Paclitaxel-induced increase in NCX activity in subpopulations of nociceptive afferents: A protective mechanism against chemotherapy-induced peripheral neuropathy?

Ion Channel and Transporter Involvement in Chemotherapy-Induced Peripheral Neurotoxicity

The peripheral nervous system can encounter alterations due to exposure to some of the most commonly used anticancer drugs (platinum drugs, taxanes, vinca alkaloids, proteasome inhibitors, thalidomide), the so-called chemotherapy-induced peripheral neurotoxicity (CIPN). CIPN can be long-lasting or even permanent, and it is detrimental for the quality of life of cancer survivors, being associated with persistent disturbances such as sensory loss and neuropathic pain at limb extremities due to a mostly sensory axonal polyneuropathy/neuronopathy. In the state of the art, there is no efficacious preventive/curative treatment for this condition. Among the reasons for this unmet clinical and scientific need, there is an uncomplete knowledge of the pathogenetic mechanisms. Ion channels and transporters are pivotal elements in both the central and peripheral nervous system, and there is a growing body of literature suggesting that they might play a role in CIPN development. In this review, we first describe the biophysical properties of these targets and then report existing data for the involvement of ion channels and transporters in CIPN, thus paving the way for new approaches/druggable targets to cure and/or prevent CIPN.

Neuropathic pain; what we know and what we should do about it

Neuropathic pain can result from injury to, or disease of the nervous system. It is notoriously difficult to treat. Peripheral nerve injury promotes Schwann cell activation and invasion of immunocompetent cells into the site of injury, spinal cord and higher sensory structures such as thalamus and cingulate and sensory cortices. Various cytokines, chemokines, growth factors, monoamines and neuropeptides effect two-way signalling between neurons, glia and immune cells. This promotes sustained hyperexcitability and spontaneous activity in primary afferents that is crucial for onset and persistence of pain as well as misprocessing of sensory information in the spinal cord and supraspinal structures. Much of the current understanding of pain aetiology and identification of drug targets derives from studies of the consequences of peripheral nerve injury in rodent models. Although a vast amount of information has been forthcoming, the translation of this information into the clinical arena has been minimal. Few, if any, major therapeutic approaches have appeared since the mid 1990's. This may reflect failure to recognise differences in pain processing in males vs. females, differences in cellular responses to different types of injury and differences in pain processing in humans vs. animals. Basic science and clinical approaches which seek to bridge this knowledge gap include better assessment of pain in animal models, use of pain models which better emulate human disease, and stratification of human pain phenotypes according to quantitative assessment of signs and symptoms of disease. This can lead to more personalized and effective treatments for individual patients. Significance statement: There is an urgent need to find new treatments for neuropathic pain. Although classical animal models have revealed essential features of pain aetiology such as peripheral and central sensitization and some of the molecular and cellular mechanisms involved, they do not adequately model the multiplicity of disease states or injuries that may bring forth neuropathic pain in the clinic. This review seeks to integrate information from the multiplicity of disciplines that seek to understand neuropathic pain; including immunology, cell biology, electrophysiology and biophysics, anatomy, cell biology, neurology, molecular biology, pharmacology and behavioral science. Beyond this, it underlines ongoing refinements in basic science and clinical practice that will engender improved approaches to pain management.

Sodium-Calcium Exchanger 2: A Pivotal Role in Oxaliplatin Induced Peripheral Neurotoxicity and Axonal Damage?

Oxaliplatin (OHP)-induced peripheral neurotoxicity (OIPN) is a frequent adverse event of colorectal cancer treatment. OIPN encompasses a chronic and an acute syndrome. The latter consists of transient axonal hyperexcitability, due to unbalance in Na+ voltage-operated channels (Na+VOC). This leads to sustained depolarisation which can activate the reverse mode of the Na+/Ca2+ exchanger 2 (NCX2), resulting in toxic Ca2+ accumulation and axonal damage (ADa). We explored the role of NCX2 in in vitro and in vivo settings. Embryonic rat Dorsal Root Ganglia (DRG) organotypic cultures treated with SEA0400 (SEA), a NCX inhibitor, were used to assess neuroprotection in a proof-of-concept and pilot study to exploit NCX modulation to prevent ADa. In vivo, OHP treated mice (7 mg/Kg, i.v., once a week for 8 weeks) were compared with a vehicle-treated group (n = 12 each). Neurophysiological and behavioural testing were performed to characterise acute and chronic OIPN, and morphological analyses were performed to detect ADa. Immunohistochemistry, immunofluorescence, and western blotting (WB) analyses were also performed to demonstrate changes in NCX2 immunoreactivity and protein expression. In vitro, NCX inhibition was matched by ADa mitigation. In the in vivo part, after verifyingboth acute and chronic OIPN had ensued, we confirmed via immunohistochemistry, immunofluorescence, and WB that a significant NCX2 alteration had ensued in the OHP group. Our data suggest NCX2 involvement in ADa development, paving the way to a new line of research to prevent OIPN.

