Probing for chemotherapy-induced peripheral neuropathy in live dorsal root ganglion neurons with atomic force microscopy

Current understanding of the molecular mechanisms of chemotherapy-induced peripheral neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is the most common off-target adverse effects caused by various chemotherapeutic agents, such as cisplatin, oxaliplatin, paclitaxel, vincristine and bortezomib. CIPN is characterized by a substantial loss of primary afferent sensory axonal fibers leading to sensory disturbances in patients. An estimated of 19-85% of patients developed CIPN during the course of chemotherapy. The lack of preventive measures and limited treatment options often require a dose reduction or even early termination of life-saving chemotherapy, impacting treatment efficacy and patient survival. In this Review, we summarized the current understanding on the pathogenesis of CIPN. One prominent change induced by chemotherapeutic agents involves the disruption of neuronal cytoskeletal architecture and axonal transport dynamics largely influenced by the interference of microtubule stability in peripheral neurons. Due to an ineffective blood-nerve barrier in our peripheral nervous system, exposure to some chemotherapeutic agents causes mitochondrial swelling in peripheral nerves, which lead to the opening of mitochondrial permeability transition pore and cytochrome c release resulting in degeneration of primary afferent sensory fibers. The exacerbated nociceptive signaling and pain transmission in CIPN patients is often linked the increased neuronal excitability largely due to the elevated expression of various ion channels in the dorsal root ganglion neurons. Another important contributing factor of CIPN is the neuroinflammation caused by an increased infiltration of immune cells and production of inflammatory cytokines. In the central nervous system, chemotherapeutic agents also induce neuronal hyperexcitability in the spinal dorsal horn and anterior cingulate cortex leading to the development of central sensitization that causes CIPN. Emerging evidence suggests that the change in the composition and diversity of gut microbiota (dysbiosis) could have direct impact on the development and progression of CIPN. Collectively, all these aspects contribute to the pathogenesis of CIPN. Recent advances in RNA-sequencing offer solid platform for in silico drug screening which enable the identification of novel therapeutic agents or repurpose existing drugs to alleviate CIPN, holding immense promises for enhancing the quality of life for cancer patients who undergo chemotherapy and improve their overall treatment outcomes.

Cell-cell and cell-matrix adhesion regulated by Piezo1 is critical for stiffness-dependent DRG neuron aggregation

The dorsal root ganglion (DRG) is characterized by the dense clustering of primary sensory neuron bodies, with their axons extending to target tissues for sensory perception. The close physical proximity of DRG neurons facilitates the integration and amplification of somatosensation, ensuring normal physiological functioning. However, the mechanism underlying DRG neuron aggregation was unclear. In our study, we culture DRG neurons from newborn rats on substrates with varying stiffness and observe that the aggregation of DRG neurons is influenced by mechanical signals arising from substrate stiffness. Moreover, we identify Piezo1 as the mechanosensor responsible for DRG neurons' ability to sense different substrate stiffness. We further demonstrate that the Piezo1-calpain-integrin-β1/E-cadherin signaling cascade regulates the aggregation of DRG neurons. These findings deepen our understanding of the mechanisms involved in histogenesis and potential disease development, as mechanical signals arising from substrate stiffness play a crucial role in these processes.

Genome-wide study reveals novel roles for formin-2 in axon regeneration as a microtubule dynamics regulator and therapeutic target for nerve repair

Peripheral nerves regenerate successfully; however, clinical outcome after injury is poor. We demonstrated that low-dose ionizing radiation (LDIR) promoted axon regeneration and function recovery after peripheral nerve injury (PNI). Genome-wide CpG methylation profiling identified LDIR-induced hypermethylation of the Fmn2 promoter, exhibiting injury-induced Fmn2 downregulation in dorsal root ganglia (DRGs). Constitutive knockout or neuronal Fmn2 knockdown accelerated nerve repair and function recovery. Mechanistically, increased microtubule dynamics at growth cones was observed in time-lapse imaging of Fmn2-deficient DRG neurons. Increased HDAC5 phosphorylation and rapid tubulin deacetylation were found in regenerating axons of neuronal Fmn2-knockdown mice after injury. Growth-promoting effect of neuronal Fmn2 knockdown was eliminated by pharmaceutical blockade of HDAC5 or neuronal Hdac5 knockdown, suggesting that Fmn2deletion promotes axon regeneration via microtubule post-translational modification. In silico screening of FDA-approved drugs identified metaxalone, administered either immediately or 24-h post-injury, accelerating function recovery. This work uncovers a novel axon regeneration function of Fmn2 and a small-molecule strategy for PNI.

