Effects of taxol on slow and fast axonal transport

Towards a mechanistic understanding of axon transport and endocytic changes underlying paclitaxel-induced peripheral neuropathy

Paclitaxel is a common chemotherapeutic agent widely used to treat solid cancer. However, it frequently causes peripheral sensory neuropathy, resulting in sensory abnormalities and pain in patients receiving treatment for cancer. As one of the most widely used chemotherapeutics, many preclinical studies on paclitaxel-induced peripheral neuropathy (PIPN) have been performed. Yet, there remain no effective options for treatment or prevention. Due to paclitaxel's ability to bind to and stabilize microtubules, a change in microtubule dynamics and subsequent disruptions in axonal transport has been predicted as a major underlying cause of paclitaxel-induced toxicity. However, the systemic understanding of PIPN mechanisms is largely incomplete, and various phenotypes have not been directly attributed to microtubule-related effects. This review aims to provide an overview of the literature involving paclitaxel-induced alteration in microtubule dynamics, axonal transport, and endocytic changes. It also aims to provide insights into how the microtubule-mediated hypothesis may relate to various phenotypes reported in PIPN studies.

Association between Low-Grade Chemotherapy-Induced Peripheral Neuropathy (CINP) and Survival in Patients with Metastatic Adenocarcinoma of the Pancreas

The combination of nab-paclitaxel and gemcitabine demonstrated greater efficacy than gemcitabine alone but resulted in higher rates of chemotherapy-induced peripheral neuropathy (CINP) in patients with metastatic pancreatic cancer (mPC). We aimed to evaluate the correlation between the development of treatment-related peripheral neuropathy and the efficacy of nab-P/Gem combination in these patients. mPC patients treated with nab-paclitaxel 125 mg/m² and gemcitabine 1000 mg/m² as a first-line therapy were included. Treatment-related adverse events, mainly peripheral neuropathy, were categorized using the National Cancer Institute Common Toxicity Criteria scale, version 4.02. Efficacy outcomes, including overall survival (OS), progression-free survival (PSF), and disease control rate (DCR), were estimated by the Kaplan–Meier model. A total of 153 patients were analyzed; of these, 47 patients (30.7%) developed grade 1–2 neuropathy. PFS was 7 months (95% CI (6–7 months)) for patients with grade 1–2 neuropathy and 6 months (95% CI (5–6 months)) for patients without peripheral neuropathy (p = 0.42). Median OS was 13 months (95% CI (10–18 months)) and 10 months (95% CI (8–13 months)) in patients with and without peripheral neuropathy, respectively (p = 0.04). DCR was achieved by 83% of patients with grade 1–2 neuropathy and by 58% of patients without neuropathy (p = 0.03). In the multivariate analysis, grade 1–2 neuropathy was independently associated with OS (HR 0.65; 95% CI, 0.45–0.98; p = 0.03). nab-P/Gem represents an optimal first-line treatment for mPC patients. Among possible treatment-related adverse events, peripheral neuropathy is the most frequent, with different grades and incidence. Our study suggests that patients experiencing CINP may have a more favorable outcome, with a higher disease control rate and prolonged median survival compared to those without neuropathy.

Neuropathic Pain: From Mechanisms to Treatment

Neuropathic pain caused by a lesion or disease of the somatosensory nervous system is a common chronic pain condition with major impact on quality of life. Examples include trigeminal neuralgia, painful polyneuropathy, postherpetic neuralgia, and central poststroke pain. Most patients complain of an ongoing or intermittent spontaneous pain of, for example, burning, pricking, squeezing quality, which may be accompanied by evoked pain, particular to light touch and cold. Ectopic activity in, for example, nerve-end neuroma, compressed nerves or nerve roots, dorsal root ganglia, and the thalamus may in different conditions underlie the spontaneous pain. Evoked pain may spread to neighboring areas, and the underlying pathophysiology involves peripheral and central sensitization. Maladaptive structural changes and a number of cell-cell interactions and molecular signaling underlie the sensitization of nociceptive pathways. These include alteration in ion channels, activation of immune cells, glial-derived mediators, and epigenetic regulation. The major classes of therapeutics include drugs acting on α₂δ subunits of calcium channels, sodium channels, and descending modulatory inhibitory pathways.

