Intersubunit interactions between mutant DEG/ENaCs induce synthetic neurotoxicity

Nanotechnology Approaches for Prevention and Treatment of Chemotherapy-Induced Neurotoxicity, Neuropathy, and Cardiomyopathy in Breast and Ovarian Cancer Survivors

Nanotechnology has emerged as a promising approach for the targeted delivery of therapeutic agents while improving their efficacy and safety. As a result, nanomaterial development for the selective targeting of cancers, with the possibility of treating off-target, detrimental sequelae caused by chemotherapy, is an important area of research. Breast and ovarian cancer are among the most common cancer types in women, and chemotherapy is an essential treatment modality for these diseases. However, chemotherapy-induced neurotoxicity, neuropathy, and cardiomyopathy are common side effects that can affect breast and ovarian cancer survivors quality of life. Therefore, there is an urgent need to develop effective prevention and treatment strategies for these adverse effects. Nanoparticles (NPs) have extreme potential for enhancing therapeutic efficacy but require continued research to elucidate beneficial interventions for women cancer survivors. In short, nanotechnology-based approaches have emerged as promising strategies for preventing and treating chemotherapy-induced neurotoxicity, neuropathy, and cardiomyopathy. NP-based drug delivery systems and therapeutics have shown potential for reducing the side effects of chemotherapeutics while improving drug efficacy. In this article, the latest nanotechnology approaches and their potential for the prevention and treatment of chemotherapy-induced neurotoxicity, neuropathy, and cardiomyopathy in breast and ovarian cancer survivors are discussed.

Pathogenesis of paclitaxel-induced peripheral neuropathy: A current review of in vitro and in vivo findings using rodent and human model systems

Paclitaxel (Brand name Taxol) is widely used in the treatment of common cancers like breast, ovarian and lung cancer. Although highly effective in blocking tumor progression, paclitaxel also causes peripheral neuropathy as a side effect in 60-70% of chemotherapy patients. Recent efforts by numerous labs have aimed at defining the underlying mechanisms of paclitaxel-induced peripheral neuropathy (PIPN). In vitro models using rodent dorsal root ganglion neurons, human induced pluripotent stem cells, and rodent in vivo models have revealed a number of molecular pathways affected by paclitaxel within axons of sensory neurons and within other cell types, such as the immune system and peripheral glia, as well skin. These studies revealed that paclitaxel induces altered calcium signaling, neuropeptide and growth factor release, mitochondrial damage and reactive oxygen species formation, and can activate ion channels that mediate responses to extracellular cues. Recent studies also suggest a role for the matrix-metalloproteinase 13 (MMP-13) in mediating neuropathy. These diverse changes may be secondary to paclitaxel-induced microtubule transport impairment. Human genetic studies, although still limited, also highlight the involvement of cytoskeletal changes in PIPN. Newly identified molecular targets resulting from these studies could provide the basis for the development of therapies with which to either prevent or reverse paclitaxel-induced peripheral neuropathy in chemotherapy patients.

Analyzing chemotherapy-induced peripheral neuropathy in vivo using non-mammalian animal models

Non-mammalian models of CIPN remain relatively sparse, but the knowledge gained from the few published studies suggest that these species have great potential to serve as a discovery platform for new pathways and underlying genetic mechanisms of CIPN. These models permit large-scale genetic and pharmacological screening, and they are highly suitable for in vivo imaging. CIPN phenotypes described in rodents have been confirmed in those models, and conversely, genetic players leading to axon de- and regeneration under conditions of chemotherapy treatment identified in these non-mammalian species have been validated in rodents. Given the need for non-traditional approaches with which to identify new CIPN mechanisms, these models bear a strong potential due to the conservation of basic mechanisms by which chemotherapeutic agents induce neurotoxicity.