Unraveling the Mechanobiology Underlying Traumatic Brain Injury with Advanced Technologies and Biomaterials

Traumatic brain injury (TBI) is a worldwide health and socioeconomic problem, associated with prolonged and complex neurological aftermaths, including a variety of functional deficits and neurodegenerative disorders. Research on the long-term effects has highlighted that TBI shall be regarded as a chronic health condition. The initiation and exacerbation of TBI involve a series of mechanical stimulations and perturbations, accompanied by mechanotransduction events within the brain tissues. Mechanobiology thus offers a unique perspective and likely promising approach to unravel the underlying molecular and biochemical mechanisms leading to neural cells dysfunction after TBI, which may contribute to the discovery of novel targets for future clinical treatment. This article investigates TBI and the subsequent brain dysfunction from a lens of neuromechanobiology. Following an introduction, the mechanobiological insights are examined into the molecular pathology of TBI, and then an overview is given of the latest research technologies to explore neuromechanobiology, with particular focus on microfluidics and biomaterials. Challenges and prospects in the current field are also discussed. Through this article, it is hoped that extensive technical innovation in biomedical devices and materials can be encouraged to advance the field of neuromechanobiology, paving potential ways for the research and rehabilitation of neurotrauma and neurological diseases.

Clinically relevant small-molecule promotes nerve repair and visual function recovery

Adult mammalian injured axons regenerate over short-distance in the peripheral nervous system (PNS) while the axons in the central nervous system (CNS) are unable to regrow after injury. Here, we demonstrated that Lycium barbarum polysaccharides (LBP), purified from Wolfberry, accelerated long-distance axon regeneration after severe peripheral nerve injury (PNI) and optic nerve crush (ONC). LBP not only promoted intrinsic growth capacity of injured neurons and function recovery after severe PNI, but also induced robust retinal ganglion cell (RGC) survival and axon regeneration after ONC. By using LBP gene expression profile signatures to query a Connectivity map database, we identified a Food and Drug Administration (FDA)-approved small-molecule glycopyrrolate, which promoted PNS axon regeneration, RGC survival and sustained CNS axon regeneration, increased neural firing in the superior colliculus, and enhanced visual target re-innervations by regenerating RGC axons leading to a partial restoration of visual function after ONC. Our study provides insights into repurposing of FDA-approved small molecule for nerve repair and function recovery.

Applicability of atomic force microscopy to determine cancer-related changes in cells

The determination of mechanical properties of living cells as an indicator of cancer progression has become possible with the development of local measurement techniques such as atomic force microscopy (AFM). Its most important advantage is a nanoscopic character, implying that very local alterations can be quantified. The results gathered from AFM measurements of various cancers show that, for most cancers, individual cells are characterized by the lower apparent Young's modulus, denoting higher cell deformability. The measured value depends on various factors, like the properties of substrates used for cell growth, force loading rate or indentation depth. Despite this, the results proved the AFM capability to recognize mechanically altered cells. This can significantly impact the development of methodological approaches toward the precise identification of pathological cells. This article is part of the theme issue 'Nanocracks in nature and industry'.

Cerebellar glutamatergic system impacts spontaneous motor recovery by regulating Gria1 expression

Peripheral nerve injury (PNI) often results in spontaneous motor recovery; however, how disrupted cerebellar circuitry affects PNI-associated motor recovery is unknown. Here, we demonstrated disrupted cerebellar circuitry and poor motor recovery in ataxia mice after PNI. This effect was mimicked by deep cerebellar nuclei (DCN) lesion, but not by damaging non-motor area hippocampus. By restoring cerebellar circuitry through DCN stimulation, and reversal of neurotransmitter imbalance using baclofen, ataxia mice achieve full motor recovery after PNI. Mechanistically, elevated glutamate-glutamine level was detected in DCN of ataxia mice by magnetic resonance spectroscopy. Transcriptomic study revealed that Gria1, an ionotropic glutamate receptor, was upregulated in DCN of control mice but failed to be upregulated in ataxia mice after sciatic nerve crush. AAV-mediated overexpression of Gria1 in DCN rescued motor deficits of ataxia mice after PNI. Finally, we found a correlative decrease in human GRIA1 mRNA expression in the cerebellum of patients with ataxia-telangiectasia and spinocerebellar ataxia type 6 patient iPSC-derived Purkinje cells, pointing to the clinical relevance of glutamatergic system. By conducting a large-scale analysis of 9,655,320 patients with ataxia, they failed to recover from carpal tunnel decompression surgery and tibial neuropathy, while aged-match non-ataxia patients fully recovered. Our results provide insight into cerebellar disorders and motor deficits after PNI.

