Modulation of CD112 by the alphaherpesvirus gD protein suppresses DNAM-1-dependent NK cell-mediated lysis of infected cells.

Natural killer (NK) cells are key players in the innate response to viruses, including herpesviruses. In particular, the variety of viral strategies to modulate the recognition of certain herpesviruses witnesses the importance of NK cells in the control of this group of viruses. Still, NK evasion strategies have remained largely elusive for the largest herpesvirus subfamily, the alphaherpesviruses. Here, we report that the gD glycoprotein of the alphaherpesviruses pseudorabies virus (PRV) and herpes simplex virus 2 (HSV-2) displays previously uncharacterized immune evasion properties toward NK cells. Expression of gD during infection or transfection led to degradation and consequent down-regulation of CD112, a ligand for the activating NK receptor DNAX accessory molecule 1 (DNAM-1). CD112 downregulation resulted in a reduced ability of DNAM-1 to bind to the surface of both virus-infected and gD-transfected cells. Consequently, expression of gD suppressed NK cell degranulation and NK cell-mediated lysis of PRV- or HSV-2-infected cells. These data identify an alphaherpesvirus evasion strategy from NK cells and point out that interactions between viral envelope proteins and host cell receptors can have biological consequences that stretch beyond virus entry.

Combination of ascorbate/epigallocatechin-3-gallate/gemcitabine synergistically induces cell cycle deregulation and apoptosis in mesothelioma cells.

Malignant mesothelioma (MMe) is a poor-prognosis tumor in need of innovative therapies. In a previous in vivo study, we showed synergistic anti-MMe properties of the ascorbate/epigallocatechin-3-gallate/gemcitabine combination. We have now focused on the mechanism of action, showing the induction of apoptosis and cell cycle arrest through measurements of caspase 3, intracellular Ca(2+), annexin V, and DNA content. StellArray™ PCR technology and Western immunoblotting revealed DAPK2-dependent apoptosis, upregulation of cell cycle promoters, downregulation of cell cycle checkpoints and repression of NFκB expression. The complex of data indicates that the mixture is synergistic in inducing cell cycle deregulation and non-inflammatory apoptosis, suggesting its possible use in MMe treatment.

Hybrid epithelial-mesenchymal status of lung cancer dictates metastatic success through differential interaction with NK cells.

BACKGROUND: Epithelial to mesenchymal transition (EMT) endows cancer cells with pro-metastatic properties, which appear most effective when cells enter an intermediate hybrid (H) state, characterized by integrated mesenchymal (M) and epithelial (E) traits. The reasons for this advantage are poorly known and, especially, it is totally unexplored whether the interplay between H-cells and NK cells could have a role. Here we characterize the pro-metastatic mechanics of non-small cell lung cancer (NSCLC) H-cells and their subset of cancer-initiating cells (CICs), dissecting crucial interactions with NK cells. METHODS: Human lung cancer cell lines and sublines representative of E, M, or H states, assessed by proteomics, were analyzed in vivo for their tumor-forming and disseminating capabilities. Interactions with NK cells were investigated in vitro using migration assays, cytotoxic degranulation assays, and evaluation of CD133+ CICs modulation after coculture, and validated in vivo through NK cell neutralization assays. Correlation between EMT status, NK cell infiltration, and survival data, was evaluated in a cohort of surgically resected NSCLC cases (n=79). RESULTS: We demonstrated that H-cells, have limited dissemination capability but show the highest potential to initiate metastases in vivo. This property was related to their ability to escape NK cell surveillance. Mechanistically, H-cells expressed low levels of NK-attracting chemokines (CXCL1 and CXCL8), generating poorly infiltrated metastases. Accordingly, proteomics and GO enrichment analysis of E, H, M cell lines showed that the related secretory processes could change during EMT.Furthermore, H-CICs uniquely expressed high levels of the inhibitory ligand B7-H3, which protected H-CIC from NK cell-mediated clearance. In vivo neutralization assays confirmed that, indeed, the pro-metastatic properties of H-cells are poorly controlled by NK cells.Finally, the analysis of patients revealed that detection of hybrid phenotypes associated with low NK infiltration in NSCLC clinical specimens could identify a subset of patients with poor prognosis. CONCLUSIONS: Our study demonstrates that H-cells play a central role in the metastatic spread in NSCLC. Such pro-metastatic advantage of H-cells is supported by their altered interaction with NK cells and by the critical role of B7-H3 in preserving their H-CIC component, indicating B7-H3 as a potential target in combined NK-based therapies.