Knock-out of a mitochondrial sirtuin protects neurons from degeneration in Caenorhabditis elegans

Sirtuins are NAD⁺-dependent deacetylases, lipoamidases, and ADP-ribosyltransferases that link cellular metabolism to multiple intracellular pathways that influence processes as diverse as cell survival, longevity, and cancer growth. Sirtuins influence the extent of neuronal death in stroke. However, different sirtuins appear to have opposite roles in neuronal protection. In Caenorhabditis elegans, we found that knock-out of mitochondrial sirtuin sir-2.3, homologous to mammalian SIRT4, is protective in both chemical ischemia and hyperactive channel induced necrosis. Furthermore, the protective effect of sir-2.3 knock-out is enhanced by block of glycolysis and eliminated by a null mutation in daf-16/FOXO transcription factor, supporting the involvement of the insulin/IGF pathway. However, data in Caenorhabditis elegans cell culture suggest that the effects of sir-2.3 knock-out act downstream of the DAF-2/IGF-1 receptor. Analysis of ROS in sir-2.3 knock-out reveals that ROS become elevated in this mutant under ischemic conditions in dietary deprivation (DD), but to a lesser extent than in wild type, suggesting more robust activation of a ROS scavenging system in this mutant in the absence of food. This work suggests a deleterious role of SIRT4 during ischemic processes in mammals that must be further investigated and reveals a novel pathway that can be targeted for the design of therapies aimed at protecting neurons from death in ischemic conditions.

Ca2+ permeability and Na+ conductance in cellular toxicity caused by hyperactive DEG/ENaC channels

Hyperactivated DEG/ENaC channels cause neuronal death mediated by intracellular Ca²⁺ overload. Mammalian ASIC1a channels and MEC-4(d) neurotoxic channels in Caenorhabditis elegans both conduct Na⁺ and Ca²⁺, raising the possibility that direct Ca²⁺ influx through these channels contributes to intracellular Ca²⁺ overload. However, we showed that the homologous C. elegans DEG/ENaC channel UNC-8(d) is not Ca²⁺ permeable, yet it is neurotoxic, suggesting that Na⁺ influx is sufficient to induce cell death. Interestingly, UNC-8(d) shows small currents due to extracellular Ca²⁺ block in the Xenopus oocyte expression system. Thus, MEC-4(d) and UNC-8(d) differ both in current amplitude and Ca²⁺ permeability. Given that these two channels show a striking difference in toxicity, we wondered how Na⁺ conductance vs. Ca²⁺ permeability contributes to cell death. To address this question, we built an UNC-8/MEC-4 chimeric channel that retains the calcium permeability of MEC-4 and characterized its properties in Xenopus oocytes. Our data support the hypothesis that for Ca²⁺-permeable DEG/ENaC channels, both Ca²⁺ permeability and Na⁺ conductance contribute to toxicity. However, for Ca²⁺-impermeable DEG/ENaCs (e.g., UNC-8), our evidence shows that constitutive Na⁺ conductance is sufficient to induce toxicity, and that this effect is enhanced as current amplitude increases. Our work further refines the contribution of different channel properties to cellular toxicity induced by hyperactive DEG/ENaC channels.

The Apoptotic Engulfment Machinery Regulates Axonal Degeneration in C. elegans Neurons

Axonal degeneration is a characteristic feature of neurodegenerative disease and nerve injury. Here, we characterize axonal degeneration in Caenorhabditis elegans neurons following laser-induced axotomy. We show that this process proceeds independently of the WLD(S) and Nmnat pathway and requires the axonal clearance machinery that includes the conserved transmembrane receptor CED-1/Draper, the adaptor protein CED-6, the guanine nucleotide exchange factor complex Crk/Mbc/dCed-12 (CED-2/CED-5/CED-12), and the small GTPase Rac1 (CED-10). We demonstrate that CED-1 and CED-6 function non-cell autonomously in the surrounding hypodermis, which we show acts as the engulfing tissue for the severed axon. Moreover, we establish a function in this process for CED-7, an ATP-binding cassette (ABC) transporter, and NRF-5, a lipid-binding protein, both associated with release of lipid-vesicles during apoptotic cell clearance. Thus, our results reveal the existence of a WLD(S)/Nmnat-independent axonal degeneration pathway, conservation of the axonal clearance machinery, and a function for CED-7 and NRF-5 in this process.

