Deleting PTEN, but not SOCS3 or myelin inhibitors, robustly boosts BRAF-elicited intraspinal regeneration of peripheral sensory axons

Abstract

Primary sensory axons fail to regenerate into the spinal cord after dorsal root (DR) injury, resulting in persistent sensory deficits. This regenerative failure occurs at the dorsal root entry zone (DREZ), the CNS-PNS interface where injured sensory axons encounter both extrinsic inhibitory cues and a limited intrinsic growth state. Although several approaches have promoted partial DR regeneration across the DREZ, sustained long-distance regeneration, particularly of large-diameter myelinated axons, remains a major challenge. We previously showed that induced expression of constitutively active B-RAF (kaBRAF) increases the regenerative competence of injured adult DRG neurons. Here, we tested whether robust intraspinal regeneration after cervical DR injury could be achieved by selective kaBRAF expression alone or in combination with removal of myelin-associated inhibitors or deletion of neuron-intrinsic growth suppressors, PTEN or SOCS3. kaBRAF promoted reproducible but limited regeneration across the DREZ and did not produce significant functional recovery by two months. Additional deletion of Nogo, MAG, and OMgp produced only a modest improvement in kaBRAF-mediated regeneration. Deletion of PTEN or SOCS3, either alone or together, failed to promote meaningful growth across the DREZ. In contrast, PTEN deletion dramatically enhanced kaBRAF-mediated regeneration, enabling many axons to penetrate the DREZ and grow deep into the spinal cord, whereas SOCS3 deletion provided little additional benefit. These findings identify combined activation of BRAF-MEK-ERK and PI3K-Akt-mTOR signaling as a powerful strategy for stimulating robust intraspinal regeneration of injured DR axons.