HSP90 is a chaperone for DLK and is required for axon injury signaling

Abstract

Peripheral nerve injury induces a robust proregenerative program that drives axon regeneration. While many regeneration-associated genes are known, the mechanisms by which injury activates them are less well-understood. To identify such mechanisms, we performed a loss-of-function pharmacological screen in cultured adult mouse sensory neurons for proteins required to activate this program. Well-characterized inhibitors were present as injury signaling was induced but were removed before axon outgrowth to identify molecules that block induction of the program. Of 480 compounds, 35 prevented injury-induced neurite regrowth. The top hits were inhibitors to heat shock protein 90 (HSP90), a chaperone with no known role in axon injury. HSP90 inhibition blocks injury-induced activation of the proregenerative transcription factor cJun and several regeneration-associated genes. These phenotypes mimic loss of the proregenerative kinase, dual leucine zipper kinase (DLK), a critical neuronal stress sensor that drives axon degeneration, axon regeneration, and cell death. HSP90 is an atypical chaperone that promotes the stability of signaling molecules. HSP90 and DLK show two hallmarks of HSP90–client relationships: (i) HSP90 binds DLK, and (ii) HSP90 inhibition leads to rapid degradation of existing DLK protein. Moreover, HSP90 is required for DLK stability in vivo, where HSP90 inhibitor reduces DLK protein in the sciatic nerve. This phenomenon is evolutionarily conserved in Drosophila. Genetic knockdown of Drosophila HSP90, Hsp83, decreases levels of Drosophila DLK, Wallenda, and blocks Wallenda-dependent synaptic terminal overgrowth and injury signaling. Our findings support the hypothesis that HSP90 chaperones DLK and is required for DLK functions, including proregenerative axon injury signaling.

Multitasking: Dual Leucine Zipper–Bearing Kinases in Neuronal Development and Stress Management

“…However, many defects of Phr mutants remain in Phr1; Dlk double KO animals (Bloom et al 2007), which is also consistent with findings in invertebrates that PHR proteins have roles in addition to regulating DLK stability (Grill et al 2016). Besides being negatively regulated by PHR, DLK stability can be regulated by the chaperone heat shock proteins under stress and injury (Daviau et al 2006, Karney-Grobe et al 2018 (Figure 2c). Whether PHR regulates LZK remains unknown.…”

Melatonin promotes Schwann cell proliferation and migration via the shh signalling pathway after peripheral nerve injury

“…These substances ensure that there is almost no Schwann cell death after axonal injury(Murai et al., 2016). Scott et al found that heat shock protein 90 (HSP90) is required as a chaperone for DLK for signalling during axonal injury, and they observed that HSP90 inhibitors strongly promote axonal growth(Karney‐Grobe et al., 2018). The studies mentioned above indicate that the response to axonal injury plays an important role in the repair of damaged peripheral nerves by melatonin.…”

Palmitoylation Couples DLK to JNK3 to Facilitate Pro-degenerative Axon-to-Soma Signaling

“…JNK-mediated feedback phosphorylation of DLK correlates with an increase in DLK protein stability following TD (Huntwork-Rodriguez et al 2013). However, this initial report found that the change in DLK stability only became significant at prolonged times (8h) after TD, and others reported that DLK is stable in DRG neurons over this same 8h timeframe (Karney-Grobe et al 2018). These findings raised the possibility that JNK-mediated feedback phosphorylation of DLK might instead affect some other aspect of DLK-JNK signaling.…”