Presenilin/gamma-secretase-mediated cleavage of the voltage-gated sodium channel beta2-subunit regulates cell adhesion and migration

Navβ2 Intracellular Fragments Contribute to Aβ1-42-Induced Cognitive Impairment and Synaptic Deficit Through Transcriptional Suppression of BDNF

A pathological hallmark of Alzheimer's disease (AD) is the region-specific accumulation of the amyloid-beta protein (Aβ), which triggers aberrant neuronal excitability, synaptic impairment, and progressive cognitive decline. Previous works have demonstrated that Aβ pathology induced aberrant elevation in the levels and excessive enzymatic hydrolysis of voltage-gated sodium channel type 2 beta subunit (Navβ2) in the brain of AD models, accompanied by alteration in excitability of hippocampal neurons, synaptic deficits, and subsequently, cognitive dysfunction. However, the mechanism is unclear. In this research, by employing cell models treated with toxic Aβ1-42 and AD mice, the possible effects and potential mechanisms induced by Navβ2. The results reveal that Aβ1-42 induces remarkable increases in Navβ2 intracellular domain (Navβ2-ICD) and decreases in both BDNF exons and protein levels, as well as phosphorylated tropomyosin-related kinase B (pTrkB) expression in cells and mice, coupled with cognitive impairments, synaptic deficits, and aberrant neuronal excitability. Administration with exogenous Navβ2-ICD further enhances these effects induced by Aβ1-42, while interfering the generation of Navβ2-ICD and/or complementing BDNF neutralize the Navβ2-ICD-conducted effects. Luciferase reporter assay verifies that Navβ2-ICD regulates BDNF transcription and expression by targeting its promoter. Collectively, our findings partially elucidate that abnormal enzymatic hydrolysis of Navβ2 induced by Aβ1-42-associated AD pathology leads to intracellular Navβ2-ICD overload, which may responsible to abnormal neuronal excitability, synaptic deficit, and cognition dysfunction, through its transcriptional suppression on BDNF. Therefore, this work supplies novel evidences that Navβ2 plays crucial roles in the occurrence and progression of cognitive impairment of AD by transcriptional regulatory activity of its cleaved ICD.

Therapeutic Potential of Targeting Regulated Intramembrane Proteolysis Mechanisms of Voltage-Gated Ion Channel Subunits and Cell Adhesion Molecules

Several integral membrane proteins undergo regulated intramembrane proteolysis (RIP), a tightly controlled process through which cells transmit information across and between intracellular compartments. RIP generates biologically active peptides by a series of proteolytic cleavage events carried out by two primary groups of enzymes: sheddases and intramembrane-cleaving proteases (iCLiPs). Following RIP, fragments of both pore-forming and non-pore-forming ion channel subunits, as well as immunoglobulin super family (IgSF) members, have been shown to translocate to the nucleus to function in transcriptional regulation. As an example, the voltage-gated sodium channel β1 subunit, which is also an IgSF-cell adhesion molecule (CAM), is a substrate for RIP. β1 RIP results in generation of a soluble intracellular domain, which can regulate gene expression in the nucleus. In this review, we discuss the proposed RIP mechanisms of voltage-gated sodium, potassium, and calcium channel subunits as well as the roles of their generated proteolytic products in the nucleus. We also discuss other RIP substrates that are cleaved by similar sheddases and iCLiPs, such as IgSF macromolecules, including CAMs, whose proteolytically generated fragments function in the nucleus. Importantly, dysfunctional RIP mechanisms are linked to human disease. Thus, we will also review how understanding RIP events and subsequent signaling processes involving ion channel subunits and IgSF proteins may lead to the discovery of novel therapeutic targets. SIGNIFICANCE STATEMENT: Several ion channel subunits and immunoglobulin superfamily molecules have been identified as substrates of regulated intramembrane proteolysis (RIP). This signal transduction mechanism, which generates polypeptide fragments that translocate to the nucleus, is an important regulator of gene transcription. RIP may impact diseases of excitability, including epilepsy, cardiac arrhythmia, and sudden death syndromes. A thorough understanding of the role of RIP in gene regulation is critical as it may reveal novel therapeutic strategies for the treatment of previously intractable diseases.

Subcellular dynamics and functional activity of the cleaved intracellular domain of the Na+ channel β1 subunit

The voltage-gated Na⁺ channel β1 subunit, encoded by SCN1B, regulates cell surface expression and gating of α subunits and participates in cell adhesion. β1 is cleaved by α/β and γ-secretases, releasing an extracellular domain and intracellular domain (ICD), respectively. Abnormal SCN1B expression/function is linked to pathologies including epilepsy, cardiac arrhythmia, and cancer. In this study, we sought to determine the effect of secretase cleavage on β1 function in breast cancer cells. Using a series of GFP-tagged β1 constructs, we show that β1-GFP is mainly retained intracellularly, particularly in the endoplasmic reticulum and endolysosomal pathway, and accumulates in the nucleus. Reduction in endosomal β1-GFP levels occurred following γ-secretase inhibition, implicating endosomes and/or the preceding plasma membrane as important sites for secretase processing. Using live-cell imaging, we also report β1ICD-GFP accumulation in the nucleus. Furthermore, β1-GFP and β1ICD-GFP both increased Na⁺ current, whereas β1STOP-GFP, which lacks the ICD, did not, thus highlighting that the β1-ICD is necessary and sufficient to increase Na⁺ current measured at the plasma membrane. Importantly, although the endogenous Na⁺ current expressed in MDA-MB-231 cells is tetrodotoxin (TTX)-resistant (carried by Na_v1.5), the Na⁺ current increased by β1-GFP or β1ICD-GFP was TTX-sensitive. Finally, we found β1-GFP increased mRNA levels of the TTX-sensitive α subunits SCN1A/Na_v1.1 and SCN9A/Na_v1.7. Taken together, this work suggests that the β1-ICD is a critical regulator of α subunit function in cancer cells. Our data further highlight that γ-secretase may play a key role in regulating β1 function in breast cancer.

Analysis of Age-Dependent Alterations in Excitability Properties of CA1 Pyramidal Neurons in an APPPS1 Model of Alzheimer's Disease

Age-dependent accumulation of amyloid-β, provoking increasing brain amyloidopathy, triggers abnormal patterns of neuron activity and circuit synchronization in Alzheimer’s disease (AD) as observed in human AD patients and AD mouse models. Recent studies on AD mouse models, mimicking this age-dependent amyloidopathy, identified alterations in CA1 neuron excitability. However, these models generally also overexpress mutated amyloid precursor protein (APP) and presenilin 1 (PS1) and there is a lack of a clear correlation of neuronal excitability alterations with progressive amyloidopathy. The active development of computational models of AD points out the need of collecting such experimental data to build a reliable disease model exhibiting AD-like disease progression. We therefore used the feature extraction tool of the Human Brain Project (HBP) Brain Simulation Platform to systematically analyze the excitability profile of CA1 pyramidal neuron in the APPPS1 mouse model. We identified specific features of neuron excitability that best correlate either with over-expression of mutated APP and PS1 or increasing Aβ amyloidopathy. Notably, we report strong alterations in membrane time constant and action potential width and weak alterations in firing behavior. Also, using a CA1 pyramidal neuron model, we evidence amyloidopathy-dependent alterations in I_h. Finally, cluster analysis of these recordings showed that we could reliably assign a trace to its correct group, opening the door to a more refined, less variable analysis of AD-affected neurons. This inter-disciplinary analysis, bringing together experimentalists and modelers, helps to further unravel the neuronal mechanisms most affected by AD and to build a biologically plausible computational model of the AD brain.