Regulatable Transgene Expression for Prevention of Chemotherapy-Induced Peripheral Neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is a debilitating complication associated with drug treatment of cancer for which there are no effective strategies of prevention or treatment. In this study, we examined the effect of intermittent expression of neurotophin-3 (NT-3) or interleukin-10 (IL-10) from replication-defective herpes simplex virus (HSV)-based regulatable vectors delivered by subcutaneous inoculation to the dorsal root ganglion (DRG) on the development of paclitaxel-induced peripheral neuropathy. We constructed two different tetracycline (tet)-on-based regulatable HSV vectors, one expressing NT-3 and the other expressing IL-10, in which the transactivator expression in the tet-on system was under the control of HSV latency-associated promoter 2 (LAP-2), and expression of the transgene was controlled by doxycycline (DOX). We examined the therapeutic effect of intermittent expression of the transgene in animals with paclitaxel-induced peripheral neuropathy modeled by intraperitoneal injection of paclitaxel (16 mg/kg) once a week for 5 weeks. Intermittent expression of either NT-3 or IL-10 3 days before and 1 day after paclitaxel administration protected animals against paclitaxel-induced peripheral neuropathy over the course of 5 weeks. These results suggest the potential of regulatable vectors for prevention of chemotherapy-induced peripheral neuropathy.

Chemotherapy-induced peripheral neuropathy: A current review

Chemotherapy-induced peripheral neuropathy (CIPN) is a common dose-limiting side effect experienced by patients receiving treatment for cancer. Approximately 30 to 40% of patients treated with neurotoxic chemotherapy will develop CIPN, and there is considerable variability in its severity between patients. It is often sensory-predominant with pain and can lead to long-term morbidity in survivors. The prevalence and burden of CIPN late effects will likely increase as cancer survival rates continue to improve. In this review, we discuss the approach to peripheral neuropathy in patients with cancer and address the clinical phenotypes and pathomechanisms of specific neurotoxic chemotherapeutic agents. Ann Neurol 2017;81:772-781.

Chemotherapy-induced neuropathy

OPINION STATEMENT

Chemotherapy-induced peripheral neurotoxicity (CIPN) is one of the most severe and unpredictable side effects of modern anticancer treatment. In recent years, a clear understanding of the importance of an integrated approach to CIPN has become evident, and efforts are increasing to better characterize its features and to identify more accurate methods to report and grade its occurrence. The clinically relevant impact of CIPN on cancer patients has been known for a long time, but knowledge of its pathogenetic aspects is still very limited. This incomplete knowledge is one of the major limitations in identifying targets for evidence-based neuroprotective strategies. Nevertheless, several studies have been devoted to the prevention or at least the effective treatment of symptoms secondary to peripheral nerve damage and to the early identification of patients at high risk of developing severe CIPN. Unfortunately, none of these studies has been successful and the optimal management of CIPN patients is still an unmet clinical need. Therefore, the modification of chemotherapy is currently the only available approach to limit the severity of neuropathy in the vast majority of patients. The indications for treatment modification are not universally accepted and they can differ among the various drugs. Generally, treatment modification should be considered as soon as symptoms and signs impair the daily life activities of the patient, but the possibility of a delayed worsening of CIPN after treatment withdrawal ("coasting") should always be considered, and delay of modification decisions should be avoided.

