A growing body of studies indicate that small noncoding RNAs, especially microRNAs (miRNA), play a crucial role in response to peripheral nerve injuries. During Wallerian degeneration and regeneration processes, they orchestrate several pathways, in particular the MAPK, AKT, and EGR2 (KROX20) pathways. Certain miRNAs show specific expression profiles upon a nerve lesion correlating with the subsequent nerve regeneration stages such as dedifferentiation and with migration of Schwann cells, uptake of debris, neurite outgrowth and finally remyelination of regenerated axons. This review highlights (a) the specific expression profiles of miRNAs upon a nerve lesion and (b) how miRNAs regulate nerve regeneration by acting on distinct pathways and linked proteins. Shedding light on the role of miRNAs associated with peripheral nerve regeneration will help researchers to better understand the molecular mechanisms and deliver targets for precision medicine.

The search for a suitable material to promote regeneration after long‐distance peripheral nerve defects turned the spotlight on spider silk. Nerve conduits enriched with native spider silk fibers as internal guiding structures previously demonstrated a regenerative outcome similar to autologous nerve grafts in animal studies. Nevertheless, spider silk is a natural material with associated limitations for clinical use. A promising alternative is the production of recombinant silk fibers that should mimic the outstanding properties of their native counterpart. However, in vitro data on the regenerative features that native silk fibers provide for cells involved in nerve regeneration are scarce. Thus, there is a lack of reference parameters to evaluate whether recombinant silk fiber candidates will be eligible for nerve repair in vivo. To gain insight into the regenerative effect of native spider silk, our study aims to define the behavioral response of primary Schwann cells (SCs), nerve‐associated fibroblasts (FBs), and dorsal root ganglion (DRG) neurons cultured on native dragline silk from the genus Nephila and on laminin coated dishes. The established multi‐color immunostaining panels together with confocal microscopy and live cell imaging enabled the analysis of cell identity, morphology, proliferation, and migration on both substrates in detail. Our findings demonstrated that native spider silk rivals laminin coating as it allowed attachment and proliferation and supported the characteristic behavior of all tested cell types. Axonal out‐growth of DRG neurons occurred along longitudinally aligned SCs that formed sustained bundled structures resembling Bungner bands present in regenerating nerves. The migration of SCs along the silk fibers achieved the reported distance of regenerating axons of about 1 mm per day, but lacked directionality. Furthermore, rFBs significantly reduced the velocity of rSCs in co‐cultures on silk fibers. In summary, this study (a) reveals features recombinant silk must possess and what modifications or combinations could be useful for enhanced nerve repair and (b) provides assays to evaluate the regenerative performance of silk fibers in vitro before being applied as internal guiding structure in nerve conduits in vivo.

The successful reconstruction of supercritical peripheral nerve injuries remains a major challenge in modern medicine. Progress in tissue engineering has enabled the development of nerve guidance conduits as an alternative to autologous nerve transplantation and the enrichment of conduits with fibrous materials or hydrogels has shown great potential in bridging nerve defects. The application of the dragline silk of spider genus Nephila as a filament for nerve guidance conduits has led to promising results. However, the use of spider silk has been phenomenological so far and the reasons for its success are still not identified. This renders a targeted tuning of synthetic fibrous luminal fillings such as recombinant silk out of reach. In this work the existing research was extended and in addition to dragline, the cocoon silk of Nephila edulis, as well as the connecting and attaching silk of Avicularia avicularia were investigated. Scanning electron microscopy revealed a difference in size and morphology of the spider silks. However, in vitro experiments indicated that Schwann cells adhere to the four fibers, independent of these two attributes. Raman spectroscopy in native state and aqueous environment demonstrated similar secondary protein structures for dragline, cocoon, and connecting silk. In contrast, the attaching silk showed a significant lower conformation of β-sheets, crucial for the stiffness of the silk. This was in line with the in vitro experiments, where the flexible attaching silk fibers adhered to each other when placed in liquid. This resulted in their inability to guide Schwann cells, leading to the generation of cell agglomerations. This direct comparison demonstrated the crucial role of β-sheets conformation for the guidance properties of natural spider silk, providing essential insights into the necessary material properties for the integration of fibrous luminal fillings in nerve guidance conduits.

Dragline silk of Trichonephila spiders has attracted attention in various applications. One of the most fascinating uses of dragline silk is in nerve regeneration as a luminal filling for nerve guidance conduits. In fact, conduits filled with spider silk can measure up to autologous nerve transplantation, but the reasons behind the success of silk fibers are not yet understood. In this study dragline fibers of Trichonephila edulis were sterilized with ethanol, UV radiation, and autoclaving and the resulting material properties were characterized with regard to the silk's suitability for nerve regeneration. Rat Schwann cells (rSCs) were seeded on these silks in vitro and their migration and proliferation were investigated as an indication for the fiber's ability to support the growth of nerves. It was found that rSCs migrate faster on ethanol treated fibers. To elucidate the reasons behind this behavior, the fiber's morphology, surface chemistry, secondary protein structure, crystallinity, and mechanical properties were studied. The results demonstrate that the synergy of dragline silk's stiffness and its composition has a crucial effect on the migration of rSCs. These findings pave the way towards understanding the response of SCs to silk fibers as well as the targeted production of synthetic alternatives for regenerative medicine applications.

Hydrogels have shown potential in replacing damaged nerve tissue, but the ideal hydrogel is yet to be found. In this study, various commercially available hydrogels were compared. Schwann cells, fibroblasts, and dorsal root ganglia neurons were seeded on the hydrogels, and their morphology, viability, proliferation, and migration were examined. Additionally, detailed analyses of the gels' rheological properties and topography were conducted. Our results demonstrate vast differences on cell elongation and directed migration on the hydrogels. Laminin was identified as the driver behind cell elongation and in combination with a porous, fibrous, and strain-stiffening matrix structure responsible for oriented cell motility. This study improves our understanding of cell-matrix interactions and thereby facilitates tailored fabrication of hydrogels in the future.