Sensory Neurons from Tau Transgenic Mice and Their Utility in Drug Screening

Time- and Concentration-Dependent Adverse Effects of Paclitaxel on Non-Neuronal Cells in Rat Primary Dorsal Root Ganglia

Paclitaxel is a chemotherapeutic agent used to treat a wide range of malignant tumors. Although it has anti-tumoral properties, paclitaxel also shows significant adverse effects on the peripheral nervous system, causing peripheral neuropathy. Paclitaxel has previously been shown to exert direct neurotoxic effects on primary DRG neurons. However, little is known about paclitaxel's effects on non-neuronal DRG cells. They provide mechanical and metabolic support and influence neuronal signaling. In the present study, paclitaxel effects on primary DRG non-neuronal cells were analyzed and their concentration or/and time dependence investigated. DRGs of Wister rats (6-8 weeks old) were isolated, and non-neuronal cell populations were separated by the density gradient centrifugation method. Different concentrations of Paclitaxel (0.01 µM-10 µM) were tested on cell viability by MTT assay, cell death by lactate dehydrogenase (LDH) assay, and propidium iodide (PI) assay, as well as cell proliferation by Bromodeoxyuridine (BrdU) assay at 24 h, 48 h, and 72 h post-treatment. Furthermore, phenotypic effects have been investigated by using immunofluorescence techniques. Paclitaxel exhibited several toxicological effects on non-neuronal cells, including a reduction in cell viability, an increase in cell death, and an inhibition of cell proliferation. These effects were concentration- and time-dependent. Cellular and nuclear changes such as shrinkage, swelling of cell bodies, nuclear condensation, chromatin fragmentation, retraction, and a loss in processes were observed. Paclitaxel showed adverse effects on primary DRG non-neuronal cells, which might have adverse functional consequences on sensory neurons of the DRG, asking for consideration in the management of peripheral neuropathy.

Palmitoylethanolamide Mitigates Paclitaxel Toxicity in Primary Dorsal Root Ganglion Neurons

Chemotherapy-induced peripheral neuropathy (CIPN) is a common side effect of several chemotherapeutic agents, such as Paclitaxel. The main symptoms of CIPN are pain and numbness in the hands and feet. Paclitaxel is believed to accumulate in the dorsal root ganglia and free nerve endings. Novel therapeutic agents might help to mitigate or prevent Paclitaxel toxicity on dorsal root ganglion (DRG) neurons. Thus, we used primary DRG neurons as a model to investigate the potential neuroprotective effects of the endocannabinoid-like substance, palmitoylethanolamide (PEA). DRG neurons were isolated from cervical to sacral segments of spinal nerves of Wister rats (6-8 weeks old). After isolation and purification of neuronal cell populations, different concentrations of Paclitaxel (0.01-10 µM) or PEA (0.1-10 µM) or their combination were tested on cell viability by MTT assay at 24 h, 48, and 72 h post-treatment. Furthermore, morphometric analyses of neurite length and soma size for DRG neurons were performed. Adverse Paclitaxel effects on cell viability were apparent at 72 h post-treatment whereas Paclitaxel significantly reduced the neurite length in a concentration-dependent manner nearly at all investigated time points. However, Paclitaxel significantly increased the size of neuronal cell bodies at all time windows. These phenotypic effects were significantly reduced in neurons additionally treated with PEA, indicating the neuroprotective effect of PEA. PEA alone led to a significant increase in neuron viability regardless of PEA concentrations, apparent improvements in neurite outgrowth as well as a significant decrease in soma size of neurons at different investigated time points. Taken together, PEA showed promising protective effects against Paclitaxel-related toxicity on DRG neurons.

Elongator promotes neuritogenesis via regulation of tau stability through acly activity

The six subunits (Elp1 to Elp6) Elongator complex promotes specific uridine modifications in tRNA’s wobble site. Moreover, this complex has been indirectly involved in the regulation of α-tubulin acetylation in microtubules (MTs) via the stabilization of ATP-Citrate Lyase (Acly), the main cytosolic source of acetyl-CoA production in cells, a key substrate used for global protein acetylation. Here, we report additional evidence that Elongator activity is important for proper cytoskeleton remodeling as cells lacking expression of Elp1 show morphology impairment; including distinct neurite process formation and disorganization and instability of MTs. Here, we show that loss of Elongator results in a reduction of expression of the microtubule associated protein Tau (MAPT). Tau, is a well-known key MT regulator in neurons whose lysines can be competitively acetylated or ubiquitylated. Therefore, we tested whether Tau is an indirect acetylation target of Elongator. We found that a reduction of Elongator activity leads to a decrease of lysine acetylation on Tau that favors its proteasomal degradation. This phenotype was prevented by using selective deacetylase or proteasomal inhibitors. Moreover, our data demonstrate that Acly’s activity regulates the mechanism underlying Tau mediated neurite morphology defects found in Elp1 KD since both Tau levels and neurites morphology are restored due to Acly overexpression. This suggests a possible involvement of both Tau and Acly dysfunction in Familial Dysautonomia (FD), which is an autosomal recessive peripheral neuropathy caused by mutation in the ELP1 gene that severely affects Elp1 expression levels in the nervous system in FD patients in a similar way as found previously in Elp1 KD neuroblastoma cells.

