Fibroblasts Colonizing Nerve Conduits Express High Levels of Soluble Neuregulin1, a Factor Promoting Schwann Cell Dedifferentiation

Abstract

Conduits for the repair of peripheral nerve gaps are a good alternative to autografts as they provide a protected environment and a physical guide for axonal re-growth. Conduits require colonization by cells involved in nerve regeneration (Schwann cells, fibroblasts, endothelial cells, macrophages) while in the autograft many cells are resident and just need to be activated. Since it is known that soluble Neuregulin1 (sNRG1) is released after injury and plays an important role activating Schwann cell dedifferentiation, its expression level was investigated in early regeneration steps (7, 14, 28 days) inside a 10 mm chitosan conduit used to repair median nerve gaps in Wistar rats. In vivo data show that sNRG1, mainly the isoform α, is highly expressed in the conduit, together with a fibroblast marker, while Schwann cell markers, including NRG1 receptors, were not. Primary culture analysis shows that nerve fibroblasts, unlike Schwann cells, express high NRG1α levels, while both express NRG1β. These data suggest that sNRG1 might be mainly expressed by fibroblasts colonizing nerve conduit before Schwann cells. Immunohistochemistry analysis confirmed NRG1 and fibroblast marker co-localization. These results suggest that fibroblasts, releasing sNRG1, might promote Schwann cell dedifferentiation to a “repair” phenotype, contributing to peripheral nerve regeneration.

Blood Vessels: The Pathway Used by Schwann Cells to Colonize Nerve Conduits

“…In a previous study [11], we detected an elevated presence of blood vessels inside the chitosan conduit at 7 days after repair, in contrast with the autograft, where blood vessels did not fill the autologous graft. This observation led us to investigate whether Schwann cells could use blood vessels inside the chitosan conduit as a track for their migration, as occurs in the nerve bridge [6].…”

“…In general, Schwann cell migration is not only necessary for axonal pathfinding in the nerve bridge, but it is one of the intrinsic steps required for nerve regeneration for all nerve injuries, regardless of the type of damage and repair [8,9]. Different factors might be involved in Schwann cell migration and proliferation after an injury, among them, the soluble isoform of Neuregulin 1 (NRG1) [10], a factor expressed not only by Schwann cells, but also by rat and human nerve fibroblasts [11,12], which also express growth arrest-specific gene 6 (GAS6), a mitogenic factor for Schwann cells [13], and fibroblast growth factor 5 (FGF5) [12], a factor regulating Schwann cell adhesion and migration [14].…”

Modified Hyaluronic Acid-Laminin-Hydrogel as Luminal Filler for Clinically Approved Hollow Nerve Guides in a Rat Critical Defect Size Model

“…In view of the fact that optimized axonal regeneration crucially depends on the invasion of the bioartificial grafts by e.g., repair Schwann cells [ 18 , 19 , 26 , 30 ], perineurial fibroblasts [ 36 ], or pro-regenerative cells from the immune system [ 14 , 37 , 38 ], it is also important to consider the interaction of the provided bioartificial graft with regeneration supporting cells types. It was recently demonstrated that collagen-based nerve guides with an optimized internal 3D structure could be enriched with autologous Schwann cells for improving regeneration of an acutely repaired 13 mm sciatic nerve defect in the rat [ 39 ].…”

Gli1 regulates the postnatal acquisition of peripheral nerve architecture

“…Finally, there is a rich array of paracrine signals released by EFs, whose expression is affected by nerve injury (Toma et al, 2020), that may likewise be altered in the Gli1 nulls impacting SCs. A recent study identified EFs in regenerating nerves as a source of soluble neuregulin (Fornasari et al, 2020), an important signal that promotes the SC repair phenotype essential for nerve regeneration (Stassart et al, 2013). Future studies utilizing RNA-seq and proteomic analyses to elucidate further the nature of the peripheral nerve signals that impinge on SCs will be of considerable interest.…”