Melanoma cells become resistant to NK-cell-mediated killing when exposed to NK-cell numbers compatible with NK-cell infiltration in the tumor.

During the past few years, a number of studies reported that different melanoma cell lines could be extensively lysed in vitro by IL-2-activated NK cells at appropriate effector/target ratios. Here, we show, by histological evaluation of different melanoma lesions, that NK/target-cell ratios compatible with those allowing efficient melanoma cell killing in vitro are hardly reached at the tumor site. We then investigated the outcome of cocultures established at low NK/melanoma cell ratios. After initial NK-mediated lysis, residual melanoma cells acquired resistance to IL-2-activated NK cells. This reflected primarily an increased expression, on melanoma cells, of classical and nonclassical HLA class I molecules, accompanied by a partial downregulation of NKG2D-ligands, and was dependent on NK-mediated IFN-γ release. Consistently, melanoma lesions showed a higher HLA class I expression on tumor cells that were proximal to infiltrating NK cells. In long-term cocultures, the "protective phenotype" acquired by melanoma cells was lost over time. However, this phenomenon was counteracted by downregulation of relevant activating receptors in cocultured NK cells. Analysis of different NK-cell-activating cytokines indicated that IL-15 can partially overcome this novel tumor escape mechanism suggesting that IL-15, rather than IL-2, may be eligible for NK-cell-based immunotherapy.

Study of the effects of NK-tumor cell interaction by proteomic analysis and imaging.

Natural Killer (NK) cells play a pivotal role in the elimination of tumor cells. The interactions that NK cells can establish with cancer cells in the tumor microenvironment (TME) are crucial for the outcome of the anti-tumor response, possibly resulting in mechanisms able to modulate NK cell effector functions on the one side, and to modify tumor cell phenotype and properties on the other side. This chapter will describe two different experimental approaches for the evaluation of NK-tumor cell interactions. First, a detailed protocol for the setting up of NK-tumor cell co-cultures will be illustrated, followed by information on cell imaging techniques, useful for assessing cell morphology and cytoskeletal changes. The second part will be focused on the description of a proteomic approach aimed at investigating the effect of this crosstalk from another point of view, i.e., characterizing the cellular and molecular pathways modulated in tumor cells following interaction with NK cells. The chapter centers on the interaction between NK and melanoma cells and refers to experimental approaches we set up to study the effects of this cross-talk on the process of the Epithelial-to-Mesenchymal Transition (EMT). Nevertheless, the described protocols can be quite easily adapted to study the interaction of NK cells with adherent tumor cell lines of different origin and histotype, as in our original study, we also analyzed possible NK-induced morphologic changes in the cervix adenocarcinoma HeLa cells and the colon cancer HT29 cells.

Murine models to study human NK cells in human solid tumors.

Since the first studies, the mouse models have provided crucial support for the most important discoveries on NK cells, on their development, function, and circulation within normal and tumor tissues. Murine tumor models were initially set to study murine NK cells, then, ever more sophisticated human-in-mice models have been developed to investigate the behavior of human NK cells and minimize the interferences from the murine environment. This review presents an overview of the models that have been used along time to study NK cells, focusing on the most popular NOG and NSG models, which work as recipients for the preparation of human-in-mice tumor models, the study of transferred human NK cells, and the evaluation of various enhancers of human NK cell function, including cytokines and chimeric molecules. Finally, an overview of the next generation humanized mice is also provided along with a discussion on how traditional and innovative in-vivo and in-vitro approaches could be integrated to optimize effective pre-clinical studies.

Cord Blood-Derived Natural Killer Cell Exploitation in Immunotherapy Protocols: More Than a Promise?

