An axon-intrinsic loop restricts nerve regeneration through axonal protein synthesis
Injured axons synthesize the RNA Binding Protein KHSRP that promotes mRNA decay and slows nerve regeneration. Axotomy-induced increase in axoplasmic Ca2+ activates axonal Khsrp translation, and while Ca2+ returns to pre-injury levels within 16 hours post-axotomy, axonal KHSRP remains elevated. Alternating translation of Reg3a and Khsrp sustains axonal KHSRP levels in regenerating axons. Nerve injury activates Reg3a expression, resulting in increased REG3A synthesis and secretion from axons. REG3A stimulates ER Ca2+ release to activate PERK, increase eIF2 phosphorylation, and increase Khsrp translation. Axoplasmic Ca2+ slowly oscillates in growth cones and Reg3A depletion attenuates growth cone Ca2+ oscillations, decreases KHSRP synthesis, reduces the axons retractive events, and accelerates peripheral nerve regeneration. Thus, REG3A to KHSRP signaling provides an axon-intrinsic loop that decelerates axon growth through localized mRNA translation.