SRGP-1 regulation, targets, and contribution to cell killing in C. elegans

The engulfment of apoptotic cell corpses is an evolutionary conserved process used by multicellular systems to remove cells with inappropriate potential (e.g., self-reactive T-cells, potentially cancerous cells). Neighboring or specialized phagocytic cells remove cell corpses through distinct steps: they first recognize the cell on the verge of death, then reorchestrate their cellular architecture toward it, actively contribute to cell killing, and eventually engulf the corpse. Thus engulfment signaling must be tightly controlled to maintain tissue homeostasis. Signaling cascades mediating cell corpse clearance likely converge at the level of the small GTPase CED-10 (Rac1); given this key position, CED-10 must be subject to a tight regulatory mechanism to prevent inappropriate phagocytic events. Here, we discuss recent work characterizing srgp-1 (nematode ortholog of mammalian srGAP), a candidate GTPase activating protein (GAP) for CED-10 involved in cell corpse clearance and "sick" cell killing in C. elegans. We additionally discuss several possible determinants of SRGP-1 function, contributing to either SRGP-1 localization and/or activation. We also survey other potential candidate GTPases that might contribute to cell corpse clearance in C. elegans, and eventually recapitulate the role of engulfment during cell killing.

Neurotoxic unc-8 mutants encode constitutively active DEG/ENaC channels that are blocked by divalent cations

Ion channels of the DEG/ENaC family can induce neurodegeneration under conditions in which they become hyperactivated. The Caenorhabditis elegans DEG/ENaC channel MEC-4(d) encodes a mutant channel with a substitution in the pore domain that causes swelling and death of the six touch neurons in which it is expressed. Dominant mutations in the C. elegans DEG/ENaC channel subunit UNC-8 result in uncoordinated movement. Here we show that this unc-8 movement defect is correlated with the selective death of cholinergic motor neurons in the ventral nerve cord. Experiments in Xenopus laevis ooctyes confirm that these mutant proteins, UNC-8(G387E) and UNC-8(A586T), encode hyperactivated channels that are strongly inhibited by extracellular calcium and magnesium. Reduction of extracellular divalent cations exacerbates UNC-8(G387E) toxicity in oocytes. We suggest that inhibition by extracellular divalent cations limits UNC-8 toxicity and may contribute to the selective death of neurons that express UNC-8 in vivo.

NRA-2, a nicalin homolog, regulates neuronal death by controlling surface localization of toxic Caenorhabditis elegans DEG/ENaC channels

Hyperactivated DEG/ENaCs induce neuronal death through excessive cation influx and disruption of intracellular calcium homeostasis. Caenorhabditis elegans DEG/ENaC MEC-4 is hyperactivated by the (d) mutation and induces death of touch neurons. The analogous substitution in MEC-10 (MEC-10(d)) co-expressed in the same neurons is only mildly neurotoxic. We exploited the lower toxicity of MEC-10(d) to identify RNAi knockdowns that enhance neuronal death. We report here that knock-out of the C. elegans nicalin homolog NRA-2 enhances MEC-10(d)-induced neuronal death. Cell biological assays in C. elegans neurons show that NRA-2 controls the distribution of MEC-10(d) between the endoplasmic reticulum and the cell surface. Electrophysiological experiments in Xenopus oocytes support this notion and suggest that control of channel distribution by NRA-2 is dependent on the subunit composition. We propose that nicalin/NRA-2 functions in a quality control mechanism to retain mutant channels in the endoplasmic reticulum, influencing the extent of neuronal death. Mammalian nicalin may have a similar role in DEG/ENaC biology, therefore influencing pathological conditions like ischemia.

CLHM-1 is a functionally conserved and conditionally toxic Ca2+-permeable ion channel in Caenorhabditis elegans