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.

From Mechanism to Cure: Renewing the Goal to Eliminate the Disease of Pain

Objective

Persistent pain causes untold misery worldwide and is a leading cause of disability. Despite its astonishing prevalence, pain is undertreated, at least in part because existing therapeutics are ineffective or cause intolerable side effects. In this review, we cover new findings about the neurobiology of pain and argue that all but the most transient forms of pain needed to avoid tissue damage should be approached as a disease where a cure can be the goal of all treatment plans, even if attaining this goal is not yet always possible.

Design

We reviewed the literature to highlight recent advances in the area of the neurobiology of pain.

Results

We discuss barriers that are currently hindering the achievement of this goal, as well as the development of new therapeutic strategies. We also discuss innovations in the field that are creating new opportunities to treat and even reverse persistent pain, some of which are in late-phase clinical trials.

Conclusion

We conclude that the confluence of new basic science discoveries and development of new technologies are creating a path toward pain therapeutics that should offer significant hope of a cure for patients and practitioners alike. Classification of Evidence. Our review points to new areas of inquiry for the pain field to advance the goal of developing new therapeutics to treat chronic pain.

Antinociceptive effectiveness of the inhibition of NCX reverse-mode action in rodent neuropathic pain model

Background

Chronic neuropathic pain is a debilitating condition that remains difficult to treat. The Na⁺-Ca²⁺ exchanger (NCX) is a transporter that can exchange Ca²⁺ with Na⁺ in either direction to maintain intracellular Ca²⁺ homeostasis. However, the effect of NCX on neuropathic pain remains unclear. Therefore, in this study, we aimed to clarify whether neuropathic pain is altered by NCX.

Methods

Adult Sprague–Dawley rats and mice (NCX2 knockout and wild type) were randomized to receive spinal nerve ligation surgery or intrathecal injection. Using behavioral testing to analyze the withdrawal thresholds and thermal withdrawal latency of rats after surgery or intrathecal injection. Immunohistochemistry and Western blotting were used to analyze the changes of NCX protein and downstream signaling pathways in rats dorsal root ganglion. We isolated the dorsal root ganglion neurons of adult rats using Fluo-4AM to detect the Ca²⁺ imaging in neurons after drug treatment.

Results

NCX was expressed in the sensory neurons of rodent dorsal root ganglia. NCX expression was altered in ipsilateral L4–6 dorsal root ganglion neurons in spinal nerve ligation rats. Intrathecal injection of an inhibitor of reverse-mode NCX activity (KB-R7943 5∼20 µg) had an antinociceptive effect in spinal nerve ligation rats, and the effect lasted for 3 h. We measured the expression of signaling pathway molecules in dorsal root ganglion neurons, and only the p-extracellular signal-regulated kinase (ERK) 1/2 level was reduced after intrathecal injection in the spinal nerve ligation group compared to the control group. In cultured dorsal root ganglion neurons, inhibitors of reverse-mode NCX activity (KB-R7943 and ORM-10103) restrained Ca²⁺ overload after tumor necrosis factor alpha (TNF-α) or lipopolysaccharide (LPS) treatment. NCX2 knockout mice presented an antinociceptive effect that lasted for more than 28 days after spinal nerve ligation surgery. The p-ERK1/2 level in NCX2 knockout mice ipsilateral L4–6 dorsal root ganglion neurons was lower than that in wild-type mice.

Conclusions

NCX proteins may mediate neuropathic pain progression via the Ca²⁺ and ERK pathways. NCX represents a potential target for the treatment of neuropathic pain.

Long-term actions of interleukin-1β on K+, Na+ and Ca2+ channel currents in small, IB4-positive dorsal root ganglion neurons; possible relevance to the etiology of neuropathic pain

Excitation of dorsal root ganglion (DRG) neurons by interleukin 1β (IL-1β) is implicated in the onset of neuropathic pain. To understand its mechanism of action, isolectin B4 positive (IB₄⁺) DRG neurons were exposed to 100pM IL-1β for 5-6d. A reversible increase in action potential (AP) amplitude reflected increased TTX-sensitive sodium current (TTX-S I_Na). An irreversible increase in AP duration reflected decreased Ca²⁺- sensitive K⁺ conductance (BK(Ca) channels). Different processes thus underlie regulation of the two channel types. Since changes in AP shape facilitated Ca²⁺ influx, this explains how IL-1β facilitates synaptic transmission in the dorsal horn; thereby provoking pain.