In the last 20 years, Natural Killer (NK) cell-based immunotherapy has become a promising approach to target various types of cancer. Indeed, NK cells play a pivotal role in the first-line defense against tumors through major histocompatibility complex-independent immunosurveillance. Their role in the control of leukemia relapse has been clearly established and, moreover, the presence of NK cells in the tumor microenvironment (TME) generally correlates with good prognosis. However, it has also been observed that, often, NK cells poorly infiltrate the tumor tissue, and, in TME, their functions may be compromised by immunosuppressive factors that contribute to the failure of anti-cancer immune response. Currently, studies are focused on the design of effective strategies to expand NK cells and enhance their cytotoxic activity, exploiting different cell sources, such as peripheral blood (PB), umbilical cord blood (UCB) and NK cell lines. Among them, UCB represents an important source of mature NK cells and CD34+ Hematopoietic Stem and Progenitor Cells (HSPCs), as precursors of NK cells. In this review, we summarize the UCB-derived NK cell activity in the tumor context, review the different in-vitro models to expand NK cells from UCB, and discuss the importance of their exploitation in anti-tumor immunotherapy protocols.

Immune Checkpoint Blockade: A Strategy to Unleash the Potential of Natural Killer Cells in the Anti-Cancer Therapy.

Immune checkpoint inhibitors (ICIs) immunotherapy has represented a breakthrough in cancer treatment. Clinical use of ICIs has shown an acceptable safety profile and promising antitumor activity. Nevertheless, some patients do not obtain clinical benefits after ICIs therapy. In order to improve and cure an increasing number of patients, the field has moved toward the discovery of new ICIs expressed by cells of innate immunity with an elevated inherent antitumor activity, such as natural killer cells. This review will focus on the recent findings concerning the role of classical and non-classical immune checkpoint molecules and receptors that regulate natural killer cell function, as potential targets, and their future clinical application.

Blocking HIF to Enhance NK Cells: Hints for New Anti-Tumor Therapeutic Strategies?

Natural Killer (NK) cells are becoming an ever more promising tool to design new anti-tumor strategies. However, two major issues are still a challenge to obtain versatile and effective NK-based therapies: the way to maximize the persistency of powerful NK effectors in the patient, and the way to overcome the multiple escape mechanisms that keep away or suppress NK cells at the tumor site. In this regard, targeting the hypoxia-inducible factors (HIFs), which is important for both tumor progression and immune suppression, may be an opportunity. Especially, in the context of the ongoing studies focused on more effective NK-based therapeutic products.

Isolation, Expansion, and Characterization of Natural Killer Cells and Their Precursors as a Tool to Study Cancer Immunosurveillance.

This chapter will describe the current methodologies to isolate and expand NK cells from Peripheral Blood (PB) or tissues for "in vitro" studies, including NK cell antitumor immune function. In addition, methods to induce NK cell maturation, differentiation, and expansion from CD34+ precursors will also be described. Finally, it will also be treated the topical issue of the characterization of new functionally and phenotypically defined NK cell subsets.

NKp44-NKp44 Ligand Interactions in the Regulation of Natural Killer Cells and Other Innate Lymphoid Cells in Humans.

Natural Killer (NK) cells are potent cytotoxic cells belonging to the family of Innate Lymphoid Cells (ILCs). Their most characterized effector functions are directed to the control of aberrant cells in the body, including both transformed and virus-infected cells. NK cell-mediated recognition of abnormal cells primarily occurs through receptor-ligand interactions, involving an array of inhibitory and activating NK receptors and different types of ligands expressed on target cells. While most of the receptors have become known over many years, their respective ligands were only defined later and their impressive complexity has only recently become evident. NKp44, a member of Natural Cytotoxicity Receptors (NCRs), is an activating receptor playing a crucial role in most functions exerted by activated NK cells and also by other NKp44+ immune cells. The large and heterogeneous panel of NKp44 ligands (NKp44L) now includes surface expressed glycoproteins and proteoglycans, nuclear proteins that can be exposed outside the cell, and molecules that can be either released in the extracellular space or carried in extracellular vesicles. Recent findings have extended our knowledge on the nature of NKp44L to soluble plasma glycoproteins, such as secreted growth factors or extracellular matrix (ECM)-derived glycoproteins. NKp44L are induced upon tumor transformation or viral infection but may also be expressed in normal cells and tissues. In addition, NKp44-NKp44L interactions are involved in the crosstalk between NK cells and different innate and adaptive immune cell types. NKp44 expression in different ILCs located in tissues further extends the potential role of NKp44-NKp44L interactions.

Hypoxia Modifies the Transcriptome of Human NK Cells, Modulates Their Immunoregulatory Profile, and Influences NK Cell Subset Migration.

