaging

Thioflavin T derivatives are used in positron-emission tomography (PET) studies to detect amyloid protein deposits in patients with Alzheimer disease. These tracers bind extensively to white matter, which suggests that they may be useful in studies of multiple sclerosis (MS), and that proteins resulting from proteolytic processing of the amyloid precursor protein (APP) may contribute to MS. This article reviews data from both clinical and preclinical studies addressing the role of these proteins, whether they are detected in CSF studies or using PET imaging. APP is widely expressed in demyelinated axons and may have a protective effect in MS and in experimental allergic encephalomyelitis in animals. Several mechanisms associated with this increased expression may affect the degree of remyelination in MS. Amyloid-PET imaging may help determine the degree of demyelination and provide information on the molecular changes linked to APP proteolytic processing experienced by patients with MS.

Neuronal ADAM10 Promotes Outgrowth of Small-Caliber Myelinated Axons in the Peripheral Nervous System

The regulation of myelination and axonal outgrowth in the peripheral nervous system is controlled by a complex signaling network involving various signaling pathways. Members of the A Disintegrin And Metalloproteinase (ADAM) family are membrane-anchored proteinases with both proteolytic and disintegrin characteristics that modulate the function of signaling molecules. One family member, ADAM17, is known to influence myelination by cleaving and thus regulating one of the key signals, neuregulin-1, which controls peripheral nervous system myelination. A similar function for ADAM10 had been suggested by previous in vitro studies. Here, we assessed whether ADAM10 exerts a similar function in vivo and deleted ADAM10 in a cell type-specific manner in either neurons or Schwann cells. We found that ADAM10 is not required in either Schwann cells or neurons for normal myelination during development or for remyelination after injury. Instead, ADAM10 is required specifically in neurons for the outgrowth of myelinated small-fiber axons in vitro and after injury in vivo. Thus, we report for the first time a neuron-intrinsic function of ADAM10 in axonal regeneration that is distinct from that of the related protein family member ADAM17 and that may have implications for targeting ADAM function in nervous system diseases.

Dysregulation of ErbB Receptor Trafficking and Signaling in Demyelinating Charcot-Marie-Tooth Disease

Charcot-Marie-Tooth (CMT) disease is the most common inherited peripheral neuropathy with the majority of cases involving demyelination of peripheral nerves. The pathogenic mechanisms of demyelinating CMT remain unclear, and no effective therapy currently exists for this disease. The discovery that mutations in different genes can cause a similar phenotype of demyelinating peripheral neuropathy raises the possibility that there may be convergent mechanisms leading to demyelinating CMT pathogenesis. Increasing evidence indicates that ErbB receptor-mediated signaling plays a major role in the control of Schwann cell-axon communication and myelination in the peripheral nervous system. Recent studies reveal that several demyelinating CMT-linked proteins are novel regulators of endocytic trafficking and/or phosphoinositide metabolism that may affect ErbB receptor signaling. Emerging data have begun to suggest that dysregulation of ErbB receptor trafficking and signaling in Schwann cells may represent a common pathogenic mechanism in multiple subtypes of demyelinating CMT. In this review, we focus on the roles of ErbB receptor trafficking and signaling in regulation of peripheral nerve myelination and discuss the emerging evidence supporting the potential involvement of altered ErbB receptor trafficking and signaling in demyelinating CMT pathogenesis and the possibility of modulating these trafficking and signaling processes for treating demyelinating peripheral neuropathy.

Neuregulin-1β Regulates the migration of Different Neurochemical Phenotypic Neurons from Organotypically Cultured Dorsal Root Ganglion Explants

Neuregulin-1β (NRG-1β) has multiple roles in the development and function in the nervous system and exhibits potent neuroprotective properties. In the present study, organotypically cultured dorsal root ganglion (DRG) explants were used to evaluate the effects of NRG-1β on migration of two major phenotypic classes of DRG neurons. The signaling pathways involved in these effects were also determined. Organotypically cultured DRG explants were exposed to NRG-1β (20 nmol/L), the phosphatidylinositol 3-kinase inhibitor LY294002 (10 μmol/L) plus NRG-1β (20 nmol/L), the extracellular signal-regulated protein kinase (ERK1/2) inhibitor PD98059 (10 μmol/L) plus NRG-1β (20 nmol/L), and LY294002 (10 μmol/L) plus PD98059 (10 μmol/L) plus NRG-1β (20 nmol/L), respectively, for 3 days. The DRG explants were continuously exposed to culture media as a control. After that, all above cultures were processed for detecting the mRNA levels of calcitonin gene-related peptide (CGRP) and neurofilament-200 (NF-200) by real-time PCR analysis. CGRP and NF-200 expression in situ was determined by fluorescent labeling technique. The results showed that NRG-1β elevated the mRNA and protein levels of CGRP and NF-200. NRG-1β also increased the number and the percentage of CGRP-immunoreactive (IR) migrating neurons and NF-200-IR migrating neurons. Inhibitors (LY294002, PD98059) either alone or in combination blocked the effects of NRG-1β. The contribution of NRG-1β on modulating distinct neurochemical phenotypic plasticity of DRG neurons suggested that NRG-1β signaling system might play an important role on the biological effects of primary sensory neurons.

