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

Abstract

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 biosynthesisassociated gene expression. UPR PCR-array analysis coupled with qRT-PCR identified and confirmed that four transcripts involved in cholesterol regulation were downregulated Ͼ2-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.

Dual leucine zipper kinase-dependent PERK activation contributes to neuronal degeneration following insult

“…Further analysis comparing nerve injury-induced mRNAs to genes known to be upregulated in an ATF4-dependent manner after neuronal oxidative stress (Lange et al, 2008) revealed that the expression changes observed in this ISR-related gene set is greater than would be expected by chance in both axonal injury models (Figure 1c,d). Together these observations suggested the hypothesis that the acute neuronal stress response may include activation of PERK signaling in multiple settings, consistent with a report that nerve crush leads to PERK phosphorylation (Ying et al, 2015).

10.7554/eLife.20725.002Figure 1.Acute neuronal insults activate the integrated stress response (ISR).( a ) Schematic of sciatic nerve crush (SNC) and optic nerve crush (ONC).

…”

Intrinsic mechanisms of neuronal axon regeneration

“…A potential area of interest will be to determine whether the endoplasmic reticulum stress pathway and the miRNA biogenesis pathway, which are known to regulate each other, do so in the context of axon regeneration 184,185 . This is possible given that the endoplasmic reticulum stress response promotes regeneration of sensory neurons 91,186 and survival of RGCs 187 .…”

Models of axon regeneration in Drosophila

“…However, this pathway may play a role in mammalian axon regeneration since retinal ganglion cells from Rtca mutant mice have a substantial increase in the number of regenerating axons (Song et al, 2015). Additionally, the UPR is activated by sciatic nerve crush in adult rat sensory neurons with enhanced expression of the core components CHOP and XBP1 (Ying et al, 2015). Finally, addition of tunicamycin, which induces the UPR, can rescue the decrease in axon regeneration seen in mutants of the ER-resident transcription factor Luman/Creb3 (Ying et al, 2015).…”

“…Additionally, the UPR is activated by sciatic nerve crush in adult rat sensory neurons with enhanced expression of the core components CHOP and XBP1 (Ying et al, 2015). Finally, addition of tunicamycin, which induces the UPR, can rescue the decrease in axon regeneration seen in mutants of the ER-resident transcription factor Luman/Creb3 (Ying et al, 2015). Hence, the RNA ligase complex may promote axon regeneration by facilitating the capacity of the UPR to promote axonal regeneration.…”