Hypoxia, which characterizes most tumor tissues, can alter the function of different immune cell types, favoring tumor escape mechanisms. In this study, we show that hypoxia profoundly acts on NK cells by influencing their transcriptome, affecting their immunoregulatory functions, and changing the chemotactic responses of different NK cell subsets. Exposure of human peripheral blood NK cells to hypoxia for 16 or 96 h caused significant changes in the expression of 729 or 1,100 genes, respectively. Gene Set Enrichment Analysis demonstrated that these changes followed a consensus hypoxia transcriptional profile. As assessed by Gene Ontology annotation, hypoxia-targeted genes were implicated in several biological processes: metabolism, cell cycle, differentiation, apoptosis, cell stress, and cytoskeleton organization. The hypoxic transcriptome also showed changes in genes with immunological relevance including those coding for proinflammatory cytokines, chemokines, and chemokine-receptors. Quantitative RT-PCR analysis confirmed the modulation of several immune-related genes, prompting further immunophenotypic and functional studies. Multiplex ELISA demonstrated that hypoxia could variably reduce NK cell ability to release IFNγ, TNFα, GM-CSF, CCL3, and CCL5 following PMA+Ionomycin or IL15+IL18 stimulation, while it poorly affected the response to IL12+IL18. Cytofluorimetric analysis showed that hypoxia could influence NK chemokine receptor pattern by sustaining the expression of CCR7 and CXCR4. Remarkably, this effect occurred selectively (CCR7) or preferentially (CXCR4) on CD56bright NK cells, which indeed showed higher chemotaxis to CCL19, CCL21, or CXCL12. Collectively, our data suggest that the hypoxic environment may profoundly influence the nature of the NK cell infiltrate and its effects on immune-mediated responses within tumor tissues.

Nidogen-1 is a novel extracellular ligand for the NKp44 activating receptor.

The release of soluble ligands of activating Natural Killer (NK) cell receptors may represent a regulatory mechanism of NK cell function both in physiologic and in pathologic conditions. Here, we identified the extracellular matrix protein Nidogen-1 (NID1) as a ligand of NKp44, an important activating receptor expressed by activated NK cells. When released as soluble molecule, NID1 regulates NK cell function by modulating NKp44-induced IFN-γ production or cytotoxicity. In particular, it also modulates IFN-γ production induced by Platelet-Derived Growth Factor (PDGF)-DD following NKp44 engagement. We also show that NID1 may be present at the cell surface. In this form or when bound to a solid support (bNID1), NID1 fails to induce NK cell cytotoxicity or cytokine release. However, analysis by mass spectrometry revealed that exposure to bNID1 can induce in human NK cells relevant changes in the proteomic profiles suggesting an effect on different biological processes.

NK-cell Editing Mediates Epithelial-to-Mesenchymal Transition via Phenotypic and Proteomic Changes in Melanoma Cell Lines.

Tumor cell plasticity is a major obstacle for the cure of malignancies as it makes tumor cells highly adaptable to microenvironmental changes, enables their phenotype switching among different forms, and favors the generation of prometastatic tumor cell subsets. Phenotype switching toward more aggressive forms involves different functional, phenotypic, and morphologic changes, which are often related to the process known as epithelial-mesenchymal transition (EMT). In this study, we report natural killer (NK) cells may increase the malignancy of melanoma cells by inducing changes relevant to EMT and, more broadly, to phenotype switching from proliferative to invasive forms. In coculture, NK cells induced effects on tumor cells similar to those induced by EMT-promoting cytokines, including upregulation of stemness and EMT markers, morphologic transition, inhibition of proliferation, and increased capacity for Matrigel invasion. Most changes were dependent on the engagement of NKp30 or NKG2D and the release of cytokines including IFNγ and TNFα. Moreover, EMT induction also favored escape from NK-cell attack. Melanoma cells undergoing EMT either increased NK-protective HLA-I expression on their surface or downregulated several tumor-recognizing activating receptors on NK cells. Mass spectrometry-based proteomic analysis revealed in two different melanoma cell lines a partial overlap between proteomic profiles induced by NK cells or by EMT cytokines, indicating that various processes or pathways related to tumor progression are induced by exposure to NK cells.Significance: NK cells can induce prometastatic properties on melanoma cells that escape from killing, providing important clues to improve the efficacy of NK cells in innovative antitumor therapies. Cancer Res; 78(14); 3913-25. ©2018 AACR.