Annonacin, a natural mitochondrial complex I inhibitor, causes tau pathology in cultured neurons

A neurodegenerative tauopathy endemic to the Caribbean island of Guadeloupe has been associated with the consumption of anonaceous plants that contain acetogenins, potent lipophilic inhibitors of complex I of the mitochondrial respiratory chain. To test the hypothesis that annonacin, a prototypical acetogenin, contributes to the etiology of the disease, we investigated whether annonacin affects the cellular distribution of the protein tau. In primary cultures of rat striatal neurons treated for 48 h with annonacin, there was a concentration-dependent decrease in ATP levels, a redistribution of tau from the axons to the cell body, and cell death. Annonacin induced the retrograde transport of mitochondria, some of which had tau attached to their outer membrane. Taxol, a drug that displaces tau from microtubules, prevented the somatic redistribution of both mitochondria and tau but not cell death. Antioxidants, which scavenged the reactive oxygen species produced by complex I inhibition, did not affect either the redistribution of tau or cell death. Both were prevented, however, by forced expression of the NDI1 nicotinamide adenine dinucleotide (NADH)-quinone-oxidoreductase of Saccharomyces cerevisiae, which can restore NADH oxidation in complex I-deficient mammalian cells and stimulation of energy production via anaerobic glycolysis. Consistently, other ATP-depleting neurotoxins (1-methyl-4-phenylpyridinium, 3-nitropropionic, and carbonyl cyanide m-chlorophenylhydrazone) reproduced the somatic redistribution of tau, whereas toxins that did not decrease ATP levels did not cause the redistribution of tau. Therefore, the annonacin-induced ATP depletion causes the retrograde transport of mitochondria to the cell soma and induces changes in the intracellular distribution of tau in a way that shares characteristics with some neurodegenerative diseases.

Chemotherapy-evoked painful peripheral neuropathy

Vincristine and paclitaxel, two of the most effective drugs in the battle against cancer, produce a dose-limiting neurotoxicity that sometimes presents as a painful peripheral neuropathy. For the first time, investigators have been able to produce these chemotherapy-evoked painful peripheral neuropathies in the laboratory rat. These new models have already begun to elucidate the causes of the neuropathic pain associated with these antineoplastic drugs, which will now make it possible to search for effective ways to prevent and treat it.

A painful peripheral neuropathy in the rat produced by the chemotherapeutic drug, paclitaxel

Paclitaxel, an effective anti-neoplastic agent in the treatment of solid tumors, produces a dose-limiting painful peripheral neuropathy in a clinically significant number of cancer patients. Prior work has demonstrated paclitaxel-induced neurodegeneration and sensory loss in laboratory rodents. We describe here an experimental paclitaxel-induced painful peripheral neuropathy. Adult male rats were given four intraperitoneal injections on alternate days of vehicle or 0.5, 1.0, or 2.0 mg/kg of paclitaxel (Taxol). Behavioral tests for pain using mechanical and thermal stimuli applied to the tail and hind paws, and tests for motor performance, were taken before, during and after dosing for 22-35 days. All three doses of paclitaxel caused heat-hyperalgesia, mechano-allodynia, mechano-hyperalgesia, and cold-allodynia, but had no effect on motor performance. Neuropathic pain began within days and lasted for several weeks. We did not detect any dose-response relationship. Tests at the distal, mid, and proximal tail failed to show evidence of a length-dependent neuropathy. Vehicle control injections had no effect on any measure. No significant systemic toxicities were noted in the paclitaxel-treated animals. Light-microscopic inspection of the sciatic nerve (mid-thigh level), L4-L5 dorsal root ganglia, and dorsal and ventral roots, and the gray and white matter of the L4-L5 spinal cord, showed no structural abnormalities. Electron microscopic examination of the sciatic nerve (mid-thigh level) and the L4-L5 dorsal root ganglia and dorsal horns demonstrated no degeneration of myelinated and unmyelinated axons in the sciatic nerve and roots, but revealed endoneurial edema. This model may be useful in understanding a significant source of pain in cancer patients, and in finding ways to avoid the neurotoxicity that limits paclitaxel therapy.