Sodium channel β1 subunits participate in regulated intramembrane proteolysis-excitation coupling

Loss-of-function (LOF) variants in SCN1B, encoding voltage-gated sodium channel β1 subunits, are linked to human diseases with high risk of sudden death, including developmental and epileptic encephalopathy and cardiac arrhythmia. β1 Subunits modulate the cell-surface localization, gating, and kinetics of sodium channel pore-forming α subunits. They also participate in cell-cell and cell-matrix adhesion, resulting in intracellular signal transduction, promotion of cell migration, calcium handling, and regulation of cell morphology. Here, we investigated regulated intramembrane proteolysis (RIP) of β1 by BACE1 and γ-secretase and show that β1 subunits are substrates for sequential RIP by BACE1 and γ-secretase, resulting in the generation of a soluble intracellular domain (ICD) that is translocated to the nucleus. Using RNA sequencing, we identified a subset of genes that are downregulated by β1-ICD overexpression in heterologous cells but upregulated in Scn1b-null cardiac tissue, which lacks β1-ICD signaling, suggesting that the β1-ICD may normally function as a molecular brake on gene transcription in vivo. We propose that human disease variants resulting in SCN1B LOF cause transcriptional dysregulation that contributes to altered excitability. Moreover, these results provide important insights into the mechanism of SCN1B-linked channelopathies, adding RIP-excitation coupling to the multifunctionality of sodium channel β1 subunits.

Amyloid Beta Hypothesis in Alzheimer's Disease: Major Culprits and Recent Therapeutic Strategies

Alzheimer's disease (AD) is one of the most common forms of dementia and has been a global concern for several years. Due to the multi-factorial nature of the disease, AD has become irreversible, fatal and imposes a tremendous socio-economic burden. Even though experimental medicines suggested moderate benefits, AD still lacks an effective treatment strategy for the management of symptoms or cure. Among the various hypotheses that describe development and progression of AD, the amyloid hypothesis has been a long-term adherent to the AD due to the involvement of various forms of Amyloid beta (Aβ) peptides in the impairment of neuronal and cognitive functions. Hence, majority of the drug discovery approaches in the past have focused on the prevention of the accumulation of Aβ peptides. Currently, there are several agents in the phase III clinical trials that target Aβ or the various macromolecules triggering Aβ deposition. In this review, we present the state of the art knowledge on the functional aspects of the key players involved in the amyloid hypothesis. Furthermore, we also discuss anti-amyloid agents present in the Phase III clinical trials.

Sodium channel β1 subunits are post-translationally modified by tyrosine phosphorylation, S-palmitoylation, and regulated intramembrane proteolysis

Voltage-gated sodium channel (VGSC) β1 subunits are multifunctional proteins that modulate the biophysical properties and cell-surface localization of VGSC α subunits and participate in cell-cell and cell-matrix adhesion, all with important implications for intracellular signal transduction, cell migration, and differentiation. Human loss-of-function variants in SCN1B, the gene encoding the VGSC β1 subunits, are linked to severe diseases with high risk for sudden death, including epileptic encephalopathy and cardiac arrhythmia. We showed previously that β1 subunits are post-translationally modified by tyrosine phosphorylation. We also showed that β1 subunits undergo regulated intramembrane proteolysis via the activity of β-secretase 1 and γ-secretase, resulting in the generation of a soluble intracellular domain, β1-ICD, which modulates transcription. Here, we report that β1 subunits are phosphorylated by FYN kinase. Moreover, we show that β1 subunits are S-palmitoylated. Substitution of a single residue in β1, Cys-162, to alanine prevented palmitoylation, reduced the level of β1 polypeptides at the plasma membrane, and reduced the extent of β1-regulated intramembrane proteolysis, suggesting that the plasma membrane is the site of β1 proteolytic processing. Treatment with the clathrin-mediated endocytosis inhibitor, Dyngo-4a, re-stored the plasma membrane association of β1-p.C162A to WT levels. Despite these observations, palmitoylation-null β1-p.C162A modulated sodium current and sorted to detergent-resistant membrane fractions normally. This is the first demonstration of S-palmitoylation of a VGSC β subunit, establishing precedence for this post-translational modification as a regulatory mechanism in this protein family.

The substrate repertoire of γ-secretase/presenilin

The intramembrane protease γ-secretase is a hetero-tetrameric protein complex with presenilin as the catalytic subunit and cleaves its membrane protein substrates within their single transmembrane domains. γ-Secretase is well known for its role in Notch signalling and in Alzheimer's disease, where it catalyzes the formation of the pathogenic amyloid β (Aβ) peptide. However, in the 21 years since its discovery many more substrates and substrate candidates of γ-secretase were identified. Although the physiological relevance of the cleavage of many substrates remains to be studied in more detail, the substrates demonstrate a broad role for γ-secretase in embryonic development, adult tissue homeostasis, signal transduction and protein degradation. Consequently, chronic γ-secretase inhibition may cause significant side effects due to inhibition of cleavage of multiple substrates. This review provides a list of 149 γ-secretase substrates identified to date and highlights by which expeirmental approach substrate cleavage was validated. Additionally, the review lists the cleavage sites where they are known and discusses the functional implications of γ-secretase cleavage with a focus on substrates identified in the recent past, such as CHL1, TREM2 and TNFR1. A comparative analysis demonstrates that γ-secretase substrates mostly have a long extracellular domain and require ectodomain shedding before γ-secretase cleavage, but that γ-secretase is also able to cleave naturally short substrates, such as the B cell maturation antigen. Taken together, the list of substrates provides a resource that may help in the future development of drugs inhibiting or modulating γ-secretase activity in a substrate-specific manner.

An alternative mechanism of early nodal clustering and myelination onset in GABAergic neurons of the central nervous system

In vertebrates, fast saltatory conduction along myelinated axons relies on the node of Ranvier. How nodes assemble on CNS neurons is not yet fully understood. We previously described that node-like clusters can form prior to myelin deposition in hippocampal GABAergic neurons and are associated with increased conduction velocity. Here, we used a live imaging approach to characterize the intrinsic mechanisms underlying the assembly of these clusters prior to myelination. We first demonstrated that their components can partially preassemble prior to membrane targeting and determined the molecular motors involved in their trafficking. We then demonstrated the key role of the protein β2Na_v for node-like clustering initiation. We further assessed the fate of these clusters when myelination proceeds. Our results shed light on the intrinsic mechanisms involved in node-like clustering prior to myelination and unravel a potential role of these clusters in node of Ranvier formation and in guiding myelination onset.

MicroRNA‑449a regulates the progression of brain aging by targeting SCN2B in SAMP8 mice

Our previous study demonstrated that the expression of sodium channel voltage-gated beta 2 (SCN2B) increased with aging in senescence-accelerated mouse prone 8 (SAMP8) mice, and was identified to be associated with a decline in learning and memory, while the underlying mechanism is unclear. In the present study, multiple differentially expressed miRNAs, which may be involved in the process of aging by regulating target genes, were identified in the prefrontal cortex and hippocampus of SAMP8 mice though miRNA microarray analysis. Using bioinformatics prediction, SCN2B was identified to be one of the potential target genes of miR-449a, which was downregulated in the hippocampus. Previous studies demonstrated that miR-449a is involved in the occurrence and progression of aging by regulating a variety of target genes. Therefore, it was hypothesized that miR-449a may be involved in the process of brain aging by targeting SCN2B. To verify this hypothesis, the following experiments were conducted: A reverse transcription-quantitative polymerase chain reaction assay revealed that the expression level of miR-449a was significantly decreased in the prefrontal cortex and hippocampus of 12-month old SAMP8 mice; a dual-luciferase reporter assay verified that miR-449a regulated SCN2B expression by binding to the 3′-UTR 'seed region'; an anti-Ago co-immunoprecipitation combined with Affymetrix micro-array analyses demonstrated that the target mRNA highly enriched with Ago-miRNPs was confirmed to be SCN2B. Finally, overexpression of miR-449a or inhibition of SCN2B promoted the extension of hippocampal neurons in vitro. The results of the present study suggested that miR-449a was downregulated in the prefrontal cortex and hippocampus of SAMP8 mice and may regulate the process of brain aging by targeting SCN2B.

Unraveling the complexity of γ-secretase

γ-Secretase was initially defined as a proteolytic activity that cleaves within the transmembrane of the amyloid precursor protein (APP) to produce the amyloid β-peptide of Alzheimer's disease. The discovery of mutations in APP and the presenilins associated with familial Alzheimer's disease and their effects on APP processing dovetailed with pharmacological studies on γ-secretase, leading to the revelation that presenilins are unprecedented membrane-embedded aspartyl proteases. Other members of what became known as the γ-secretase complex were subsequently identified. In parallel with these advances, connections between presenilins and Notch receptors essential to metazoan development became evident, resulting in the concurrent realization that γ-secretase also carries out intramembrane proteolysis of Notch as part of its signaling mechanism. Substantial progress has been made toward elucidating how γ-secretase carries out complex processing of transmembrane domains, how it goes awry in familial Alzheimer's disease, the scope of its substrates, and the atomic details of its structure. Critical questions remain for future study, toward further unraveling the complexity of this unique membrane-embedded proteolytic machine and its roles in biology and disease.