Disruption of neuronal Ca(2+) homeostasis contributes to neurodegenerative diseases through mechanisms that are not fully understood. A polymorphism in CALHM1, a recently described ion channel that regulates intracellular Ca(2+) levels, is a possible risk factor for late-onset Alzheimer's disease. Since there are six potentially redundant CALHM family members in humans, the physiological and pathophysiological consequences of CALHM1 function in vivo remain unclear. The nematode Caenorhabditis elegans expresses a single CALHM1 homolog, CLHM-1. Here we find that CLHM-1 is expressed at the plasma membrane of sensory neurons and muscles. Like human CALHM1, C. elegans CLHM-1 is a Ca(2+)-permeable ion channel regulated by voltage and extracellular Ca(2+). Loss of clhm-1 in the body-wall muscles disrupts locomotory kinematics and biomechanics, demonstrating that CLHM-1 has a physiologically significant role in vivo. The motility defects observed in clhm-1 mutant animals can be rescued by muscle-specific expression of either C. elegans CLHM-1 or human CALHM1, suggesting that the function of these proteins is conserved in vivo. Overexpression of either C. elegans CLHM-1 or human CALHM1 in neurons is toxic, causing degeneration through a necrotic-like mechanism that is partially Ca(2+) dependent. Our data show that CLHM-1 is a functionally conserved ion channel that plays an important but potentially toxic role in excitable cell function.

The genetic architecture of degenerin/epithelial sodium channels in Drosophila

Degenerin/epithelial sodium channels (DEG/ENaC) represent a large family of animal-specific membrane proteins. Although the physiological functions of most family members are not known, some have been shown to act as nonvoltage gated, amiloride-sensitive sodium channels. The DEG/ENaC family is exceptionally large in genomes of Drosophila species relative to vertebrates and other insects. To elucidate the evolutionary history of the DEG/ENaC family in Drosophila, we took advantage of the genomic and genetic information available for 12 Drosophila species that represent all the major species groups in the Drosophila clade. We have identified 31 family members (termed pickpocket genes) in Drosophila melanogaster, which can be divided into six subfamilies, which are represented in all 12 species. Structure prediction analyses suggested that some subunits evolved unique structural features in the large extracellular domain, possibly supporting mechanosensory functions. This finding is further supported by experimental data that show that both ppk1 and ppk26 are expressed in multidendritic neurons, which can sense mechanical nociceptive stimuli in larvae. We also identified representative genes from five of the six DEG/ENaC subfamilies in a mosquito genome, suggesting that the core DEG/ENaC subfamilies were already present early in the dipteran radiation. Spatial and temporal analyses of expression patterns of the various pickpocket genes indicated that paralogous genes often show very different expression patterns, possibly indicating that gene duplication events have led to new physiological or cellular functions rather than redundancy. In summary, our analyses support a rapid early diversification of the DEG/ENaC family in Diptera followed by physiological and/or cellular specialization. Some members of the family may have diversified to support the physiological functions of a yet unknown class of ligands.

Insight into DEG/ENaC channel gating from genetics and structure

The founding members of the superfamily of DEG/ENaC ion channel proteins are C. elegans proteins that form mechanosensitive channels in touch and pain receptors. For more than a decade, the research community has used mutagenesis to identify motifs that regulate gating. This review integrates insight derived from unbiased in vivo mutagenesis screens with recent crystal structures to develop new models for activation of mechanically gated DEGs.

Sensory functions for degenerin/epithelial sodium channels (DEG/ENaC)

All animals use a sophisticated array of receptor proteins to sense their external and internal environments. Major advances have been made in recent years in understanding the molecular and genetic bases for sensory transduction in diverse modalities, indicating that both metabotropic and ionotropic pathways are important in sensory functions. Here, I review the historical background and recent advances in understanding the roles of a relatively newly discovered family of receptors, the degenerin/epithelial sodium channels (DEG/ENaC). These animal-specific cation channels show a remarkable sequence and functional diversity in different species and seem to exert their functions in diverse sensory modalities. Functions for DEG/ENaC channels have been implicated in mechanosensation as well as chemosensory transduction pathways. In spite of overall sequence diversity, all family members share a unique protein topology that includes just two transmembrane domains and an unusually large and highly structured extracellular domain, that seem to be essential for both their mechanical and chemical sensory functions. This review will discuss many of the recent discoveries and controversies associated with sensory function of DEG/ENaC channels in both vertebrate and invertebrate model systems, covering the role of family members in taste, mechanosensation, and pain.