Morphological evidence of the inhibitory effect of taxol on the fast axonal transport

The short term effects of taxol, a stabilizing drug of microtubules, on the peripheral nerves in the rat was investigated using a new chamber system which can be applied to incubate a sciatic nerve with various solutions in vivo. A functional analysis of retrograde axonal transport using rhodamine-labeled wheat germ agglutinin (WGA-rhodamine) showed the inhibitory effect of the drug. An electron microscopic study also revealed that a variety of vesicles were observed to accumulate on both the proximal and the distal sides of the chamber, however, no significant increase in the number of microtubules in the axons, based on the pharmacological effect of the drug, was observed even though one had been expected. These findings support the inhibitory effect of taxol on the fast axonal transport of the neurons. Furthermore, the accumulated vesicles were morphologically different from those accumulated by ligation. These results suggest that a special component of the fast axonal transport was thus selectively blocked by the drug.

Assembly of microfilaments and microtubules from axonally transported actin and tubulin after axotomy

The slow component (SC) of axonal transport conveys structural proteins, regulatory proteins, and glycolytic enzymes toward the axon tip at 1-6 mm/day. Following axon interruption (axotomy), the rate of outgrowth corresponds to the rate of SCb-the fastest subcomponent of SC. Both axonal outgrowth and SCb accelerate 20-25% after axotomy. Tubulin and actin are the major proteins being carried by SCb. To further characterize the acceleration of SCb, we measured the equilibrium between subunits and polymers for both actin and tubulin. We radiolabeled newly synthesized proteins in rat motor neurons by microinjecting [35S]methionine into the spinal cord 7 days after crushing the sciatic nerve (85 mm from the spinal cord). Nerves were removed 7 days later for homogenization in polymer-stabilizing buffer (PSB) and centrifugation, followed by SDS-PAGE of supernatants (S) and pellets (P). We removed beta-tubulin, actin, and the medium-weight neurofilament protein (NF-M) from each gel by using the fluorogram as a template. After solubilizing gel segments for liquid scintillation spectrometry, we expressed counts as a polymerization ratio: P/[S+P]. In the nerve segments that contained radiolabeled Scb proteins, located 24-36 mm from the spinal cord, axotomy increased the polymerization ratio of SCb actin from 0.23 to 0.36 (P < 0.05) but had no effect on SCb beta-tubulin. In a separate experiment, we added 12 microM taxol to PSB to stabilize newly assembled microtubules. Adding taxol did not alter the polymerization ratio for SCb beta-tubulin in sham-axotomized nerves but aid increase the ratio in axotomized nerves, from 0.44 to 0.63 (P < 0.05); polymerization ratios for SCb actin were unaffected. We conclude that the assembly of microfilaments and microtubules increases to provide cytoskeletal elements for axon sprouts. The resulting loss of actin and tubulin subunits may play a role in the acceleration of SCb.

Cytoplasmic mechanisms of axonal and dendritic growth in neurons

The structural mechanisms responsible for the gradual elaboration of the cytoplasmic elongation of neurons are reviewed. In addition to discussing recent work, important older work is included to inform newcomers to the field how the current perspective arose. The highly specialized axon and the less exaggerated dendrite both result from the advance of the motile growth cone. In the area of physiology, studies in the last decade have directly confirmed the classic model of the growth cone pulling forward and the axon elongating from this tension. Particularly in the case of the axon, cytoplasmic elongation is closely linked to the formation of an axial microtubule bundle from behind the advancing growth cone. Substantial progress has been made in understanding the expression of microtubule-associated proteins during neuronal differentiation to stiffen and stabilize axonal microtubules, providing specialized structural support. Studies of membrane organelle transport along the axonal microtubules produced an explosion of knowledge about ATPase molecules serving as motors driving material along microtubule rails. However, most aspects of the cytoplasmic mechanisms responsible for neurogenesis remain poorly understood. There is little agreement on mechanisms for the addition of new plasma membrane or the addition of new cytoskeletal filaments in the growing axon. Also poorly understood are the mechanisms that couple the promiscuous motility of the growth cone to the addition of cytoplasmic elements.