Mechanisms of hyperexcitability in Alzheimer's disease hiPSC-derived neurons and cerebral organoids vs isogenic controls

Human Alzheimer’s disease (AD) brains and transgenic AD mouse models manifest hyperexcitability. This aberrant electrical activity is caused by synaptic dysfunction that represents the major pathophysiological correlate of cognitive decline. However, the underlying mechanism for this excessive excitability remains incompletely understood. To investigate the basis for the hyperactivity, we performed electrophysiological and immunofluorescence studies on hiPSC-derived cerebrocortical neuronal cultures and cerebral organoids bearing AD-related mutations in presenilin-1 or amyloid precursor protein vs. isogenic gene corrected controls. In the AD hiPSC-derived neurons/organoids, we found increased excitatory bursting activity, which could be explained in part by a decrease in neurite length. AD hiPSC-derived neurons also displayed increased sodium current density and increased excitatory and decreased inhibitory synaptic activity. Our findings establish hiPSC-derived AD neuronal cultures and organoids as a relevant model of early AD pathophysiology and provide mechanistic insight into the observed hyperexcitability.

Trafficking and Function of the Voltage-Gated Sodium Channel β2 Subunit

The voltage-gated sodium channel is vital for cardiomyocyte function, and consists of a protein complex containing a pore-forming α subunit and two associated β subunits. A fundamental, yet unsolved, question is to define the precise function of β subunits. While their location in vivo remains unclear, large evidence shows that they regulate localization of α and the biophysical properties of the channel. The current data support that one of these subunits, β2, promotes cell surface expression of α. The main α isoform in an adult heart is Na_V1.5, and mutations in SCN5A, the gene encoding Na_V1.5, often lead to hereditary arrhythmias and sudden death. The association of β2 with cardiac arrhythmias has also been described, which could be due to alterations in trafficking, anchoring, and localization of Na_V1.5 at the cardiomyocyte surface. Here, we will discuss research dealing with mechanisms that regulate β2 trafficking, and how β2 could be pivotal for the correct localization of Na_V1.5, which influences cellular excitability and electrical coupling of the heart. Moreover, β2 may have yet to be discovered roles on cell adhesion and signaling, implying that diverse defects leading to human disease may arise due to β2 mutations.

Neuroprotection induced by Navβ2‑knockdown in APP/PS1 transgenic neurons is associated with NEP regulation

Voltage‑gated sodium channel β2 (Navβ2), as an unconventional substrate of β‑site amyloid precursor protein cleaving enzyme 1, is involved in regulating the neuronal surface expression of sodium channels. A previous study demonstrated that knockdown of Navβ2 protected neurons and induced spatial cognition improvement by partially reducing pathological amyloidogenic processing of amyloid precursor protein (APP) in aged APP/presenilin 1 (PS1) transgenic mice. The present study aimed to investigate whether Navβ2 knockdown altered APP metabolism via regulation of the Aβ‑degrading enzyme neprilysin (NEP). APPswe/PS1ΔE9 mice (APP/PS1 transgenic mice with a C57BL/6J genetic background) carrying a Navβ2‑knockdown mutation (APP/PS1/Navβ2‑kd) or without Navβ2 knockdown (APP/PS1) were used for cell culture and further analysis. The present results demonstrated that in APP/PS1 mouse‑derived neurons, Navβ2 knockdown partially reversed the reduction in pathological APP cleavage, and the recovery of neurite extension and neuron area. Additionally, Navβ2 knockdown increased NEP activity and levels, and the levels of intracellular domain fragment binding to the NEP promoter. The present findings suggested that knockdown of Navβ2 reversed the APP/PS1 mutation‑induced deficiency in amyloid β degradation by regulating NEP.

Emerging roles for multifunctional ion channel auxiliary subunits in cancer

Several superfamilies of plasma membrane channels which regulate transmembrane ion flux have also been shown to regulate a multitude of cellular processes, including proliferation and migration. Ion channels are typically multimeric complexes consisting of conducting subunits and auxiliary, non-conducting subunits. Auxiliary subunits modulate the function of conducting subunits and have putative non-conducting roles, further expanding the repertoire of cellular processes governed by ion channel complexes to processes such as transcellular adhesion and gene transcription. Given this expansive influence of ion channels on cellular behaviour it is perhaps no surprise that aberrant ion channel expression is a common occurrence in cancer. This review will focus on the conducting and non-conducting roles of the auxiliary subunits of various Ca2+, K+, Na+ and Cl- channels and the burgeoning evidence linking such auxiliary subunits to cancer. Several subunits are upregulated (e.g. Cavβ, Cavγ) and downregulated (e.g. Kvβ) in cancer, while other subunits have been functionally implicated as oncogenes (e.g. Navβ1, Cavα2δ1) and tumour suppressor genes (e.g. CLCA2, KCNE2, BKγ1) based on in vivo studies. The strengthening link between ion channel auxiliary subunits and cancer has exposed these subunits as potential biomarkers and therapeutic targets. However further mechanistic understanding is required into how these subunits contribute to tumour progression before their therapeutic potential can be fully realised.

pHluorin-BACE1-mCherry Acts as a Reporter for the Intracellular Distribution of Active BACE1 In Vitro and In Vivo

β-site APP-cleaving enzyme 1 (BACE1) initiates amyloid precursor protein (APP) cleavage and β-amyloid (Aβ) production, a critical step in the pathogenesis of Alzheimer's disease (AD). It is thus of considerable interest to investigate how BACE1 activity is regulated. BACE1 has its maximal activity at acidic pH and GFP variant-pHluorin-displays pH dependence. In light of these observations, we generated three tandem fluorescence-tagged BACE1 fusion proteins, named pHluorin-BACE1-mCherry, BACE1-mCherry-pHluorin and BACE1-mCherry-EGFP. Comparing the fluorescence characteristics of these proteins in response to intracellular pH changes induced by chloroquine or bafilomycin A1, we found that pHluorin-BACE1-mCherry is a better pH sensor for BACE1 because its fluorescence intensity responds to pH changes more dramatically and more quickly. Additionally, we found that (pro)renin receptor (PRR), a subunit of the v-ATPase complex, which is critical for maintaining vesicular pH, regulates pHluorin's fluorescence and BACE1 activity in pHluorin-BACE1-mCherry expressing cells. Finally, we found that the expression of Swedish mutant APP (APPswe) suppresses pHluorin fluorescence in pHluorin-BACE1-mCherry expressing cells in culture and in vivo, implicating APPswe not only as a substrate but also as an activator of BACE1. Taken together, these results suggest that the pHluorin-BACE1-mCherry fusion protein may serve as a useful tool for visualizing active/inactive BACE1 in culture and in vivo.

Exploring the influence of the surface proteins on probiotic effects performed by Lactobacillus pentosus HC-2 using transcriptome analysis in Litopenaeus vannamei midgut

In order to understand the mediation function of surface proteins in probiotic effects executed by Lactobacillus pentosus HC-2 in midgut of Litopenaeus vannamei, the immune and digestion related enzymes and the transcriptome expression were analyzed after shrimp fed with normal HC-2 or with stripped surface proteins HC-2 by lithium chloride (LiCl) treatment. The results showed that the shrimp fed with normal HC-2 produced much higher immune and digestion related enzymes than the control group or LiCl-treated HC-2 group to defense the Vibrio parahaemolyticus E1 infection. We obtained total over 275,099 unigenes from L. vannamei midgut, 981 genes were significant differentially expressed in normal HC-2 group compared with control, 1314 genes were significant differentially expressed in LiCl-treated HC-2 group compared with control, and 1689 genes were significant differentially expressed in LiCl-treated HC-2 group compared with normal HC-2 group. The GO/KEGG enrichment analysis of the significantly different genes demonstrated that L. vannamei fed with normal HC-2 induced immune-related, signal transduction, ion homeostasis, cell-cell adhesion, response stress/stimulus, vascular endothelial growth factor and peritrophin genes up-regulation, which were important genes involved in improving the shrimp intestine immune response, nutrition and growth performance, and bacteria adhesion and colonization, but these genes were suppressed in the midgut of shrimp fed with deprived surface proteins bacteria. Taken together, these results indicated that the surface proteins were essential for HC-2 executing probiotic effects in midgut of shrimp. Our data contribute to improve the current understanding of host - Lactobacillus interaction and the probiotic mechanisms in shrimps.