P2X(7) receptors exert a permissive role on the activation of release-enhancing presynaptic alpha7 nicotinic receptors co-existing on rat neocortex glutamatergic terminals.

Adenosine triphosphate (ATP) has been reported to enhance the release of glutamate by acting at P2X presynaptic receptors. Acetylcholine (ACh) can elicit glutamate release through presynaptic nicotinic cholinergic receptors (nAChRs) of the alpha7 subtype situated on glutamatergic axon terminals, provided that the terminal membrane is weakly depolarized. Considering that ATP and ACh are co-transmitters, we here investigate on the possibility that P2X and nAChRs co-exist and interact on the same glutamatergic nerve endings using purified rat neocortex synaptosomes in superfusion. ATP evoked Ca(2+)-dependent release of pre-accumulated D-[(3)H]aspartate ([(3)H]D-ASP) as well as of endogenous glutamate; (-)-nicotine, inactive on its own, potentiated the ATP-evoked release. The ATP analogue benzoylbenzoylATP (BzATP) behaved like ATP, but was approximately 30 times more potent; the potentiation of the BzATP-evoked release was blocked by methyllycaconitine or alpha-bungarotoxin. Adding inactive concentrations of (-)-nicotine, epibatidine or choline together with inactive concentrations of BzATP resulted in significant elevation of the [(3)H]D-ASP release mediated by alpha7 nAChRs. To conclude, P2X(7) receptors and alpha7 nAChRs seem to co-exist and interact on rat neocortex glutamatergic terminals; in particular, P2X(7) receptors exert a permissive role on the activation of alpha7 nAChRs, suggesting that ATP may not only evoke glutamate release on its own, but may also regulate the release of the amino acid elicited by ACh.

Nicotine has a permissive role on the activation of metabotropic glutamate 5 receptors coexisting with nicotinic receptors on rat hippocampal noradrenergic nerve terminals.

The existence of metabotropic glutamate receptors (mGluRs) on hippocampal noradrenergic nerve terminals and their interaction with coexisting nicotinic acetylcholine receptors (nAChRs) were investigated in superfused rat synaptosomes using [(3)H]-noradrenaline ([(3)H]-NA) release as a readout. The selective agonist of group I mGluRs, (S)-3,5-dihydroxyphenylglycine (DHPG), inactive on its own, acquired ability to release [(3)H]-NA when added together with (-)-nicotine. The effect of DHPG was prevented by 2-methyl-6-(phenylethynyl)-pyridine (MPEP), a selective antagonist of mGluR5, but not by 7-(hydroxyimino)cyclopropane[b]chromen-1-carboxylate ethyl ester (CPCCOEt), selective antagonist of mGluR1. The [(3)H]-NA release evoked by (-)-nicotine plus DHPG was totally abrogated by the nAChR antagonist mecamylamine. Veratrine mimicked the permissive role of (-)-nicotine on the activation of mGluR5 mediating [(3)H]-NA release. The mGluR5-mediated component of the [(3)H]-NA release provoked by DHPG plus (-)-nicotine was blocked by xestospongin C, a selective antagonist of inositoltrisphosphate (IP(3)) receptors. It can be concluded that (i) release-enhancing mGluRs of subtype 5 exist on hippocampal noradrenergic axon terminals; (ii) activation of mGluR5 to mediate IP(3)-dependent NA release requires activation of depolarizing nAChRs coexisting on the same terminals.

NK Cells, Tumor Cell Transition, and Tumor Progression in Solid Malignancies: New Hints for NK-Based Immunotherapy?

Several evidences suggest that NK cells can patrol the body and eliminate tumors in their initial phases but may hardly control established solid tumors. Multiple factors, including the transition of tumor cells towards a proinvasive/prometastatic phenotype, the immunosuppressive effect of the tumor microenvironment, and the tumor structure complexity, may account for limited NK cell efficacy. Several putative mechanisms of NK cell suppression have been defined in these last years; conversely, the cross talk between NK cells and tumor cells undergoing different transitional phases remains poorly explored. Nevertheless, recent in vitro studies and immunohistochemical analyses on tumor biopsies suggest that NK cells could not only kill tumor cells but also influence their evolution. Indeed, NK cells may induce tumor cells to change the expression of HLA-I, PD-L1, or NKG2D-L and modulate their susceptibility to the immune response. Moreover, NK cells may be preferentially located in the borders of tumor masses, where, indeed, tumor cells can undergo Epithelial-to-Mesenchymal Transition (EMT) acquiring prometastatic phenotype. Finally, the recently highlighted role of HMGB1 both in EMT and in amplifying the recruitment of NK cells provides further hints on a possible effect of NK cells on tumor progression and fosters new studies on this issue.