Dynamic cellular biomechanics in responses to chemotherapeutic drug in hypoxia probed by atomic force spectroscopy

The changes in cellular structure play an important role in cancer cell development, progression, and metastasis. By exploiting single-cell, force spectroscopy methods, we probed biophysical and biomechanical kinetics (stiffness, morphology, roughness, adhesion) of brain, breast, prostate, and pancreatic cancer cells with standard chemotherapeutic drugs in normoxia and hypoxia over 12–24 hours. After exposure to the drugs, we found that brain, breast, and pancreatic cancer cells became approximately 55–75% less stiff, while prostate cancer cells became more stiff, due to either drug-induced disruption or reinforcement of cytoskeletal structure. However, the rate of the stiffness change decreased up to 2-folds in hypoxia, suggesting a correlation between cellular stiffness and drug resistance of cancer cells in hypoxic tumor microenvironment. Also, we observed significant changes in the cell body height, surface roughness, and cytoadhesion of cancer cells after exposure to drugs, which followed the trend of stiffness. Our results show that a degree of chemotherapeutic drug effects on biomechanical and biophysical properties of cancer cells is distinguishable in normoxia and hypoxia, which are correlated with alteration of cytoskeletal structure and integrity during drug-induced apoptotic process.

Stiffening of DU145 prostate cancer cells driven by actin filaments - microtubule crosstalk conferring resistance to microtubule-targeting drugs

The crucial role of microtubules in the mitotic-related segregation of chromosomes makes them an excellent target for anticancer microtubule targeting drugs (MTDs) such as vinflunine (VFL), colchicine (COL), and docetaxel (DTX). MTDs affect mitosis by directly perturbing the structural organisation of microtubules. By a direct assessment of the biomechanical properties of prostate cancer DU145 cells exposed to different MTDs using atomic force microscopy, we show that cell stiffening is a response to the application of all the studied MTDs (VFL, COL, DTX). Changes in cellular rigidity are typically attributed to remodelling of the actin filaments in the cytoskeleton. Here, we demonstrate that cell stiffening can be driven by crosstalk between actin filaments and microtubules in MTD-treated cells. Our findings improve the interpretation of biomechanical data obtained for living cells in studies of various physiological and pathological processes.

Nanomechanics in Monitoring the Effectiveness of Drugs Targeting the Cancer Cell Cytoskeleton

Increasing attention is devoted to the use of nanomechanics as a marker of various pathologies. Atomic force microscopy (AFM) is one of the techniques that could be applied to quantify the nanomechanical properties of living cells with a high spatial resolution. Thus, AFM offers the possibility to trace changes in the reorganization of the cytoskeleton in living cells. Impairments in the structure, organization, and functioning of two main cytoskeletal components, namely, actin filaments and microtubules, cause severe effects, leading to cell death. That is why these cytoskeletal components are targets for antitumor therapy. This review intends to describe the gathered knowledge on the capability of AFM to trace the alterations in the nanomechanical properties of living cells induced by the action of antitumor drugs that could translate into their effectiveness.

Effects of paclitaxel on the viscoelastic properties of mouse sensory nerves

Paclitaxel is an effective and widely used chemotherapeutic, but also causes debilitating peripheral sensory neuropathy. Due to its influence on microtubule stability, we and others have hypothesized that paclitaxel alters neuromechanical properties. A prior study suggested that paclitaxel increases the tensile moduli of rat sensory nerves. However, the effects of paclitaxel on tissue level viscoelasticity have not been tested. In this study, sural branches of C57BL/6J mouse sciatic nerves were bilaterally excised. One nerve was treated with Ringer's solution containing paclitaxel, and the contralateral nerve with Ringer's alone. Nerves were then subject to a passive loading protocol in which peak stress, relaxed stress, and stress-relaxation dynamics were monitored at increasing strain. Elastic and tangent tensile moduli were calculated from both peak and relaxed stress-strain curves as well as failure stress were significantly elevated in paclitaxel-treated nerves compared to controls. Double-exponential fits (with τ_m and τ_n indicating fast and slow time constants, respectively) were successfully applied to model stress-relaxation. Though no significant differences in the τ_m and τ_n were found between groups, paclitaxel treatment significantly increased the variability of τ_m, suggesting heterogeneous effects on nerve biomechanical properties. Our data establish that paclitaxel effects at the cellular level influence tensile viscoelastic properties of nerves at the tissue level. These results have implications for understanding biomechanical influences on the progression and physical rehabilitation of paclitaxel-induced neuropathy.