A cryo-ET survey of microtubules and intracellular compartments in mammalian axons

The neuronal axon is packed with cytoskeletal filaments, membranes, and organelles, many of which move between the cell body and axon tip. Here, we used cryo-electron tomography to survey the internal components of mammalian sensory axons. We determined the polarity of the axonal microtubules (MTs) by combining subtomogram classification and visual inspection, finding MT plus and minus ends are structurally similar. Subtomogram averaging of globular densities in the MT lumen suggests they have a defined structure, which is surprising given they likely contain the disordered protein MAP6. We found the endoplasmic reticulum in axons is tethered to MTs through multiple short linkers. We surveyed membrane-bound cargos and describe unexpected internal features such as granules and broken membranes. In addition, we detected proteinaceous compartments, including numerous virus-like capsid particles. Our observations outline novel features of axonal cargos and MTs, providing a platform for identification of their constituents.

Tau associated peripheral and central neurodegeneration: Identification of an early imaging marker for tauopathy

Pathological hyperphosphorylated tau is a key feature of Alzheimer's disease (AD) and Frontotemporal dementia (FTD). Using transgenic mice overexpressing human non-mutated tau (htau mice), we assessed the contribution of tau to peripheral and central neurodegeneration. Indices of peripheral small and large fiber neuropathy and learning and memory performances were assessed at 3 and 6 months of age. Overexpression of human tau is associated with peripheral neuropathy at 6 months of age. Our study also provides evidence that non-mutated tau hyperphosphorylation plays a critical role in memory deficits. In addition, htau mice had reduced stromal corneal nerve length with preservation of sub-basal corneal nerves, consistent with a somatofugal degeneration. Corneal nerve degeneration occurred prior to any cognitive deficits and peripheral neuropathy. Stromal corneal nerve loss was observed in patients with FTD but not AD. Corneal confocal microscopy may be used to identify early neurodegeneration and differentiate FTD from AD.

Electrophysiological characterization of acutely isolated spiral ganglion neurons in neonatal and mature sonic hedgehog knock-in mice

Spiral ganglion neurons (SGNs) are primary afferent auditory neurons activated by inner hair cells in mammalian cochlea. Here, for the convenience of SGN studies such as patch-clamp or single cell RNA-sequence studies, a knock-in mouse (Shh^CreEGFP/+; Rosa26-Tdtomato^loxp/+) was generated for the purpose of obtaining fluorescence SGNs. Auditory brainstem response (ABR) and Tuj1 immunohistochemistry staining were performed to verify the hearing function and the morphological characteristics. The results showed that there was no significant difference between shh and wild type mice. In electrophysiological studies, we verified a series of electrophysiological characteristics including the amplitude of sodium and potassium currents and action potential characteristics of shh and wild type mice and no significant differences were found either. From the above, shh mice have the same cell function and morphology as their littermate control wild type mice and could be used as an ideal tool to study the function and characteristics of spiral ganglion neurons. Potassium channels of SGNs play an important role in resolving time accuracy. We obtained similar amplitude of I_K⁺ in neonatal and mature mice in the aging competition experiment, however, the density of I_K⁺ from mature mice were significantly different from those of neonatal mice, a phenomenon that may play a key role in the nervous system. Potassium channels have been shown to contribute to apoptosis induced by cisplatin administration in various cell lines. Here we used cisplatin administration to study the ototoxicity and found that the effects of a low dose of cisplatin (0.5 mM correspond to therapeutic doses) causes a decrease in currents and is reversible after a short administration time. Moreover, we propose the activated state of potassium channels has changed but the characteristic and number remain still after cisplatin administration. The excess potassium ions may accumulate in the cell body, which had affected the firing properties and induce cytotoxicity and apoptosis. We suggest that the electrophysiological properties of acutely isolated SGNs may support further research on the mechanics of auditory propagation and ion channel pharmacology.