Changes of fast axonal transport by taxol injected subepineurally into the rat sciatic nerve

In contrast to the complete and long-lasting inhibition of tubulin transport, taxol has no effect on fast axonal transport examined immediately after its sub-epineural application to rat sciatic nerve. However, a significant accumulation of rapidly migrating radioactivity appears at the site proximal to the injection when examined a few weeks after treatment, probably due to mechanical obstruction caused by abnormal aggregation of a huge number of intra-axonal microtubules. It also decreases slightly in amount within a few weeks post-treatment, which may be due to reduction of the number of axons caused by degeneration.

Organization and slow axonal transport of cytoskeletal proteins under normal and regenerating conditions

The organization of the axonal cytoskeleton was investigated by analyzing the solubility and transport profile of the major cytoskeletal proteins in motor axons of the rat sciatic nerve under normal and regenerating conditions. When extracted with the Triton-containing buffer at low temperature, 50% of tubulin and 30% of actin were recovered in the insoluble form resistant to further depolymerizing treatments. Most of this cold-insoluble form was transported in slow component a (SCa), the slower of the two subcomponents of slow axonal transport, whereas the cold-soluble form showed a biphasic distribution between SCa and SCb (slow component b). Changes in slow transport during regeneration were studied by injuring the nerve either prior to (experiment I) or after (experiment II) radioactive labeling. In experiment I where the transport of proteins synthesized in response to injury was examined, selective acceleration of SCb was detected together with an increase in the relative proportion of this component. In experiment II where the response of the preexisting cytoskeleton was examined, a shift from SCa to SCb of the cold-soluble form was observed. The differential distribution and response of the two forms of tubulin and actin suggest that the cold-soluble form may be more directly involved in axonal transport.

Association of glycolytic enzymes with the cytoskeleton

The diverse physical associations of the glycolytic enzymes with structural components of the cell suggest that the glycolytic enzymes are not entirely soluble in the cell. The relatively low affinities of the associations are likely responsible for the apparently transient interactions. The binding phenomenon is suggested to regulate metabolism through changes in enzymatic activity and facilitates localized enrichment of the enzymes.

Maturation and aging of the axonal cytoskeleton: biochemical analysis of transported tubulin

Changes in solubility and axonal transport of tubulin during maturation and aging have been investigated using sciatic motor fibers of rats at 4, 7, 14, 30, and 80 weeks of age. One to six weeks after injection of L-[35S]methionine into the spinal cord, labeled cytoskeletal proteins in consecutive segments of the sciatic nerve and the ventral roots were fractionated into soluble and insoluble forms by extraction in 1% Triton at low temperature. In 4-week-old rats, the two forms of tubulin were transported coordinately in a single wave with the average rate of 2 mm/day. At 7 weeks of age, two components in tubulin transport were observed to develop, possibly reflecting the maturation of the axonal cytoskeleton. The slower main component (1.5 mm/day) contained most of the insoluble form together with the neurofilament proteins and the faster component (3 mm/day) was enriched in the soluble form. Though significantly different in composition, the two components correspond to slow component a (SCa) and slow component b (SCb) originally defined in the optic system. A progressive decrease in transport rates of both SCa and SCb was observed with rats at 14, 30, and 80 weeks of age. In addition, there was a large decrease in the proportion of insoluble tubulin during the course of transport in animals older than 30 weeks. This loss of the insoluble form seems to be accounted for partly by the proteolytic degradation of the severely retarded SCa proteins. Changes in axonal transport of tubulin may thus reflect age-related changes in dynamics and turnover of the axonal cytoskeleton.