The Complex Work of Proteases and Secretases in Wallerian Degeneration: Beyond Neuregulin-1

After damage, axons in the peripheral nervous system (PNS) regenerate and regrow following a process termed Wallerian degeneration, but the regenerative process is often incomplete and usually the system does not reach full recovery. Key steps to the creation of a permissive environment for axonal regrowth are the trans-differentiation of Schwann cells and the remodeling of the extracellular matrix (ECM). In this review article, we will discuss how proteases and secretases promote effective regeneration and remyelination. We will detail how they control neuregulin-1 (NRG-1) activity at the post-translational level, as the concerted action of alpha, beta and gamma secretases cooperates to balance activating and inhibitory signals necessary for physiological myelination and remyelination. In addition, we will discuss the role of other proteases in nerve repair, among which A Disintegrin And Metalloproteinases (ADAMs) and gamma-secretases substrates. Moreover, we will present how matrix metalloproteinases (MMPs) and proteases of the blood coagulation cascade participate in forming newly synthetized myelin and in regulating axonal regeneration. Overall, we will highlight how a deeper comprehension of secretases and proteases mechanism of action in Wallerian degeneration might be useful to develop new therapies with the potential of readily and efficiently improve the regenerative process.

Contribution of voltage-gated sodium channel β-subunits to cervical cancer cells metastatic behavior

Background

Voltage-gated sodium (Na_V) channels are heteromeric proteins consisting of a single pore forming α-subunit associated with one or two auxiliary β-subunits. These channels are classically known for being responsible of action potential generation and propagation in excitable cells; but lately they have been reported as widely expressed and regulated in several human cancer types. We have previously demonstrated the overexpression of Na_V1.6 channel in cervical cancer (CeCa) biopsies and primary cultures, and its contribution to cell migration and invasiveness. Here, we investigated the expression of Na_V channels β-subunits (Na_Vβs) in the CeCa cell lines HeLa, SiHa and CaSki, and determined their contribution to cell proliferation, migration and invasiveness.

Methods

We assessed the expression of Na_Vβs in CeCa cell lines by performing RT-PCR and western blotting experiments. We also evaluated CeCa cell lines proliferation, migration, and invasion by in vitro assays, both in basal conditions and after inducing changes in Na_Vβs levels by transfecting specific cDNAs or siRNAs. The potential role of Na_Vβs in modulating the expression of Na_V α-subunits in the plasma membrane of CeCa cells was examined by the patch-clamp whole-cell technique. Furthermore, we investigated the role of Na_Vβ1 on cell cycle in SiHa cells by flow cytometry.

Results

We found that the four Na_Vβs are expressed in the three CeCa cell lines, even in the absence of functional Na_V α-subunit expression in the plasma membrane. Functional in vitro assays showed differential roles for Na_Vβ1 and Na_Vβ4, the latter as a cell invasiveness repressor and the former as a migration abolisher in CeCa cells. In silico analysis of Na_Vβ4 expression in cervical tissues corroborated the downregulation of this protein expression in CeCa vs normal cervix, supporting the evidence of Na_Vβ4’s role as a cell invasiveness repressor.

Conclusions

Our results contribute to the recent conception about Na_Vβs as multifunctional proteins involved in cell processes like ion channel regulation, cell adhesion and motility, and even in metastatic cell behaviors. These non-canonical functions of Na_Vβs are independent of the presence of functional Na_V α-subunits in the plasma membrane and might represent a new therapeutic target for the treatment of cervical cancer.

Electronic supplementary material

The online version of this article (10.1186/s12935-019-0757-6) contains supplementary material, which is available to authorized users.

Voltage-gated sodium channel β subunits: The power outside the pore in brain development and disease

Voltage gated sodium channels (VGSCs) were first identified in terms of their role in the upstroke of the action potential. The underlying proteins were later identified as saxitoxin and scorpion toxin receptors consisting of α and β subunits. We now know that VGSCs are heterotrimeric complexes consisting of a single pore forming α subunit joined by two β subunits; a noncovalently linked β1 or β3 and a covalently linked β2 or β4 subunit. VGSC α subunits contain all the machinery necessary for channel cell surface expression, ion conduction, voltage sensing, gating, and inactivation, in one central, polytopic, transmembrane protein. VGSC β subunits are more than simple accessories to α subunits. In the more than two decades since the original cloning of β1, our knowledge of their roles in physiology and pathophysiology has expanded immensely. VGSC β subunits are multifunctional. They confer unique gating mechanisms, regulate cellular excitability, affect brain development, confer distinct channel pharmacology, and have functions that are independent of the α subunits. The vast array of functions of these proteins stems from their special station in the channelome: being the only known constituents that are cell adhesion and intra/extracellular signaling molecules in addition to being part of channel complexes. This functional trifecta and how it goes awry demonstrates the power outside the pore in ion channel signaling complexes, broadening the term channelopathy beyond defects in ion conduction. This article is part of the Special Issue entitled 'Channelopathies.'

Do sodium channel proteolytic fragments regulate sodium channel expression?

The cardiac voltage-gated sodium channel (gene: SCN5A, protein: Na_V1.5) is responsible for the sodium current that initiates the cardiomyocyte action potential. Research into the mechanisms of SCN5A gene expression has gained momentum over the last few years. We have recently described the transcriptional regulation of SCN5A by GATA4 transcription factor. In this addendum to our study, we report our observations that 1) the linker between domains I and II (L_DI-DII) of Na_V1.5 contains a nuclear localization signal (residues 474-481) that is necessary to localize L_DI-DII into the nucleus, and 2) nuclear L_DI-DII activates the SCN5A promoter in gene reporter assays using cardiac-like H9c2 cells. Given that voltage-gated sodium channels are known targets of proteases such as calpain, we speculate that Na_V1.5 degradation is signaled to the cell transcriptional machinery via nuclear localization of L_DI-DII and subsequent stimulation of the SCN5A promoter.

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

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.

Intramembranous processing by γ-secretase regulates reverse signaling of ephrin-B2 in migration of microglia

The Eph-ephrin system plays pivotal roles in cell adhesion and migration. The receptor-like functions of the ephrin ligands allow the regulation of intracellular processes via reverse signaling. γ-Secretase mediated processing of ephrin-B has previously been linked to activation of Src, a kinase crucial for focal adhesion and podosome phosphorylation. Here, we analyzed the role of γ-secretase in the stimulation of reverse ephrin-B2 signaling in the migration of mouse embryonic stem cell derived microglia. The proteolytic generation of the ephrin-B2 intracellular domain (ICD) by γ-secretase stimulates Src and focal adhesion kinase (FAK). Inhibition of γ-secretase decreased the phosphorylation of Src and FAK, and reduced cell motility. These effects were associated with enlargement of the podosomal surface. Interestingly, expression of ephrin-B2 ICD could rescue these effects, indicating that this proteolytic fragment mediates the activation of Src and FAK, and thereby regulates podosomal dynamics in microglial cells. Together, these results identify γ-secretase as well as ephrin-B2 as regulators of microglial migration.

