Role of Piezo2 in Schwann Cell Volume Regulation and Its Impact on Neurotrophic Release Regulation

Background and Aims
Tactile perception depends on mechanoreceptors and various types of nerve fibers, including C-fibers, Aβ-fibers, and Aδ-fibers. Schwann cells (SCs) are essential in supporting these nerve fibers: non-myelinating SCs envelop C-fibers, while myelinating SCs surround Aβ- and Aδ-fibers. Recent studies have revealed additional roles for cutaneous sensory SCs, identifying nociceptive SCs as contributors to pain perception and Meissner corpuscle SCs as important for touch sensation. Notably, Piezo2—previously linked to tactile responses in Merkel cells—has also been detected in SCs. This study aimed to explore the mechanosensitive channels involved in SC function and to examine how these channels influence the release of neurotrophic factors.

Methods
Two Schwann cell subtypes—undifferentiated and differentiated—were generated from immortalized IFRS1 cells and human primary SCs. Quantitative PCR was used to assess the expression of differentiation markers and mechanosensitive channels, including TRP channels (TRPV4, TRPM7, TRPA1) and Piezo channels (Piezo1 and Piezo2). To determine the functional activity of these channels, calcium imaging and electronic cell sizing were performed under hypotonic conditions. Inhibitors and siRNAs were applied to further clarify channel involvement. Protein expression levels were evaluated by Western blot and immunostaining. Additionally, secretome analysis was carried out to investigate the release of neurotrophic factors under hypotonic stress, with BDNF (brain-derived neurotrophic factor) measured by ELISA as a representative marker.

Results
Differentiation led to increased Piezo2 mRNA expression in both IFRS1 and human primary SCs. Both undifferentiated and differentiated cells responded to hypotonic stress, though differentiated SCs exhibited a more substantial response. The calcium transients induced by hypotonicity in differentiated SCs were effectively suppressed by Gd³⁺ and FM1-43, suggesting the involvement of Piezo2 channels. In contrast, inhibition of Piezo1 and TRPM7 with Dooku1 and NS8593 had no significant effect. Piezo2 also appeared to contribute to regulatory volume decrease (RVD) in differentiated SCs following hypotonic-induced cell swelling. When Piezo2 was downregulated, RVD was impaired, resulting in sustained cell swelling and an increased release of BDNF. This effect was likely due to compensatory upregulation of TRPV4, a calcium-permeable channel endogenously expressed in SCs.

Conclusion
This study identifies Piezo2 as a key mechanosensitive channel in Schwann cells, involved in both cellular volume regulation and the release of neurotrophic factors. Piezo2 appears to facilitate RVD, supporting cellular homeostasis, and may act as a negative regulator of neurotrophic factor secretion. These findings highlight a novel functional role for Piezo2 in SC physiology and point to the need for further research into its regulatory mechanisms in neurotrophic signaling.