Changes in organization and axonal transport of cytoskeletal proteins during regeneration

Changes in solubility and transport rate of cytoskeletal proteins during regeneration were studied in the motor fibers of the rat sciatic nerve. Nerves were injured by freezing at the midthigh level either 1-2 weeks before (experiment I) or 1 week after radioactive labeling of the spinal cord with L-[35S]methionine (experiment II). Labeled proteins in 6-mm consecutive segments of the nerve 2 weeks after labeling were analyzed following fractionation into soluble and insoluble populations with 1% Triton at 4 degrees C. When axonal transport of newly synthesized cytoskeleton was examined in the regenerating nerve in experiment I, a new faster component enriched in soluble tubulin and actin was observed that was not present in the control nerve. The rate of the slower main component containing most of the insoluble tubulin and actin together with neurofilament proteins was not affected. A smaller but significant peak of radioactivity enriched in soluble tubulin and actin was also detected ahead of the main peak when the response of the preexisting cytoskeleton was examined in experiment II. It is thus concluded that during regeneration changes in the organization take place in both the newly synthesized and the preexisting axonal cytoskeleton, resulting in a selective acceleration in rate of transport of soluble tubulin and actin.

Two 68-kDa proteins in slow axonal transport belong to the 70-kDa heat shock protein family and the annexin family

The major 68-kDa protein found selectively in the faster of the two subcomponents of slow axonal transport [group IV or slow component b (SCb)] in the rat sciatic nerve has been characterized. It was found to contain two distinct classes of proteins, S1 and S2, both of which have isoelectric points of 5.7, but differ in their solubility in the presence of calcium. The S1 protein, which contributes up to 70% of the 68-kDa component, was soluble in the presence or absence of calcium, whereas the S2 protein was bound to the cytoskeleton in a calcium-dependent manner. Further characterization of the two proteins by peptide mapping and immunological methods revealed that the S1 protein belonged to a family of proteins related to the 70-kDa heat shock protein, whereas the S2 protein was identical to 68-kDa calelectrin (annexin VI). Selective occurrence in SCb of these proteins with potential abilities to regulate protein-protein or protein-membrane interactions suggests that they may play important roles in the control of cytoskeletal organization in the axon, because SCb contains mainly cytoskeletal proteins in a more dynamic form compared with the slowest rate component, slow component a, which is enriched in the stably polymerized form of these proteins.

Tyrosinated and detyrosinated microtubules in axonal processes of cerebellar macroneurons grown in culture

We have used the monoclonal antibody YL 1/2 (Tyr) specific for tyrosinated tubulin, and a polyclonal antibody (Glu) specific for detyrosinated tubulin to visualize the distribution of microtubules and microtubule assembly sites during axonal outgrowth. Cerebellar macroneurons growing in culture initially extend several short and thin neurites which have the potential to differentiate either as axons or dendrites (Ferreira and Caceres: Developmental Brain Research 49:205-213, 1989). At the onset of axonal outgrowth the Tyr antibody labels the minor neurites, the axon, and its growth cone, while the Glu antibody only shows immunoreactivity in the axonal shaft. After nocodazole treatment, the Tyr staining disappears, whereas that produced by the Glu antibody remains practically unchanged. When nocodazole was removed, tyrosinated microtubules reappeared first at the tip of the axon, in a more distal region than that occupied by detyrosinated microtubules; another focal site of tyrosinated tubulin incorporation was detected in the cell body. Incorporation of tyrosinated tubulin into growing axons was also studied after taxol treatment. After long incubation periods in the presence of taxol, the Tyr staining disappeared from the axon but remained in the cell body; however, immunoreactivity in this site was negative when the cells were preincubated in the presence of protein synthesis inhibitors. Release from taxol results in the reappearance of Tyr immunoreactivity at the distal end of the axon. Taken collectively, the present results indicate 1) that in cerebellar macroneurons axonal differentiation is accompanied by a temporal and spatial differentiation of microtubules and 2) that there is an active site of tyrosinated tubulin assembly at the tip of axonal processes, and they suggest that the highly tyrosinated domain in this region is a consequence of rapid microtubule turnover and tubulin tyrosine ligase activity.