Mining Protein Evolution for Insights into Mechanisms of Voltage-Dependent Sodium Channel Auxiliary Subunits

Voltage-gated sodium channel (VGSC) beta (β) subunits have been called the "overachieving" auxiliary ion channel subunit. Indeed, these subunits regulate the trafficking of the sodium channel complex at the plasma membrane and simultaneously tune the voltage-dependent properties of the pore-forming alpha-subunit. It is now known that VGSC β-subunits are capable of similar modulation of multiple isoforms of related voltage-gated potassium channels, suggesting that their abilities extend into the broader voltage-gated channels. The gene family for these single transmembrane immunoglobulin beta-fold proteins extends well beyond the traditional VGSC β1-β4 subunit designation, with deep roots into the cell adhesion protein family and myelin-related proteins - where inherited mutations result in a myriad of electrical signaling disorders. Yet, very little is known about how VGSC β-subunits support protein trafficking pathways, the basis for their modulation of voltage-dependent gating, and, ultimately, their role in shaping neuronal excitability. An evolutionary approach can be useful in yielding new clues to such functions as it provides an unbiased assessment of protein residues, folds, and functions. An approach is described here which indicates the greater emergence of the modern β-subunits roughly 400 million years ago in the early neurons of Bilateria and bony fish, and the unexpected presence of distant homologues in bacteriophages. Recent structural breakthroughs containing α and β eukaryotic sodium channels containing subunits suggest a novel role for a highly conserved polar contact that occurs within the transmembrane segments. Overall, a mixture of approaches will ultimately advance our understanding of the mechanism for β-subunit interactions with voltage-sensor containing ion channels and membrane proteins.

Voltage-Gated Sodium Channel β Subunits and Their Related Diseases

Voltage-gated sodium channels are protein complexes comprised of one pore forming α subunit and two, non-pore forming, β subunits. The voltage-gated sodium channel β subunits were originally identified to function as auxiliary subunits, which modulate the gating, kinetics, and localization of the ion channel pore. Since that time, the five β subunits have been shown to play crucial roles as multifunctional signaling molecules involved in cell adhesion, cell migration, neuronal pathfinding, fasciculation, and neurite outgrowth. Here, we provide an overview of the evidence implicating the β subunits in their conducting and non-conducting roles. Mutations in the β subunit genes (SCN1B-SCN4B) have been linked to a variety of diseases. These include cancer, epilepsy, cardiac arrhythmias, sudden infant death syndrome/sudden unexpected death in epilepsy, neuropathic pain, and multiple neurodegenerative disorders. β subunits thus provide novel therapeutic targets for future drug discovery.

Dual roles of voltage-gated sodium channels in development and cancer

Voltage-gated Na(+) channels (VGSCs) are heteromeric protein complexes containing pore-forming α subunits together with non-pore-forming β subunits. There are nine α subunits, Nav1.1-Nav1.9, and four β subunits, β1-β4. The β subunits are multifunctional, modulating channel activity, cell surface expression, and are members of the immunoglobulin superfamily of cell adhesion molecules. VGSCs are classically responsible for action potential initiation and conduction in electrically excitable cells, including neurons and muscle cells. In addition, through the β1 subunit, VGSCs regulate neurite outgrowth and pathfinding in the developing central nervous system. Reciprocal signalling through Nav1.6 and β1 collectively regulates Na(+) current, electrical excitability and neurite outgrowth in cerebellar granule neurons. Thus, α and β subunits may have diverse interacting roles dependent on cell/tissue type. VGSCs are also expressed in non-excitable cells, including cells derived from a number of types of cancer. In cancer cells, VGSC α and β subunits regulate cellular morphology, migration, invasion and metastasis. VGSC expression associates with poor prognosis in several studies. It is hypothesised that VGSCs are up-regulated in metastatic tumours, favouring an invasive phenotype. Thus, VGSCs may have utility as prognostic markers, and/or as novel therapeutic targets for reducing/preventing metastatic disease burden. VGSCs appear to regulate a number of key cellular processes, both during normal postnatal development of the CNS and during cancer metastasis, by a combination of conducting (i.e. via Na(+) current) and non-conducting mechanisms.

Proteinases, Their Extracellular Targets, and Inflammatory Signaling

Given that over 2% of the human genome codes for proteolytic enzymes and their inhibitors, it is not surprising that proteinases serve many physiologic-pathophysiological roles. In this context, we provide an overview of proteolytic mechanisms regulating inflammation, with a focus on cell signaling stimulated by the generation of inflammatory peptides; activation of the proteinase-activated receptor (PAR) family of G protein-coupled receptors (GPCR), with a mechanism in common with adhesion-triggered GPCRs (ADGRs); and by proteolytic ion channel regulation. These mechanisms are considered in the much wider context that proteolytic mechanisms serve, including the processing of growth factors and their receptors, the regulation of matrix-integrin signaling, and the generation and release of membrane-tethered receptor ligands. These signaling mechanisms are relevant for inflammatory, neurodegenerative, and cardiovascular diseases as well as for cancer. We propose that the inflammation-triggering proteinases and their proteolytically generated substrates represent attractive therapeutic targets and we discuss appropriate targeting strategies.

Arm Selection Preference of MicroRNA-193a Varies in Breast Cancer

MicroRNAs (miRNAs) are short noncoding RNAs derived from the 3′ and 5′ ends of the same precursor. However, the biological function and mechanism of miRNA arm expression preference remain unclear in breast cancer. We found significant decreases in the expression levels of miR-193a-5p but no significant differences in those of miR-193a-3p in breast cancer. MiR-193a-3p suppressed breast cancer cell growth and migration and invasion abilities, whereas miR-193a-5p suppressed cell growth but did not influence cell motility. Furthermore, NLN and CCND1, PLAU, and SEPN1 were directly targeted by miR-193a-5p and miR-193a-3p, respectively, in breast cancer cells. The endogenous levels of miR-193a-5p and miR-193a-3p were significantly increased by transfecting breast cancer cells with the 3′UTR of their direct targets. Comprehensive analysis of The Cancer Genome Atlas database revealed significant differences in the arm expression preferences of several miRNAs between breast cancer and adjacent normal tissues. Our results collectively indicate that the arm expression preference phenomenon may be attributable to the target gene amount during breast cancer progression. The miRNA arm expression preference may be a means of modulating miRNA function, further complicating the mRNA regulatory network. Our findings provide a new insight into miRNA regulation and an application for breast cancer therapy.

Ion channel regulation by β-secretase BACE1 - enzymatic and non-enzymatic effects beyond Alzheimer's disease

β-site APP-cleaving enzyme 1 (BACE1) has become infamous for its pivotal role in the pathogenesis of Alzheimer's disease (AD). Consequently, BACE1 represents a prime target in drug development. Despite its detrimental involvement in AD, it should be quite obvious that BACE1 is not primarily present in the brain to drive mental decline. In fact, additional functions have been identified. In this review, we focus on the regulation of ion channels, specifically voltage-gated sodium and KCNQ potassium channels, by BACE1. These studies provide evidence for a highly unexpected feature in the functional repertoire of BACE1. Although capable of cleaving accessory channel subunits, BACE1 exerts many of its physiologically significant effects through direct, non-enzymatic interactions with main channel subunits. We discuss how the underlying mechanisms can be conceived and develop scenarios how the regulation of ion conductances by BACE1 might shape electric activity in the intact and diseased brain and heart.

Voltage-Gated Na+ Channels: Not Just for Conduction

Voltage-gated sodium channels (VGSCs), composed of a pore-forming α subunit and up to two associated β subunits, are critical for the initiation of the action potential (AP) in excitable tissues. Building on the monumental discovery and description of sodium current in 1952, intrepid researchers described the voltage-dependent gating mechanism, selectivity of the channel, and general structure of the VGSC channel. Recently, crystal structures of bacterial VGSC α subunits have confirmed many of these studies and provided new insights into VGSC function. VGSC β subunits, first cloned in 1992, modulate sodium current but also have nonconducting roles as cell-adhesion molecules and function in neurite outgrowth and neuronal pathfinding. Mutations in VGSC α and β genes are associated with diseases caused by dysfunction of excitable tissues such as epilepsy. Because of the multigenic and drug-resistant nature of some of these diseases, induced pluripotent stem cells and other novel approaches are being used to screen for new drugs and further understand how mutations in VGSC genes contribute to pathophysiology.

