KLF9 and JNK3 Interact to Suppress Axon Regeneration in the Adult CNS

Abstract

Neurons in the adult mammalian CNS decrease in intrinsic axon growth capacity during development in concert with changes in Krüppel-like transcription factors (KLFs). KLFs regulate axon growth in CNS neurons including retinal ganglion cells (RGCs). Here, we found that knock-down of KLF9, an axon growth suppressor that is normally upregulated 250-fold in RGC development, promotes long-distance optic nerve regeneration in adult rats of both sexes. We identified a novel binding partner, MAPK10/JNK3 kinase, and found that JNK3 (c-Jun N-terminal kinase 3) is critical for KLF9's axon-growth-suppressive activity. Interfering with a JNK3-binding domain or mutating two newly discovered serine phosphorylation acceptor sites, Ser106 and Ser110, effectively abolished KLF9's neurite growth suppression and promoted axon regeneration These findings demonstrate a novel, physiologic role for the interaction of KLF9 and JNK3 in regenerative failure in the optic nerve and suggest new therapeutic strategies to promote axon regeneration in the adult CNS. Injured CNS nerves fail to regenerate spontaneously. Promoting intrinsic axon growth capacity has been a major challenge in the field. Here, we demonstrate that knocking down Krüppel-like transcription factor 9 (KLF9) via shRNA promotes long-distance axon regeneration after optic nerve injury and uncover a novel and important KLF9-JNK3 interaction that contributes to axon growth suppression and regenerative failure These studies suggest potential therapeutic approaches to promote axon regeneration in injury and other degenerative diseases in the adult CNS.

Intrinsic mechanisms of neuronal axon regeneration

“…For example, overexpression of KLF4, the expression of which increases during development in RGCs, potently inhibits axon regeneration in RGCs, whereas its deletion promotes axon regeneration in vivo after optic nerve injury 131 . Similarly, deletion of KLF9, the expression of which also increases during development, promotes long-distance optic nerve regeneration in a JNK3-dependent manner 134 . Conversely, both KLF6 (REFS 131,135 ) and KLF7 (REFS 131,132 ), which are developmentally downregulated, promote growth when overexpressed in CNS neurons in both mammals and zebrafish 136 (TABLE 1).…”

The Role of Axon Transport in Neuroprotection and Regeneration

“…Identifying genes or pathways whose pattern of expression in immature, highly regenerative CNS neurons are drastically altered after maturation may lead to candidate regulators of intrinsic growth potential. In the case of KLFs, where developmental upregulation of KLF9 and KLF4 and downregulation of KLF6 and KLF7 are coincident with the reduction of intrinsic regenerative capacity of RGCs, reversing these expression patterns in adult RGCs or in other CNS neuron pathways after injury allows sprouting or long‐distance regeneration (Moore et al, ; Blackmore et al, ; Apara et al, ; Wang et al, ). Exploring the molecular mechanisms of the KLFs further, identification of co‐factors such as JNK3 and STAT3, and downstream targets such as serotonin receptors and dual‐specificity phosphatase 14 (DUSP14) (Qin et al, ; Apara et al, ; Trakhtenberg et al, ; Galvao et al, ) have led to broader understanding of the biology of intrinsic capacity for axon growth.…”

Construction and reconstruction of brain circuits: normal and pathological axon guidance

“…The transcription factors that are expressed after injury appear to determine whether a specific sub‐type of RGC will regrow. The Kruppel‐like factors 4 and 9 (KLF4 and KLF9), for example, play a major role suppressing axon development (Qin et al ; Apara et al ). KLF4 interacts with Tyr705‐phosphorylated signal transducer and activator of transcription 3 (STAT3) suppressing its activity and function as an intrinsic barrier for regeneration of damaged adult RGC axons (Qin et al ).…”

“…KLF4 interacts with Tyr705‐phosphorylated signal transducer and activator of transcription 3 (STAT3) suppressing its activity and function as an intrinsic barrier for regeneration of damaged adult RGC axons (Qin et al ). KLF9 functions as another intrinsic inhibitor for axon regeneration as shRNA mediated knockdown of KLF9 promote RGC survival and axon regeneration following optic nerve injury in vivo (Apara et al ). This KLF9‐mediated inhibition is via interaction of the upstream kinase c‐Jun N‐terminal kinase 3 (JNK3) (Apara et al ).…”

“…KLF9 functions as another intrinsic inhibitor for axon regeneration as shRNA mediated knockdown of KLF9 promote RGC survival and axon regeneration following optic nerve injury in vivo (Apara et al ). This KLF9‐mediated inhibition is via interaction of the upstream kinase c‐Jun N‐terminal kinase 3 (JNK3) (Apara et al ). Recent findings indicate that micro RNAs 135a and 135b (miRNA135s) could regulate KLF4 expression during axon development.…”