A Novel In Vitro Assay Using Human iPSC-Derived Sensory Neurons to Evaluate the Effects of External Chemicals on Neuronal Morphology: Possible Implications in the Prediction of Abnormal Skin Sensation

Abstract

Neuronal morphological changes in the epidermis are considered to be one of causes of abnormal skin sensations in dry skin-based skin diseases. The present study aimed to develop an in vitro model optimised for human skin to test the external factors that lead to its exacerbation. Human-induced pluripotent stem cell-derived sensory neurons (hiPSC-SNs) were used as a model of human sensory neurons. The effects of chemical substances on these neurons were evaluated by observing the elongation of nerve fibers, incidence of blebs (bead-like swellings), and the expression of nicotinamide mononucleotide adenylyl transferase 2 (NMNAT2). The nerve fiber length increased upon exposure to two common cosmetic preservatives—methylparaben and phenoxyethanol—but not to benzo[a]pyrene, an air pollutant at the estimated concentrations in the epidermis. Furthermore, the incidence of blebs increased upon exposure to benzo[a]pyrene. However, there was a decrease in the expression of NMNAT2 in nerve fibers, suggesting degenerative changes. No such degeneration was found after methylparaben or phenoxyethanol at the estimated concentrations in the epidermis. These findings suggest that methylparaben and phenoxyethanol promote nerve elongation in hiPSC-SNs, whereas benzo[a]pyrene induces nerve degeneration. Such alterations may be at least partly involved in the onset and progression of sensitive skin.

In vitro models for investigating itch

“…With special focus on itch receptors, a similar, shortened approach (19 days from NCCs to SNs) was insufficient to generate a full DRG-like phenotype but still allowed for small reactions to capsaicin, AITC (TRPA1 agonist), IL-31, IL-4, and BAM8-22 ( Umehara et al, 2020 ). With even less differentiated sensory neurons, another group have claimed to be able to test skin sensitizing substances, such as the irritant methylparaben (a possible TRPA1 agonist) by analyzing neuronal outgrowth and blebbing ( Satoh et al, 2021 ). In addition to sensory neurons, more types of nerve cells are needed for signal transmission to the CNS, such as spinal cord interneurons, that can also be generated from iPSCs, providing a chance for downstream itch investigation and co-culture systems building the bridge to the CNS ( Gupta et al, 2018 ).…”