Chemotherapeutic resistance: a nano-mechanical point of view

Chemotherapeutic resistance is one of the main obstacles for cancer remission. To understand how cancer cells acquire chemotherapeutic resistance, biochemical studies focusing on drug target alteration, altered cell proliferation, and reduced susceptibility to apoptosis were performed. Advances in nano-mechanobiology showed that the enhanced mechanical deformability of cancer cells accompanied by cytoskeletal alteration is a decisive factor for cancer development. Furthermore, atomic force microscopy (AFM)–based nano-mechanical studies showed that chemotherapeutic treatments reinforced the mechanical stiffness of drug-sensitive cancer cells. However, drug-resistant cancer cells did not show such mechanical responses following chemotherapeutic treatments. Interestingly, drug-resistant cancer cells are mechanically heterogeneous, with a subpopulation of resistant cells showing higher stiffness than their drug-sensitive counterparts. The signaling pathways involving Rho, vinculin, and myosin II were found to be responsible for these mechanical alterations in drug-resistant cancer cells. In the present review, we highlight the mechanical aspects of chemotherapeutic resistance, and suggest how mechanical studies can contribute to unravelling the multifaceted nature of chemotherapeutic resistance.

Effect of dacarbazine on CD44 in live melanoma cells as measured by atomic force microscopy-based nanoscopy

CD44 ligand-receptor interactions are known to be involved in regulating cell migration and tumor cell metastasis. High expression levels of CD44 correlate with a poor prognosis of melanoma patients. In order to understand not only the mechanistic basis for dacarbazine (DTIC)-based melanoma treatment but also the reason for the poor prognosis of melanoma patients treated with DTIC, dynamic force spectroscopy was used to structurally map single native CD44-coupled receptors on the surface of melanoma cells. The effect of DTIC treatment was quantified by the dynamic binding strength and the ligand-binding free-energy landscape. The results demonstrated no obvious effect of DTIC on the unbinding force between CD44 ligand and its receptor, even when the CD44 nanodomains were reduced significantly. However, DTIC did perturb the kinetic and thermodynamic interactions of the CD44 ligand-receptor, with a resultant greater dissociation rate, lower affinity, lower binding free energy, and a narrower energy valley for the free-energy landscape. For cells treated with 25 and 75 μg/mL DTIC for 24 hours, the dissociation constant for CD44 increased 9- and 70-fold, respectively. The CD44 ligand binding free energy decreased from 9.94 for untreated cells to 8.65 and 7.39 kcal/mol for DTIC-treated cells, which indicated that the CD44 ligand-receptor complexes on DTIC-treated melanoma cells were less stable than on untreated cells. However, affinity remained in the micromolar range, rather than the millimolar range associated with nonaffinity ligands. Hence, the CD44 receptor could still be activated, resulting in intracellular signaling that could trigger a cellular response. These results demonstrate DTIC perturbs, but not completely inhibits, the binding of CD44 ligand to membrane receptors, suggesting a basis for the poor prognosis associated with DTIC treatment of melanoma. Overall, atomic force microscopy-based nanoscopic methods offer thermodynamic and kinetic insight into the effect of DTIC on the CD44 ligand-binding process.