Elevated Neuronal Excitability Due to Modulation of the Voltage-Gated Sodium Channel Nav1.6 by Aβ1-42

Aberrant increases in neuronal network excitability may contribute to the cognitive deficits in Alzheimer's disease (AD). However, the mechanisms underlying hyperexcitability are not fully understood. Such overexcitation of neuronal networks has been detected in the brains of APP/PS1 mice. In the present study, using current-clamp recording techniques, we observed that 12 days in vitro (DIV) primary cultured pyramidal neurons from P0 APP/PS1 mice exhibited a more prominent action potential burst and a lower threshold than WT littermates. Moreover, after treatment with Aβ₁₋₄₂ peptide, 12 DIV primary cultured neurons showed similar changes, to a greater degree than in controls. Voltage-clamp recordings revealed that the voltage-dependent sodium current density of neurons incubated with Aβ₁₋₄₂ was significantly increased, without change in the voltage-dependent sodium channel kinetic characteristics. Immunohistochemistry and western blot results showed that, after treatment with Aβ₁₋₄₂, expressions of Nav and Nav1.6 subtype increased in cultured neurons or APP/PS1 brains compared to control groups. The intrinsic neuronal hyperexcitability of APP/PS1 mice might thus be due to an increased expression of voltage-dependent sodium channels induced by Aβ₁₋₄₂. These results may illuminate the mechanism of aberrant neuronal networks in AD.

Binary architecture of the Nav1.2-β2 signaling complex

To investigate the mechanisms by which β-subunits influence Nav channel function, we solved the crystal structure of the β2 extracellular domain at 1.35Å. We combined these data with known bacterial Nav channel structural insights and novel functional studies to determine the interactions of specific residues in β2 with Nav1.2. We identified a flexible loop formed by (72)Cys and (75)Cys, a unique feature among the four β-subunit isoforms. Moreover, we found that (55)Cys helps to determine the influence of β2 on Nav1.2 toxin susceptibility. Further mutagenesis combined with the use of spider toxins reveals that (55)Cys forms a disulfide bond with (910)Cys in the Nav1.2 domain II pore loop, thereby suggesting a 1:1 stoichiometry. Our results also provide clues as to which disulfide bonds are formed between adjacent Nav1.2 (912/918)Cys residues. The concepts emerging from this work will help to form a model reflecting the β-subunit location in a Nav channel complex.

An update on transcriptional and post-translational regulation of brain voltage-gated sodium channels

Voltage-gated sodium channels are essential proteins in brain physiology, as they generate the sodium currents that initiate neuronal action potentials. Voltage-gated sodium channels expression, localisation and function are regulated by a range of transcriptional and post-translational mechanisms. Here, we review our understanding of regulation of brain voltage-gated sodium channels, in particular SCN1A (Na_V1.1), SCN2A (Na_V1.2), SCN3A (Na_V1.3) and SCN8A (Na_V1.6), by transcription factors, by alternative splicing, and by post-translational modifications. Our focus is strongly centred on recent research lines, and newly generated knowledge.

An aberrant sugar modification of BACE1 blocks its lysosomal targeting in Alzheimer's disease

The β-site amyloid precursor protein cleaving enzyme-1 (BACE1), an essential protease for the generation of amyloid-β (Aβ) peptide, is a major drug target for Alzheimer's disease (AD). However, there is a concern that inhibiting BACE1 could also affect several physiological functions. Here, we show that BACE1 is modified with bisecting N-acetylglucosamine (GlcNAc), a sugar modification highly expressed in brain, and demonstrate that AD patients have higher levels of bisecting GlcNAc on BACE1. Analysis of knockout mice lacking the biosynthetic enzyme for bisecting GlcNAc, GnT-III (Mgat3), revealed that cleavage of Aβ-precursor protein (APP) by BACE1 is reduced in these mice, resulting in a decrease in Aβ plaques and improved cognitive function. The lack of this modification directs BACE1 to late endosomes/lysosomes where it is less colocalized with APP, leading to accelerated lysosomal degradation. Notably, other BACE1 substrates, CHL1 and contactin-2, are normally cleaved in GnT-III-deficient mice, suggesting that the effect of bisecting GlcNAc on BACE1 is selective to APP. Considering that GnT-III-deficient mice remain healthy, GnT-III may be a novel and promising drug target for AD therapeutics.

MicroRNA-9 induces defective trafficking of Nav1.1 and Nav1.2 by targeting Navβ2 protein coding region in rat with chronic brain hypoperfusion

Background

Previous studies have demonstrated that the trafficking defects of Nav1.1/Nav1.2 are involved in the dementia pathophysiology. However, the detailed mechanisms are not fully understood. Moreover, whether the impaired miRNAs regulation linked to dementia is a key player in sodium channel trafficking disturbance remains unclear. The cognitive impairment induced by chronic cerebral ischemia through chronic brain hypoperfusion (CBH) is likely reason to precede dementia. Therefore, our goal in the present study was to examine the role of microRNA-9 (miR-9) in regulating Nav1.1/Nav1.2 trafficking under CBH generated by bilateral common carotid artery occlusion (2VO).

Results

The impairment of Nav1.1/Nav1.2 trafficking and decreased expression of Navβ2 were found in the hippocampi and cortices of rats following CBH generated by bilateral 2VO. MiR-9 was increased in both the hippocampi and cortices of rats following CBH by qRT-PCR. Intriguingly, miR-9 suppressed, while AMO-miR-9 enhanced, the trafficking of Nav1.1/Nav1.2 from cytoplasm to cell membrane. Further study showed that overexpression of miR-9 inhibited the Navβ2 expression by targeting on its coding sequence (CDS) domain by dual luciferase assay. However, binding-site mutation or miR-masks failed to influence Navβ2 expression as well as Nav1.1/Nav1.2 trafficking process, indicating that Navβ2 is a potential target for miR-9. Lentivirus-mediated miR-9 overexpression also inhibited Navβ2 expression and elicited translocation deficits to cell membrane of Nav1.1/Nav1.2 in rats, whereas injection of lentivirus-mediated miR-9 knockdown could reverse the impaired trafficking of Nav1.1/Nav1.2 triggered by 2VO.

Conclusions

We conclude that miR-9 may play a key role in regulating the process of Nav1.1/Nav1.2 trafficking via targeting on Navβ2 protein in 2VO rats at post-transcriptional level, and inhibition of miR-9 may be a potentially valuable approach to prevent Nav1.1/Nav1.2 trafficking disturbance induced by CBH.

Diverse impact of acute and long-term extracellular proteolytic activity on plasticity of neuronal excitability

Learning and memory require alteration in number and strength of existing synaptic connections. Extracellular proteolysis within the synapses has been shown to play a pivotal role in synaptic plasticity by determining synapse structure, function, and number. Although synaptic plasticity of excitatory synapses is generally acknowledged to play a crucial role in formation of memory traces, some components of neural plasticity are reflected by nonsynaptic changes. Since information in neural networks is ultimately conveyed with action potentials, scaling of neuronal excitability could significantly enhance or dampen the outcome of dendritic integration, boost neuronal information storage capacity and ultimately learning. However, the underlying mechanism is poorly understood. With this regard, several lines of evidence and our most recent study support a view that activity of extracellular proteases might affect information processing in neuronal networks by affecting targets beyond synapses. Here, we review the most recent studies addressing the impact of extracellular proteolysis on plasticity of neuronal excitability and discuss how enzymatic activity may alter input-output/transfer function of neurons, supporting cognitive processes. Interestingly, extracellular proteolysis may alter intrinsic neuronal excitability and excitation/inhibition balance both rapidly (time of minutes to hours) and in long-term window. Moreover, it appears that by cleavage of extracellular matrix (ECM) constituents, proteases may modulate function of ion channels or alter inhibitory drive and hence facilitate active participation of dendrites and axon initial segments (AISs) in adjusting neuronal input/output function. Altogether, a picture emerges whereby both rapid and long-term extracellular proteolysis may influence some aspects of information processing in neurons, such as initiation of action potential, spike frequency adaptation, properties of action potential and dendritic backpropagation.

The normal and pathologic roles of the Alzheimer's β-secretase, BACE1

As the most common neurodegenerative disease, therapeutic avenues for the treatment and prevention of Alzheimer's Disease are highly sought after. The aspartic protease BACE1 is the initiator enzyme for the formation of Aβ, a major constituent of amyloid plaques that represent one of the hallmark pathological features of this disorder. Thus, targeting BACE1 for disease-modifying AD therapies represents a rationale approach. The collective knowledge acquired from investigations of BACE1 deletion mutants and characterization of BACE1 substrates has downstream significance not only for the discovery of AD drug therapies but also for predicting side effects of BACE1 inhibition. Here we discuss the identification and validation of BACE1 as the β-secretase implicated in AD, in addition to information regarding BACE1 cell biology, localization, substrates and potential physiological functions derived from BACE1 knockout models.

