The Unfolded Protein Response and Cholesterol Biosynthesis Link Luman/CREB3 to Regenerative Axon Growth in Sensory Neurons.

We recently revealed that the axon endoplasmic reticulum resident transcription factor Luman/CREB3 (herein called Luman) serves as a unique retrograde injury signal in regulation of the intrinsic elongating form of sensory axon regeneration. Here, evidence supports that Luman contributes to axonal regeneration through regulation of the unfolded protein response (UPR) and cholesterol biosynthesis in adult rat sensory neurons. One day sciatic nerve crush injury triggered a robust increase in UPR-associated mRNA and protein expression in both neuronal cell bodies and the injured axons. Knockdown of Luman expression in 1 d injury-conditioned neurons by siRNA attenuated axonal outgrowth to 48% of control injured neurons and was concomitant with reduced UPR- and cholesterol biosynthesis-associated gene expression. UPR PCR-array analysis coupled with qRT-PCR identified and confirmed that four transcripts involved in cholesterol regulation were downregulated >2-fold by the Luman siRNA treatment of the injury-conditioned neurons. Further, the Luman siRNA-attenuated outgrowth could be significantly rescued by either cholesterol supplementation or 2 ng/ml of the UPR inducer tunicamycin, an amount determined to elevate the depressed UPR gene expression to a level equivalent of that observed with crush injury. Using these approaches, outgrowth increased significantly to 74% or 69% that of injury-conditioned controls, respectively. The identification of Luman as a regulator of the injury-induced UPR and cholesterol at levels that benefit the intrinsic ability of axotomized adult rat sensory neurons to undergo axonal regeneration reveals new therapeutic targets to bolster nerve repair.