Ciguatoxin reduces regenerative capacity of axotomized peripheral neurons and delays functional recovery in pre-exposed mice after peripheral nerve injury

Ciguatera fish poisoning (CFP) results from consumption of tropical reef fish containing ciguatoxins (CTXs). Pacific (P)-CTX-1 is among the most potent known CTXs and the predominant source of CFP in the endemic region responsible for the majority of neurological symptoms in patients. Chronic and persistent neurological symptoms occur in some CFP patients, which often result in incomplete functional recovery for years. However, the direct effects of exposure to CTXs remain largely unknown. In present study, we exposed mice to CTX purified from ciguatera fish sourced from the Pacific region. P-CTX-1 was detected in peripheral nerves within hours and persisted for two months after exposure. P-CTX-1 inhibited axonal regrowth from axotomized peripheral neurons in culture. P-CTX-1 exposure reduced motor function in mice within the first two weeks of exposure before returning to baseline levels. These pre-exposed animals exhibited delayed sensory and motor functional recovery, and irreversible motor deficits after peripheral nerve injury in which formation of functional synapses was impaired. These findings are consistent with reduced muscle function, as assessed by electromyography recordings. Our study provides strong evidence that the persistence of P-CTX-1 in peripheral nerves reduces the intrinsic growth capacity of peripheral neurons, resulting in delayed functional recovery after injury.

Modeling the mechanics of cells in the cell-spreading process driven by traction forces

Mechanical properties of cells and their mechanical interaction with the extracellular environments are main factors influencing cellular function, thus indicating the progression of cells in different disease states. By considering the mechanical interactions between cell adhesion molecules and the extracellular environment, we developed a cell mechanical model that can characterize the mechanical changes in cells during cell spreading. A cell model was established that consisted of various main subcellular components, including cortical cytoskeleton, nuclear envelope, actin filaments, intermediate filaments, and microtubules. We demonstrated the structural changes in subcellular components and the changes in spreading areas during cell spreading driven by traction forces. The simulation of nanoindentation tests was conducted by integrating the indenting force to the cell model. The force-indentation curve of the cells at different spreading states was simulated, and the results showed that cell stiffness increased with increasing traction forces, which were consistent with the experimental results. The proposed cell mechanical model provides a strategy to investigate the mechanical interactions of cells with the extracellular environments through the adhesion molecules and to reveal the cell mechanical properties at the subcellular level as cells shift from the suspended state to the adherent state.

Paclitaxel alters sensory nerve biomechanical properties

Paclitaxel is an effective chemotherapeutic that, despite its common use, frequently causes debilitating peripheral sensory neuropathy. Paclitaxel binds to and stabilizes microtubules, and through unknown mechanisms, causes abnormal microtubule aggregation. Given that microtubules contribute to the mechanical properties of cells, we tested the hypothesis that paclitaxel treatment would alter the stiffness of sensory nerves. Rat sural nerves were excised and soaked in Ringer's solution with or without paclitaxel. Nerves were secured between a force transducer and actuator, and linearly strained. Stress-strain curves were generated, from which elastic moduli were calculated. Paclitaxel treated nerves exhibited significantly higher moduli in both linear and transition regions of the curve. A composite-tissue model was then generated to estimate the stiffness increase in the cellular fraction of the nerve following paclitaxel treatment. This model was supported experimentally by data on mechanical properties of sural nerves stripped of their epineurium, and area fractions of the cellular and connective tissue components of the rat sural nerve, calculated from immunohistochemical images. Model results revealed that the cellular components of the nerve must stiffen 12x to 115x, depending on the initial axonal modulus assumed, in order to achieve the observed tissue level mechanical changes. Consistent with such an increase, electron microscopy showed increased microtubule aggregation and cytoskeletal packing, suggestive of a more cross-linked cytoskeleton. Overall, our data suggests that paclitaxel treatment induces increased microtubule bundling in axons, which leads to alterations in tissue-level mechanical properties.

Atomic force microscopy as an advanced tool in neuroscience

This review highlights relevant issues about applications and improvements of atomic force microscopy (AFM) toward a better understanding of neurodegenerative changes at the molecular level with the hope of contributing to the development of effective therapeutic strategies for neurodegenerative illnesses. The basic principles of AFM are briefly discussed in terms of evaluation of experimental data, including the newest PeakForce Quantitative Nanomechanical Mapping (QNM) and the evaluation of Young’s modulus as the crucial elasticity parameter. AFM topography, revealed in imaging mode, can be used to monitor changes in live neurons over time, representing a valuable tool for high-resolution detection and monitoring of neuronal morphology. The mechanical properties of living cells can be quantified by force spectroscopy as well as by new AFM. A variety of applications are described, and their relevance for specific research areas discussed. In addition, imaging as well as non-imaging modes can provide specific information, not only about the structural and mechanical properties of neuronal membranes, but also on the cytoplasm, cell nucleus, and particularly cytoskeletal components. Moreover, new AFM is able to provide detailed insight into physical structure and biochemical interactions in both physiological and pathophysiological conditions.