The role of non-pore-forming β subunits in physiology and pathophysiology of voltage-gated sodium channels

Voltage-gated sodium channel β1 and β2 subunits were discovered as auxiliary proteins that co-purify with pore-forming α subunits in brain. The other family members, β1B, β3, and β4, were identified by homology and shown to modulate sodium current in heterologous systems. Work over the past 2 decades, however, has provided strong evidence that these proteins are not simply ancillary ion channel subunits, but are multifunctional signaling proteins in their own right, playing both conducting (channel modulatory) and nonconducting roles in cell signaling. Here, we discuss evidence that sodium channel β subunits not only regulate sodium channel function and localization but also modulate voltage-gated potassium channels. In their nonconducting roles, VGSC β subunits function as immunoglobulin superfamily cell adhesion molecules that modulate brain development by influencing cell proliferation and migration, axon outgrowth, axonal fasciculation, and neuronal pathfinding. Mutations in genes encoding β subunits are linked to paroxysmal diseases including epilepsy, cardiac arrhythmia, and sudden infant death syndrome. Finally, β subunits may be targets for the future development of novel therapeutics.

The E280A presenilin mutation reduces voltage-gated sodium channel levels in neuronal cells

BACKGROUND

Familial Alzheimer's disease (FAD) mutations in presenilin (PS) modulate PS/γ-secretase activity and therefore contribute to AD pathogenesis. Previously, we found that PS/γ-secretase cleaves voltage-gated sodium channel β2-subunits (Navβ2), releases the intracellular domain of Navβ2 (β2-ICD), and thereby, increases intracellular sodium channel α-subunit Nav1.1 levels. Here, we tested whether FAD-linked PS1 mutations modulate Navβ2 cleavages and Nav1.1 levels.

OBJECTIVE

It was the aim of this study to analyze the effects of PS1-linked FAD mutations on Navβ2 processing and Nav1.1 levels in neuronal cells.

METHODS

We first generated B104 rat neuroblastoma cells stably expressing Navβ2 and wild-type PS1 (wtPS1), PS1 with one of three FAD mutations (E280A, M146L or ΔE9), or PS1 with a non-FAD mutation (D333G). Navβ2 processing and Nav1.1 protein and mRNA levels were then analyzed by Western blot and real-time RT-PCR, respectively.

RESULTS

The FAD-linked E280A mutation significantly decreased PS/γ-secretase-mediated processing of Navβ2 as compared to wtPS1 controls, both in cells and in a cell-free system. Nav1.1 mRNA and protein levels, as well as the surface levels of Nav channel α-subunits, were also significantly reduced in PS1(E280A) cells.

CONCLUSION

Our data indicate that the FAD-linked PS1(E280A) mutation decreases Nav channel levels by partially inhibiting the PS/γ-secretase-mediated cleavage of Navβ2 in neuronal cells.

The Alzheimer's β-secretase BACE1 localizes to normal presynaptic terminals and to dystrophic presynaptic terminals surrounding amyloid plaques

β-Site amyloid precursor protein (APP) cleaving enzyme-1 (BACE1) is the β-secretase that initiates Aβ production in Alzheimer’s disease (AD). BACE1 levels are increased in AD, which could contribute to pathogenesis, yet the mechanism of BACE1 elevation is unclear. Furthermore, the normal function of BACE1 is poorly understood. We localized BACE1 in the brain at both the light and electron microscopic levels to gain insight into normal and pathophysiologic roles of BACE1 in health and AD, respectively. Our findings provide the first ultrastructural evidence that BACE1 localizes to vesicles (likely endosomes) in normal hippocampal mossy fiber terminals of both non-transgenic and APP transgenic (5XFAD) mouse brains. In some instances, BACE1-positive vesicles were located near active zones, implying a function for BACE1 at the synapse. In addition, BACE1 accumulated in swollen dystrophic autophagosome-poor presynaptic terminals surrounding amyloid plaques in 5XFAD cortex and hippocampus. Importantly, accumulations of BACE1 and APP co-localized in presynaptic dystrophies, implying increased BACE1 processing of APP in peri-plaque regions. In primary cortical neuron cultures, treatment with the lysosomal protease inhibitor leupeptin caused BACE1 levels to increase; however, exposure of neurons to the autophagy inducer trehalose did not reduce BACE1 levels. This suggests that BACE1 is degraded by lysosomes but not by autophagy. Our results imply that BACE1 elevation in AD could be linked to decreased lysosomal degradation of BACE1 within dystrophic presynaptic terminals. Elevated BACE1 and APP levels in plaque-associated presynaptic dystrophies could increase local peri-plaque Aβ generation and accelerate amyloid plaque growth in AD.

CRMP2 protein SUMOylation modulates NaV1.7 channel trafficking

Voltage-gated sodium channel (NaV) trafficking is incompletely understood. Post-translational modifications of NaVs and/or auxiliary subunits and protein-protein interactions have been posited as NaV-trafficking mechanisms. Here, we tested if modification of the axonal collapsin response mediator protein 2 (CRMP2) by a small ubiquitin-like modifier (SUMO) could affect NaV trafficking; CRMP2 alters the extent of NaV slow inactivation conferred by the anti-epileptic (R)-lacosamide, implying NaV-CRMP2 functional coupling. Expression of a CRMP2 SUMOylation-incompetent mutant (CRMP2-K374A) in neuronal model catecholamine A differentiated (CAD) cells did not alter lacosamide-induced NaV slow inactivation compared with CAD cells expressing wild type CRMP2. Like wild type CRMP2, CRMP2-K374A expressed robustly in CAD cells. Neurite outgrowth, a canonical CRMP2 function, was moderately reduced by the mutation but was still significantly higher than enhanced GFP-transfected cortical neurons. Notably, huwentoxin-IV-sensitive NaV1.7 currents, which predominate in CAD cells, were significantly reduced in CAD cells expressing CRMP2-K374A. Increasing deSUMOylation with sentrin/SUMO-specific protease SENP1 or SENP2 in wild type CRMP2-expressing CAD cells decreased NaV1.7 currents. Consistent with a reduction in current density, biotinylation revealed a significant reduction in surface NaV1.7 levels in CAD cells expressing CRMP2-K374A; surface NaV1.7 expression was also decreased by SENP1 + SENP2 overexpression. Currents in HEK293 cells stably expressing NaV1.7 were reduced by CRMP2-K374A in a manner dependent on the E2-conjugating enzyme Ubc9. No decrement in current density was observed in HEK293 cells co-expressing CRMP2-K374A and NaV1.1 or NaV1.3. Diminution of sodium currents, largely NaV1.7, was recapitulated in sensory neurons expressing CRMP2-K374A. Our study elucidates a novel regulatory mechanism that utilizes CRMP2 SUMOylation to choreograph NaV1.7 trafficking.

BACE1 and presenilin/γ-secretase regulate proteolytic processing of KCNE1 and 2, auxiliary subunits of voltage-gated potassium channels

BACE1 and presenilin (PS)/γ-secretase play a major role in Alzheimer's disease pathogenesis by regulating amyloid-β peptide generation. We recently showed that these secretases also regulate the processing of voltage-gated sodium channel auxiliary β-subunits and thereby modulate membrane excitability. Here, we report that KCNE1 and KCNE2, auxiliary subunits of voltage-gated potassium channels, undergo sequential cleavage mediated by either α-secretase and PS/γ-secretase or BACE1 and PS/γ-secretase in cells. Elevated α-secretase or BACE1 activities increased C-terminal fragment (CTF) levels of KCNE1 and 2 in human embryonic kidney (HEK293T) and rat neuroblastoma (B104) cells. KCNE-CTFs were then further processed by PS/γ-secretase to KCNE intracellular domains. These KCNE cleavages were specifically blocked by chemical inhibitors of the secretases in the same cell models. We also verified our results in mouse cardiomyocytes and cultured primary neurons. Endogenous KCNE1- and KCNE2-CTF levels increased by 2- to 4-fold on PS/γ-secretase inhibition or BACE1 overexpression in these cells. Furthermore, the elevated BACE1 activity increased KCNE1 processing and shifted KCNE1/KCNQ1 channel activation curve to more positive potentials in HEK cells. KCNE1/KCNQ1 channel is a cardiac potassium channel complex, and the positive shift would lead to a decrease in membrane repolarization during cardiac action potential. Together, these results clearly showed that KCNE1 and KCNE2 cleavages are regulated by BACE1 and PS/γ-secretase activities under physiological conditions. Our results also suggest a functional role of KCNE cleavage in regulating voltage-gated potassium channels.