Role of NK cells in immunotherapy and virotherapy of solid tumors.

Although natural killer (NK) cells are endowed with powerful cytolytic activity against cancer cells, their role in different therapies against solid tumors has not yet been fully elucidated. Their interactions with various elements of the tumor microenvironment as well as their possible effects in contributing to and/or limiting oncolytic virotherapy render this potential immunotherapeutic tool still difficult to exploit at the bedside. Here, we will review the current literature with the aim of providing new hints to manage this powerful cell type in future innovative therapies, such as the use of NK cells in combination with new cytokines, specific mAbs (inducing ADCC), Tyr-Kinase inhibitors, immunomodulatory drugs and/or the design of oncolytic viruses aimed at optimizing the effect of NK cells in virotherapy.

Natural Killer (NK)/melanoma cell interaction induces NK-mediated release of chemotactic High Mobility Group Box-1 (HMGB1) capable of amplifying NK cell recruitment.

In this study we characterize a new mechanism by which Natural Killer (NK) cells may amplify their recruitment to tumors. We show that NK cells, upon interaction with melanoma cells, can release a chemotactic form of High Mobility Group Box-1 (HMGB1) protein capable of attracting additional activated NK cells. We first demonstrate that the engagement of different activating NK cell receptors, including those mainly involved in tumor cell recognition can induce the active release of HMGB1. Then we show that during NK-mediated tumor cell killing two HMGB1 forms are released, each displaying a specific electrophoretic mobility possibly corresponding to a different redox status. By the comparison of normal and perforin-defective NK cells (which are unable to kill target cells) we demonstrate that, in NK/melanoma cell co-cultures, NK cells specifically release an HMGB1 form that acts as chemoattractant, while dying tumor cells passively release a non-chemotactic HMGB1. Finally, we show that Receptor for Advanced Glycation End products is expressed by NK cells and mediates HMGB1-induced NK cell chemotaxis. Proteomic analysis of NK cells exposed to recombinant HMGB1 revealed that this molecule, besides inducing immediate chemotaxis, also promotes changes in the expression of proteins involved in the regulation of the cytoskeletal network. Importantly, these modifications could be associated with an increased motility of NK cells. Thus, our findings allow the definition of a previously unidentified mechanism used by NK cells to amplify their response to tumors, and provide additional clues for the emerging role of HMGB1 in immunomodulation and tumor immunity.

Nicotine exerts a permissive role on NMDA receptor function in hippocampal noradrenergic terminals.

The coexistence of nicotinic cholinergic receptors (nAChRs) and of N-methyl-D-aspartate (NMDA) receptors on the same noradrenergic axon terminals and the nAChR/NMDA receptor cross-talk were investigated by monitoring the release of noradrenaline (NA) evoked in superfused rat hippocampal synaptosomes by (-)-nicotine and NMDA alone or in combination. In medium containing a physiological concentration (1.2 mM) of Mg2+, the release of [3H]NA was very slightly increased by NMDA plus glycine, whereas it was significantly enhanced by (-)-nicotine. The (-)-nicotine/NMDA combination elicited supraadditive release which was totally abolished by the nAChR blocker mecamylamine and partly prevented by selectively blocking NMDA receptors. Supraadditive [3H]NA release was also observed by exposing synaptosomes to veratrine, but not to ionomycin. The supraadditive release elicited by the (-)-nicotine/NMDA or the veratrine/NMDA combination was sensitive to the protein kinase A/C inhibitor staurosporine and the selective protein kinase A inhibitor H89, but insensitive to the protein kinase C inhibitor Ro 31-8220. It is concluded that (i) release-modulating nAChRs and NMDA receptors coexist on hippocampal noradrenergic axon terminals; and (ii) nicotine permits NMDA receptor activation in the presence of Mg2+, possibly because the nicotine-induced influx of Na+ depolarizes the nerve ending membrane sufficiently to remove the Mg2+ block.