Roles of SMAD and SMAD-Associated Signaling Pathways in Nerve Regeneration Following Peripheral Nerve Injury: A Narrative Literature Review.

Although several methods are being applied to treat peripheral nerve injury, a perfect treatment that leads to full functional recovery has not yet been developed. SMAD (Suppressor of Mothers Against Decapentaplegic Homolog) plays a crucial role in nerve regeneration by facilitating the survival and growth of nerve cells following peripheral nerve injury. We conducted a systematic literature review on the role of SMAD in this context. Following peripheral nerve injury, there was an increase in the expression of SMAD1, -2, -4, -5, and -8, while SMAD5, -6, and -7 showed no significant changes; SMAD8 expression was decreased. Specifically, SMAD1 and SMAD4 were found to promote nerve regeneration, whereas SMAD2 and SMAD6 inhibited it. SMAD exerts its effects by promoting neuronal survival and growth through BMP/SMAD1, BMP/SMAD4, and BMP/SMAD7 signaling pathways. Furthermore, it activates nerve regeneration programs via the PI3K/GSK3/SMAD1 pathway, facilitating active regeneration of nerve cells and subsequent functional recovery after peripheral nerve damage. By leveraging these mechanisms of SMAD, novel strategies for treating peripheral nerve damage could potentially be developed. We aim to further elucidate the precise mechanisms of nerve regeneration mediated by SMAD and explore the potential for developing targeted nerve treatments based on these findings.