cJun promotes CNS axon growth

Abstract

A number of genes regulate regeneration of peripheral axons, but their ability to drive axon growth and regeneration in the central nervous system (CNS) remains largely untested. To address this question we overexpressed eight transcription factors and one small GTPase alone and in pairwise combinations to test whether combinatorial overexpression would have a synergistic impact on CNS neuron neurite growth. The Jun oncogene/signal transducer and activator of transcription 6 (JUN/STAT6) combination increased neurite growth in dissociated cortical neurons and in injured cortical slices. In injured cortical slices, JUN overexpression increased axon growth to a similar extent as JUN and STAT6 together. Interestingly, JUN overexpression was not associated with increased growth associated protein 43 (GAP43) or integrin alpha 7 (ITGA7) expression, though these are predicted transcriptional targets. This study demonstrates that JUN overexpression in cortical neurons stimulates axon growth, but does so independently of changes in expression of genes thought to be critical for JUN’s effects on axon growth. We conclude that JUN activity underlies this CNS axonal growth response, and that it is mechanistically distinct from peripheral regeneration responses, in which increases in JUN expression coincide with increases in GAP43 expression.

The tumor suppressor HHEX inhibits axon growth when prematurely expressed in developing central nervous system neurons

“…Of these, 166 TFs are expressed in the adult and/or developing cortex according to the Allen Brain Atlas (Supplementary Table 1). Intriguingly, these 166 cancer-related TFs included many genes that are already known to regulate axon growth, including 9 out of 10 factors discussed in a recent review of the transcriptional control of axon growth (ATF3, CREB, JUN, KLF4 and −6, NFKB1, SNON, the SOXC family, STAT3, and p53/TP53) (Gao et al, 2004; Qiu et al, 2005; Stegmuller et al, 2006; Gallagher et al, 2007; Seijffers et al, 2007; Jankowski et al, 2009; Moore et al, 2009; Tedeschi et al, 2009; Lerch et al, 2014; Wang et al, 2015). This high degree of overlap supports the notion of common transcriptional mechanisms that influence growth across diverse cell types, and raised the question of whether other cancer-implicated TFs on the list, many of which are unstudied in a neuronal context, might also act as regulators of axon growth.…”

“…We therefore used an established high content screening approach to quantify how cancer-related TFs affected neurite outgrowth when expressed in cortical neurons (Moore et al, 2009; Blackmore et al, 2010; Lerch et al, 2014). Sixty-nine cancer-related TFs were examined.…”

“…Each gene was tested in three independent experiments, and neurite lengths were normalized to mCherry control (Supplementary Table 1). The set of “cancer” genes included JUN, a known promoter of neurite growth, as well as KLF4 and KLF6, shown previously to inhibit and promote neurite growth, respectively (Moore et al, 2009; Blackmore et al, 2010; Lerch et al, 2014). These genes therefore functioned as positive controls.…”

“…Prominent examples include JUN, ATF3, KLF7, SOX11, STAT3, and CREB, all of which are active in neurons that mount successful axon regeneration but are missing or inactive in non-regenerating cell types (Gao et al, 2004; Qiu et al, 2005 ; Seijffers et al, 2007; Jankowski et al, 2009; Blackmore et al, 2012; Lerch et al, 2014). Accordingly, we and others have pursued forced expression of these TFs as a strategy to enhance axon growth (Qiu et al, 2005; Seijffers et al, 2007; Jankowski et al, 2009; Blackmore et al, 2012; Lerch et al, 2014; Wang et al, 2015). It is well appreciated, however, that regenerative ability can also be held in check by the presence of cell-intrinsic factors that limit growth potential.…”

Intrinsic mechanisms of neuronal axon regeneration

“…Although overexpression of ATF3 can promote peripheral nerve regeneration 109 , it fails to do so in several models of CNS injury 110,111 . Conversely, JUN overexpression can promote regeneration in cultured cortical 110,112,113 and DRG neurons 114 ; however, as described above, its activation downstream of DLK also promotes cell death in RGCs 66 . Whereas neuronal JUN is clearly necessary for peripheral axon regeneration 115 , JNK-mediated phosphorylation of JUN appears to not be required for its full function in facial nerve regeneration 116 .…”