Oxidative stress induces overgrowth of theDrosophilaneuromuscular junction

Abstract

Synaptic terminals are known to expand and contract throughout an animal's life. The physiological constraints and demands that regulate appropriate synaptic growth and connectivity are currently poorly understood. In previous work, we identified a Drosophila model of lysosomal storage disease (LSD), spinster (spin), with larval neuromuscular synapse overgrowth. Here we identify a reactive oxygen species (ROS) burden in spin that may be attributable to previously identified lipofuscin deposition and lysosomal dysfunction, a cellular hallmark of LSD. Reducing ROS in spin mutants rescues synaptic overgrowth and electrophysiological deficits. Synapse overgrowth was also observed in mutants defective for protection from ROS and animals subjected to excessive ROS. ROS are known to stimulate JNK and fos signaling. Furthermore, JNK and fos in turn are known potent activators of synapse growth and function. Inhibiting JNK and fos activity in spin rescues synapse overgrowth and electrophysiological deficits. Similarly, inhibiting JNK, fos, and jun activity in animals with excessive oxidative stress rescues the overgrowth phenotype. These data suggest that ROS, via activation of the JNK signaling pathway, are a major regulator of synapse overgrowth. In LSD, increased autophagy contributes to lysosomal storage and, presumably, elevated levels of oxidative stress. In support of this suggestion, we report here that impaired autophagy function reverses synaptic overgrowth in spin. Our data describe a previously unexplored link between oxidative stress and synapse overgrowth via the JNK signaling pathway.

Autophagy in synaptic development, function, and pathology

“…ROS not only leads to apoptotic cell death, but also regulates synaptic growth and function [28,88] . Autophagy is the main cellular response to OS burden.…”

“…The activation of JNK/AP-1 regulates synaptic development under oxidative stress by activating autophagy [25,27,28] .…”

“…Autophagy is the main cellular response to OS burden. In a Drosophila model of lysosomal storage disease, spinster (spin), OS induces synaptic overgrowth [28] . Autophagy-related genes, such as atg1 and atg18, are required for OS burden-triggered synaptic overgrowth in this model, and disturbance of autophagy is able to reverse synaptic overgrowth [28] .…”

“…In a Drosophila model of lysosomal storage disease, spinster (spin), OS induces synaptic overgrowth [28] . Autophagy-related genes, such as atg1 and atg18, are required for OS burden-triggered synaptic overgrowth in this model, and disturbance of autophagy is able to reverse synaptic overgrowth [28] . Therefore, it is hypothesized that upon OS burden, the overproduction of ROS may activate autophagy which plays a key role in mediating synaptic growth, function, and senescence [25] .…”

Dendritic growth gated by a steroid hormone receptor underlies increases in activity in the developing Drosophila locomotor system

“…At the Drosophila neuromuscular junction, two principal adjustment mechanisms are thought to compensate for size-related changes in muscle physiology. First, the neuromuscular junction grows proportionately with the postsynaptic muscle (8), regulated by the balance of factors that promote and constrain presynaptic terminal growth (13)(14)(15)(16)(17)(18)(19)(20). Second, synaptic strength is enhanced by increasing the number of release sites per presynaptic bouton (21) and glutamate receptor-containing densities at postsynaptic specializations (22).…”