Sodium channel β2 subunit promotes filopodia-like processes and expansion of the dendritic tree in developing rat hippocampal neurons

The β2 auxiliary subunit of voltage-gated sodium channels (VGSCs) appears at early stages of brain development. It is abundantly expressed in the mammalian central nervous system where it forms complexes with different channel isoforms, including Na_v1.2. From the structural point of view, β2 is a transmembrane protein: at its extracellular N-terminus an Ig-like type C2 domain mediates the binding to the pore-forming alpha subunit with disulfide bonds and the interactions with the extracellular matrix. Given this structural versatility, β2 has been suggested to play multiple functions ranging from channel targeting to the plasma membrane and gating modulation to control of cell adhesion. We report that, when expressed in Chinese Hamster Ovary cells CHO-K1, the subunit accumulates at the perimetral region of adhesion and particularly in large lamellipodia-like membrane processes where it induces formation of filopodia-like structures. When overexpressed in developing embryonic rat hippocampal neurons in vitro, β2 specifically promotes formation of filopodia-like processes in dendrites leading to expansion of the arborization tree, while axonal branching remains unaltered. In contrast to this striking and highly specific effect on dendritic morphology, the targeting of functional sodium channels to the plasma membrane, including the preferential localization of Na_v1.2 at the axon, and their gating properties are only minimally affected. From these and previously reported observations it is suggested that β2, among its multiple functions, may contribute to promote dendritic outgrowth and to regulate neuronal wiring at specific stages of neuronal development.

Is the amyloid hypothesis of Alzheimer's disease therapeutically relevant?

The conventional view of AD (Alzheimer's disease) is that much of the pathology is driven by an increased load of β-amyloid in the brain of AD patients (the 'Amyloid Hypothesis'). Yet, many therapeutic strategies based on lowering β-amyloid have so far failed in clinical trials. This failure of β-amyloid-lowering agents has caused many to question the Amyloid Hypothesis itself. However, AD is likely to be a complex disease driven by multiple factors. In addition, it is increasingly clear that β-amyloid processing involves many enzymes and signalling pathways that play a role in a diverse array of cellular processes. Thus the clinical failure of β-amyloid-lowering agents does not mean that the hypothesis itself is incorrect; it may simply mean that manipulating β-amyloid directly is an unrealistic strategy for therapeutic intervention, given the complex role of β-amyloid in neuronal physiology. Another possible problem may be that toxic β-amyloid levels have already caused irreversible damage to downstream cellular pathways by the time dementia sets in. We argue in the present review that a more direct (and possibly simpler) approach to AD therapeutics is to rescue synaptic dysfunction directly, by focusing on the mechanisms by which elevated levels of β-amyloid disrupt synaptic physiology.

Voltage-gated sodium channels and metastatic disease

Voltage-gated Na (+) channels (VGSCs) are macromolecular protein complexes containing a pore-forming α subunit and smaller non-pore-forming β subunits. VGSCs are expressed in metastatic cells from a number of cancers. In these cells, Na (+) current carried by α subunits enhances migration, invasion and metastasis in vivo. In contrast, the β subunits mediate cellular adhesion and process extension. The prevailing hypothesis is that VGSCs are upregulated in cancer, in general favoring an invasive/metastatic phenotype, although the mechanisms are still not fully clear. Expression of the Nav 1.5 α subunit associates with poor prognosis in clinical breast cancer specimens, suggesting that VGSCs may have utility as prognostic markers for cancer progression. Furthermore, repurposing existing VGSC-blocking therapeutic drugs may provide a new strategy to improve outcomes in patients suffering from metastatic disease, which is the major cause of cancer-related deaths, and for which there is currently no cure.

Brain excitability and connectivity of neuronal assemblies in Alzheimer's disease: from animal models to human findings

The human brain contains about 100 billion neurons forming an intricate network of innumerable connections, which continuously adapt and rewire themselves following inputs from external and internal environments as well as the physiological synaptic, dendritic and axonal sculpture during brain maturation and throughout the life span. Growing evidence supports the idea that Alzheimer's disease (AD) targets selected and functionally connected neuronal networks and, specifically, their synaptic terminals, affecting brain connectivity well before producing neuronal loss and compartmental atrophy. The understanding of the molecular mechanisms underlying the dismantling of neuronal circuits and the implementation of 'clinically oriented' methods to map-out the dynamic interactions amongst neuronal assemblies will enhance early/pre-symptomatic diagnosis and monitoring of disease progression. More important, this will open the avenues to innovative treatments, bridging the gap between molecular mechanisms and the variety of symptoms forming disease phenotype. In the present review a set of evidence supports the idea that altered brain connectivity, exhausted neural plasticity and aberrant neuronal activity are facets of the same coin linked to age-related neurodegenerative dementia of Alzheimer type. Investigating their respective roles in AD pathophysiology will help in translating findings from basic research to clinical applications.

Lysosomal calcium homeostasis defects, not proton pump defects, cause endo-lysosomal dysfunction in PSEN-deficient cells

Presenilin (PSEN) deficiency is accompanied by accumulation of endosomes and autophagosomes, likely caused by impaired endo-lysosomal fusion. Recently, Lee et al. (2010. Cell. doi: http://dx.doi.org/10.1016/j.cell.2010.05.008) attributed this phenomenon to PSEN1 enabling the transport of mature V0a1 subunits of the vacuolar ATPase (V-ATPase) to lysosomes. In their view, PSEN1 mediates the N-glycosylation of V0a1 in the endoplasmic reticulum (ER); consequently, PSEN deficiency prevents V0a1 glycosylation, compromising the delivery of unglycosylated V0a1 to lysosomes, ultimately impairing V-ATPase function and lysosomal acidification. We show here that N-glycosylation is not a prerequisite for proper targeting and function of this V-ATPase subunit both in vitro and in vivo in Drosophila melanogaster. We conclude that endo-lysosomal dysfunction in PSEN(-/-) cells is not a consequence of failed N-glycosylation of V0a1, or compromised lysosomal acidification. Instead, lysosomal calcium storage/release is significantly altered in PSEN(-/-) cells and neurons, thus providing an alternative hypothesis that accounts for the impaired lysosomal fusion capacity and accumulation of endomembranes that accompanies PSEN deficiency.

Overexpression of the VSSC-associated CAM, β-2, enhances LNCaP cell metastasis associated behavior

BACKGROUND

Prostate cancer (PCa) is the second-leading cause of cancer death in American men. This is due largely to the "silent" nature of the disease until it has progressed to a highly metastatic and castrate resistant state. Voltage sensitive sodium channels (VSSCs) are multimeric transmembrane protein complexes comprised of a pore-forming α subunit and one or two β subunits. The β-subunits modulate surface expression and gating kinetics of the channels but also have inherent cell adhesion molecule (CAM) functions. We hypothesize that PCa cells use VSSC β-subunits as CAMs during PCa progression and metastasis.

METHODS

We overexpressed the beta-2 isoform as a C-terminal fusion protein with enhanced cyan fluorescence protein (ECFP) in the weakly metastatic LNCaP cells. The effect of beta-2 overexpression on cell morphology was examined using confocal microscopy while metastasis-associated behavior was tested by performing several in vitro metastatic functional assays and in vivo subcutaneous tumor studies.

RESULTS

We found that cells overexpressing beta-2 (2BECFP) converted to a bipolar fibroblastic morphology. 2BECFP cells were more adhesive than control (ECFP) to vitronectin (twofold) and Matrigel® (1.3-fold), more invasive through Matrigel® (3.6-fold in 72 hr), and had enhanced migration (2.1-fold in 96 hr) independent of proliferation in wound-healing assays. In contrast, 2BECFP cells have a reduced tumor-take and tumor volume in vivo even though the overexpression of beta-2 was maintained.

CONCLUSIONS

Functional overexpression of VSSC β-subunits in PCa may be one mechanism leading to increased metastatic behavior while decreasing the ability to form localized tumor masses.