Axon injury and stress trigger a microtubule-based neuroprotective pathway

Abstract

Axon injury elicits profound cellular changes, including axon regeneration. However, the full range of neuronal injury responses remains to be elucidated. Surprisingly, after axons of Drosophila dendritic arborization neurons were severed, dendrites were more resistant to injury-induced degeneration. Concomitant with stabilization, microtubule dynamics in dendrites increased. Moreover, dendrite stabilization was suppressed when microtubule dynamics was dampened, which was achieved by lowering levels of the microtubule nucleation protein γ-tubulin. Increased microtubule dynamics and global neuronal stabilization were also activated by expression of expanded polyglutamine (poly-Q) proteins SCA1, SCA3, and huntingtin. In all cases, dynamics were increased through microtubule nucleation and depended on JNK signaling, indicating that acute axon injury and long-term neuronal stress activate a common cytoskeleton-based stabilization program. Reducing levels of γ-tubulin exacerbated long-term degeneration induced by SCA3 in branched sensory neurons and in a well established Drosophila eye model of poly-Q-induced neurodegeneration. Thus, increased microtubule dynamics can delay short-term injury-induced degeneration, and, in the case of poly-Q proteins, can counteract progressive longer-term degeneration. We conclude that axon injury or stress triggers a microtubulebased neuroprotective pathway that stabilizes neurons against degeneration.M any animals generate a single set of neurons that must function for the entire life of the individual. Each neuron typically has a single axon that transmits signals to other neurons or output cells such as muscle. As axons can extend long distances, they are at risk for injury, and, if the single axon is damaged, the cell can no longer function. Many neurons thus mount major responses to axon injury. The best characterized of these responses is axon regeneration, the process in which a neuron extends the stump of the existing axon or grows a new axon from a dendrite (1-3).In addition to the regenerative response, axon injury can cause other less well-understood changes. For example, in mammalian dorsal root ganglion cells, injury of the peripheral axon causes a transcriptional response that increases the capacity of the central axon to regenerate if it is subsequently injured (4, 5). In Drosophila sensory neurons, axon injury causes cytoskeletal changes in the entire dendrite arbor, specifically the number of growing microtubules is up-regulated (6). In this study, we investigated the functional significance of the cytoskeletal changes in the dendrite arbor. We present results that suggest the altered microtubule dynamics in dendrites acts to stabilize them, and thus axon injury seems to trigger a neuroprotective pathway that acts on the rest of the cell. However, this neuroprotective pathway is turned on only transiently after axon injury and subsides as axon regeneration initiates.Axon injury is a very acute neuronal stress. Neurons are also subject to a variety of long-term stress...

The microtubule-severing protein fidgetin acts after dendrite injury to promote their degeneration

“…This longer term (6-48-h time range) global increase in microtubule dynamics after axon injury is also observed in mouse neurons (Kleele et al, 2014). We have been able to show in Drosophila that these global increases in microtubule dynamics on the cell body side of the axon injury are mediated by microtubule nucleation and not severing (Chen et al, 2012). In addition to being mechanistically distinct from the increase in dynamics associated with degeneration here, this nucleation-induced increase in dynamics acts to stabilize the injured neuron (Chen et al, 2012) rather than as part of the disassembly process.…”

“…We have been able to show in Drosophila that these global increases in microtubule dynamics on the cell body side of the axon injury are mediated by microtubule nucleation and not severing (Chen et al, 2012). In addition to being mechanistically distinct from the increase in dynamics associated with degeneration here, this nucleation-induced increase in dynamics acts to stabilize the injured neuron (Chen et al, 2012) rather than as part of the disassembly process. Thus, increases in the number of microtubule plus ends after injury can be part of either stabilizing or destabilizing pathways, depending on the mechanism in which they are induced and probably also on the other microtubule regulators, for example depolymerases, active at the same time.…”

FoxO regulates microtubule dynamics and polarity to promote dendrite branching in Drosophila sensory neurons

“…For overexpression of FoxO, we used UAS-FoxO WT (listed as FoxO WT #1; a gift from Robert Tjian [University of California, Berkeley, Berkeley, CA, US]) in class I and II and UAS-FoxO WT f19-5 (listed as FoxO WT #2; a gift from Marc Tatar [Brown University, Providence, RI, US]) in class IV. For control RNAi we used Vienna Drosophila RNAi Center lines 25271 (gamma-tubulin 37C RNAi, listed as control RNAi #1) and 33320 (Rtnl2 RNAi, listed as control RNAi #2) (Chen et al, 2012; Hill et al, 2012). For FoxO/Futsch interaction, we used Oregon R, futsch k68 ; a gift from Christian Klämbt (University of Muenster, Muenster, Germany; Hummel et al, 2000), futsch k68 ;; foxO Δ 94 , and ppk-CD4-GFP (BDSC 35842).…”

Bidirectional cargo transport: moving beyond tug of war

“…C-Jun N-terminal Kinases (JNKs) and JNK-interacting proteins (JIPs) are important regulators of bidirectional transport in neurons, with JIPs acting as scaffolding proteins that link motors to cargo and JNK phosphorylation leading to some combination of reduced cargo binding, reduced activity, and diminished mechanical performance of kinesin-1 motors 16, 31, 156–159 . JNK kinases are also thought to enhance microtubule dynamics and promote microtubule stability, providing another avenue for altering transport 160–163 .…”