Neurological dysfunctions associated with altered BACE1-dependent Neuregulin-1 signaling

Inhibition of BACE1 is being pursued as a therapeutic target to treat patients suffering from Alzheimer's disease because BACE1 is the sole β-secretase that generates β-amyloid peptide. Knowledge regarding other cellular functions of BACE1 is therefore critical for the safe use of BACE1 inhibitors in human patients. Neuregulin-1 (Nrg1) is a BACE1 substrate and BACE1 cleavage of Nrg1 is critical for signaling functions in myelination, remyelination, synaptic plasticity, normal psychiatric behaviors, and maintenance of muscle spindles. This review summarizes the most recent discoveries associated with BACE1-dependent Nrg1 signaling in these areas. This body of knowledge will help to provide guidance for preventing unwanted Nrg1-based side effects following BACE1 inhibition in humans. To initiate its signaling cascade, membrane anchored Neuregulin (Nrg), mainly type I and III β1 Nrg1 isoforms and Nrg3, requires ectodomain shedding. BACE1 is one of such indispensable sheddases to release the functional Nrg signaling fragment. The dependence of Nrg on the cleavage by BACE1 is best manifested by disrupting the critical role of Nrg in the control of axonal myelination, schizophrenic behaviors as well as the formation and maintenance of muscle spindles.

Nicastrin functions to sterically hinder γ-secretase-substrate interactions driven by substrate transmembrane domain

γ-Secretase is an intramembrane-cleaving protease that processes many type-I integral membrane proteins within the lipid bilayer, an event preceded by shedding of most of the substrate's ectodomain by α- or β-secretases. The mechanism by which γ-secretase selectively recognizes and recruits ectodomain-shed substrates for catalysis remains unclear. In contrast to previous reports that substrate is actively recruited for catalysis when its remaining short ectodomain interacts with the nicastrin component of γ-secretase, we find that substrate ectodomain is entirely dispensable for cleavage. Instead, γ-secretase-substrate binding is driven by an apparent tight-binding interaction derived from substrate transmembrane domain, a mechanism in stark contrast to rhomboid--another family of intramembrane-cleaving proteases. Disruption of the nicastrin fold allows for more efficient cleavage of substrates retaining longer ectodomains, indicating that nicastrin actively excludes larger substrates through steric hindrance, thus serving as a molecular gatekeeper for substrate binding and catalysis.

Proteolytic Processing of Neuregulin 1 Type III by Three Intramembrane-cleaving Proteases

Numerous membrane-bound proteins undergo regulated intramembrane proteolysis. Regulated intramembrane proteolysis is initiated by shedding, and the remaining stubs are further processed by intramembrane-cleaving proteases (I-CLiPs). Neuregulin 1 type III (NRG1 type III) is a major physiological substrate of β-secretase (β-site amyloid precursor protein-cleaving enzyme 1 (BACE1)). BACE1-mediated cleavage is required to allow signaling of NRG1 type III. Because of the hairpin nature of NRG1 type III, two membrane-bound stubs with a type 1 and a type 2 orientation are generated by proteolytic processing. We demonstrate that these stubs are substrates for three I-CLiPs. The type 1-oriented stub is further cleaved by γ-secretase at an ϵ-like site five amino acids N-terminal to the C-terminal membrane anchor and at a γ-like site in the middle of the transmembrane domain. The ϵ-cleavage site is only one amino acid N-terminal to a Val/Leu substitution associated with schizophrenia. The mutation reduces generation of the NRG1 type III β-peptide as well as reverses signaling. Moreover, it affects the cleavage precision of γ-secretase at the γ-site similar to certain Alzheimer disease-associated mutations within the amyloid precursor protein. The type 2-oriented membrane-retained stub of NRG1 type III is further processed by signal peptide peptidase-like proteases SPPL2a and SPPL2b. Expression of catalytically inactive aspartate mutations as well as treatment with 2,2'-(2-oxo-1,3-propanediyl)bis[(phenylmethoxy)carbonyl]-l-leucyl-l-leucinamide ketone inhibits formation of N-terminal intracellular domains and the corresponding secreted C-peptide. Thus, NRG1 type III is the first protein substrate that is not only cleaved by multiple sheddases but is also processed by three different I-CLiPs.