Nociceptor Translational Profiling Reveals the Ragulator-Rag GTPase Complex as a Critical Generator of Neuropathic Pain

Nociceptors, sensory neurons in the DRG that detect damaging or potentially damaging stimuli, are key drivers of neuropathic pain. Injury to these neurons causes activation of translation regulation signaling, including the mechanistic target of rapamycin complex 1 (mTORC1) and mitogen-activated protein kinase interacting kinase (MNK) eukaryotic initiation factor (eIF) 4E pathways. This is a mechanism driving changes in excitability of nociceptors that is critical for the generation of chronic pain states; however, the mRNAs that are translated to lead to this plasticity have not been elucidated. To address this gap in knowledge, we used translating ribosome affinity purification in male and female mice to comprehensively characterize mRNA translation in Scn10a-positive nociceptors in chemotherapy-induced neuropathic pain (CIPN) caused by paclitaxel treatment. This unbiased method creates a new resource for the field, confirms many findings in the CIPN literature and also find extensive evidence for new target mechanisms that may cause CIPN. We provide evidence that an underlying mechanism of CIPN is sustained mTORC1 activation driven by MNK1-eIF4E signaling. RagA, a GTPase controlling mTORC1 activity, is identified as a novel target of MNK1-eIF4E signaling. This demonstrates a novel translation regulation signaling circuit wherein MNK1-eIF4E activity drives mTORC1 via control of RagA translation. CIPN and RagA translation are strongly attenuated by genetic ablation of eIF4E phosphorylation, MNK1 elimination or treatment with the MNK inhibitor eFT508. We identify a novel translational circuit for the genesis of neuropathic pain caused by chemotherapy with important implications for therapeutics.SIGNIFICANCE STATEMENT Neuropathic pain affects up to 10% of the population, but its underlying mechanisms are incompletely understood, leading to poor treatment outcomes. We used translating ribosome affinity purification technology to create a comprehensive translational profile of DRG nociceptors in naive mice and at the peak of neuropathic pain induced by paclitaxel treatment. We reveal new insight into how mechanistic target of rapamycin complex 1 is activated in neuropathic pain pointing to a key role of MNK1-eIF4E-mediated translation of a complex of mRNAs that control mechanistic target of rapamycin complex 1 signaling at the surface of the lysosome. We validate this finding using genetic and pharmacological techniques. Our work strongly suggests that MNK1-eIF4E signaling drives CIPN and that a drug in human clinical trials, eFT508, may be a new therapeutic for neuropathic pain.

An updated understanding of the mechanisms involved in chemotherapy-induced neuropathy

The burdensome condition of chemotherapy-induced peripheral neuropathy occurs with various chemotherapeutics, including bortezomib, oxaliplatin, paclitaxel and vincristine. The symptoms, which include pain, numbness, tingling and loss of motor function, can result in therapy titrations that compromise therapy efficacy. Understanding the mechanisms of chemotherapy-induced peripheral neuropathy is therefore essential, yet incompletely understood. The literature presented here will address a multitude of molecular and cellular mechanisms, beginning with the most well-understood cellular and molecular-level changes. These modifications include alterations in voltage-gated ion channels, neurochemical transmission, organelle function and intracellular pathways. System-level alterations, including changes to glial cells and cytokine activation are also explored. Finally, we present research on the current understanding of genetic contributions to this condition. Suggestions for future research are provided.

Paclitaxel-induced increase in mitochondrial volume mediates dysregulation of intracellular Ca2+ in putative nociceptive glabrous skin neurons from the rat

We have recently demonstrated that in a rat model of chemotherapy-induced peripheral neuropathy (CIPN), there is a significant decrease in the duration of the depolarization-evoked Ca²⁺ transient in small diameter, IB4+, and capsaicin-responsive neurons innervating the glabrous skin of the hindpaw. This change was specific to the transient duration and significantly smaller if not undetectable in neurons innervating the dorsal skin of the hindpaw or the skin of the inner thigh. Given the importance of mitochondria in intracellular Ca²⁺ regulation and the findings of chemotherapy-associated increase in mitotoxicity along the sensory neuron axons, we hypothesized that CIPN is due to both increases and decreases in mitochondria function, with changes manifest in distinct subpopulations of afferents. To begin to test this hypothesis, we used confocal microscopy and Ca²⁺ imaging in combination with pharmacological manipulations to study paclitaxel-induced changes in retrograde tracer-labeled neurons from naïve, vehicle-treated, and paclitaxel-treated rats. Paclitaxel treatment was not associated with decreased mitochondrial membrane potential or increased superoxide levels in the somata of putative nociceptive glabrous skin neurons. However, it was associated with significant increases in the relative contribution of mitochondria to the control of the evoked Ca²⁺ transient duration in putative nociceptive glabrous skin neurons, as well as increases in mitotracker and Tom20 staining which reflected an increase in mitochondrial volume. Furthermore, the relative contribution of the sarco-endoplasmic reticulum Ca²⁺ ATPase to the regulation of the duration of the depolarization evoked Ca²⁺ transient was also increased in this subpopulation of neurons from paclitaxel treated rats. Our results indicate that the paclitaxel-induced decrease in the duration of the evoked Ca²⁺ transient is due to both direct and indirect influences of mitochondria. It remains to be determined if and how these changes contribute to the manifestation of CIPN.