Loss of the RhoGAP SRGP-1 promotes the clearance of dead and injured cells in Caenorhabditis elegans

Multicellular animals rapidly clear dying cells from their bodies. Many of the pathways that mediate this cell removal are conserved through evolution. Here, we identify srgp-1 as a negative regulator of cell clearance in both Caenorhabditis elegans and mammalian cells. Loss of srgp-1 function results in improved engulfment of apoptotic cells, whereas srgp-1 overexpression inhibits apoptotic cell corpse removal. We show that SRGP-1 functions in engulfing cells and functions as a GTPase activating protein (GAP) for CED-10 (Rac1). Interestingly, loss of srgp-1 function promotes not only the clearance of already dead cells, but also the removal of cells that have been brought to the verge of death through sublethal apoptotic, necrotic or cytotoxic insults. In contrast, impaired engulfment allows damaged cells to escape clearance, which results in increased long-term survival. We propose that C. elegans uses the engulfment machinery as part of a primitive, but evolutionarily conserved, survey mechanism that identifies and removes unfit cells within a tissue.

Spatial asymmetry in the mechanosensory phenotypes of the C. elegans DEG/ENaC gene mec-10

DEG/ENaC channels have been broadly implicated in mechanosensory transduction, yet many questions remain about how these proteins contribute to complexes that sense mechanical stimuli. In C. elegans, two DEG/ENaC channel subunits are thought to contribute to a gentle touch transduction complex: MEC-4, which is essential for gentle touch sensation, and MEC-10, whose importance is less well defined. By characterizing a mec-10 deletion mutant, we have found that MEC-10 is important, but not essential, for gentle touch responses in the body touch neurons ALM, PLM, and PVM. Surprisingly, the requirement for MEC-10 in ALM and PLM is spatially asymmetric; mec-10 animals show significant behavioral and physiological responses to stimulation at the distal end of touch neuron dendrites, but respond poorly to stimuli applied near the neuronal cell body. The subcellular distribution of a rescuing MEC-10::GFP translational fusion was found to be restricted to the neuronal cell body and proximal dendrite, consistent with the hypothesis that MEC-10 protein is asymmetrically distributed within the touch neuron process. These results suggest that MEC-10 may contribute to only a subset of gentle touch mechanosensory complexes found preferentially at the proximal dendrite.

Insights into the molecular determinants of proton inhibition in an acid-inactivated degenerins and mammalian epithelial Na(+) channel

Mammalian ASIC channels of the DEG/ENaC superfamily are gated by extracellular protons and function to mediate touch and pain sensitivity, learning and memory, and fear conditioning. The recently solved crystal structure of chicken ASIC1a and preliminary functional studies suggested that a highly negatively charged pocket in the extracellular domain of the channel might be the primary proton binding domain. However, more recent extensive mutagenesis analysis paints a more complex mechanism of channel gating, involving binding of protons at sites immediately after the first transmembrane domain (TM1) and displacement of inhibitory Ca(2+) ions from the acidic pocket in the extracellular domain and from another Ca(2+) binding site at the mouth of the pore. We recently identified and functionally characterized Caenorhabditis elegans ACD-1, the first acid-inactivated DEG/ENaC channel. ACD-1 is expressed in C. elegans amphid glia and functions with neuronal DEG/ENaC channel DEG-1 to mediate acid avoidance and chemotaxis to the amino acid lysine. The post-TM1 residues that were proposed to bind protons in ASIC1a are not conserved in ACD-1, but some of the amino acids constituting the acidic pocket are. However, ACD-1 proton sensitivity is completely independent from extracellular Ca(2+), and protons appear to bind the channel in a less cooperative manner. We thus wondered if residues in the acidic pocket might contribute to ACD-1 acid sensitivity. We show here that while ACD-1 sensitivity to extracellular protons is influenced by mutations in the acidic pocket, other sites are likely to participate. We also report that one histidine at the base of the thumb and residues in the channel pore influence proton inhibition in a voltage-independent manner, suggesting that they affect the coupling of proton binding with the gating rather than proton binding itself. We conclude that ACD-1 inhibition by protons is likely mediated by binding of proton ions to multiple sites throughout the extracellular domain of the channel. Our data also support a model in which residues in the acidic pocket contribute to determining the channel state perhaps by changing the strength of the interaction between adjacent thumb and finger domains.