The alpha secretase ADAM10: A metalloprotease with multiple functions in the brain

Proteins belonging to the 'A Disintegrin And Metalloproteinase' (ADAM) family are membrane-anchored proteases that are able to cleave the extracellular domains of several membrane-bound proteins in a process known as 'ectodomain shedding'. In the central nervous system, ADAM10 has attracted the most attention, since it was described as the amyloid precursor protein α-secretase over ten years ago. Despite the excitement over the potential of ADAM10 as a novel drug target in Alzheimer disease, the physiological functions of ADAM10 in the brain are not yet well understood. This is largely because of the embryonic lethality of ADAM10-deficient mice, which results from the loss of cleavage and signaling of the Notch receptor, another ADAM10 substrate. However, the recent generation of conditional ADAM10-deficient mice and the identification of further ADAM10 substrates in the brain has revealed surprisingly numerous and fundamental functions of ADAM10 in the development of the embryonic brain and also in the homeostasis of adult neuronal networks. Mechanistically, ADAM10 controls these functions by utilizing unique postsynaptic substrates in the central nervous system, in particular synaptic cell adhesion molecules, such as neuroligin-1, N-cadherin, NCAM, Ephrin A2 and A5. Consequently, a dysregulation of ADAM10 activity is linked to psychiatric and neurological diseases, such as epilepsy, fragile X syndrome and Huntington disease. This review highlights the recent progress in understanding the substrates and function as well as the regulation and cell biology of ADAM10 in the central nervous system and discusses the value of ADAM10 as a drug target in brain diseases.

Neuregulin-1-mediated ErbB2-ErbB3 signalling protects human trophoblasts against apoptosis to preserve differentiation

During placentation, foetal trophoblasts invade deeply into maternal tissue to establish a foeto–maternal circulation. We have previously shown that extravillous trophoblast (EVT) lineage cells express ErbB2 and ErbB3, of which the potential as an oncogenic unit is well established. However, a physiological function of this receptor combination in humans remains a puzzling question. Here, we demonstrate neuregulin 1 (NRG1) expression and secretion by human decidual stromal cells. Stimulation of human primary trophoblasts with exogenous NRG1 induced phosphorylation of ErbB2, ErbB3 and related downstream effectors. Co-immunoprecipitation experiments confirmed the formation of ErbB2–ErbB3 dimers upon ligand engagement. Along this line, receptor knockdown and ErbB3 neutralization strongly diminished NRG1-dependent activation of the signalling complex. Functional studies revealed that NRG1 promotes EVT formation in placental explant cultures. Although, in the presence of NRG1, basal and camptothecin-induced trophoblast apoptosis was significantly repressed, this effect was abolished upon ErbB3 inhibition. Notably, camptothecin provoked a strong reduction of trophoblast cell column size, whereas NRG1-treated explants were refractory to the compound. Taken together, our findings newly identify a physiological function of the NRG1–ErbB2–ErbB3 axis in trophoblast survival during human placental development.

Highlighted Article: Decidual-stromal-cell-derived neuregulin-1 induces ErbB2–ErbB3 heterodimerization in extravillous trophoblasts and stabilizes trophoblast differentiation through suppression of apoptosis.

Calcyon stimulates neuregulin 1 maturation and signaling

Neuregulin1 (NRG1) is a single transmembrane protein that plays a critical role in neural development and synaptic plasticity. Both NRG1 and its receptor, ErbB4, are well-established risk genes of schizophrenia. The NRG1 ecto-domain (ED) binds and activates ErbB4 following proteolytic cleavage of pro-NRG1 precursor protein. Although several studies have addressed the function of NRG1 in brain, very little is known about the cleavage and shedding mechanism. Here we show that the neuronal vesicular protein calcyon is a potent activator and key determinant of NRG1 ED cleavage and shedding. Calcyon stimulates clathrin-mediated endocytosis and endosomal targeting; and its levels are elevated in postmortem brains of schizophrenics. Overexpression of calcyon stimulates NRG1 cleavage and signaling in vivo, and as a result, GABA transmission is enhanced in calcyon overexpressing mice. Conversely, NRG1 cleavage, ErbB4 activity and GABA transmission are decreased in calcyon null mice. Moreover, stimulation of NRG1 cleavage by calcyon was recapitulated in HEK 293 cells suggesting the mechanism involved is cell-autonomous. Finally, studies with site-specific mutants in calcyon and inhibitors for the major sheddases indicate that the stimulatory effects of calcyon on NRG1 cleavage and shedding depend on clathrin-mediated endocytosis, β-secretase 1, and interaction with clathrin adaptor proteins. Together these results identify a novel mechanism for NRG1 cleavage and shedding.