Date Published: May 29, 2018
Publisher: Public Library of Science
Author(s): Diana Luong, Luselena Perez, Jennifer C. Jemc, Amit Singh.
Glial cells perform numerous functions to support neuron development and function, including axon wrapping, formation of the blood brain barrier, and enhancement of synaptic transmission. We have identified a novel gene, raw, which functions in glia of the central and peripheral nervous systems in Drosophila. Reducing Raw levels in glia results in morphological defects in the brain and ventral nerve cord, as well as defects in neuron function, as revealed by decreased locomotion in crawling assays. Examination of the number of glia along peripheral nerves reveals a reduction in glial number upon raw knockdown. The reduced number of glia along peripheral nerves occurs as a result of decreased glial proliferation. As Raw has been shown to negatively regulate Jun N-terminal kinase (JNK) signaling in other developmental contexts, we examined the expression of a JNK reporter and the downstream JNK target, matrix metalloproteinase 1 (mmp1), and found that raw knockdown results in increased reporter activity and Mmp1 levels. These results are consistent with previous studies showing increased Mmp levels lead to nerve cord defects similar to those observed upon raw knockdown. In addition, knockdown of puckered, a negative feedback regulator of JNK signaling, also causes a decrease in glial number. Thus, our studies have resulted in the identification of a new regulator of gliogenesis, and demonstrate that increased JNK signaling negatively impacts glial development.
The precise interaction between neurons and glia is essential for the establishment and maintenance of nervous system structure and function. A failure of neurons and glia to interact properly or a breakdown in interactions can result in neuropathy or neurodegeneration [1–3]. Neurons play a critical role in sensing environmental stimuli and conveying responses to these stimuli. However, their function depends on their close association with different types of glia. Glia perform a variety of roles in the nervous system, from regulating blood brain barrier (BBB) formation and synapse structure to insulating neurons, clearing debris, and providing trophic support (reviewed in ).
Our studies have resulted in the identification of Raw as a regulator of glial development. We show that reducing raw levels in glia affects the morphology of the CNS, resulting in a smaller brain and an elongated VNC. In addition, examination of glial cell number along the peripheral nerves revealed that raw knockdown led to a reduction in the number of glia along the A8/9 peripheral nerves, which is due in part to reduced glial proliferation. Previous studies demonstrated that Raw functions as a negative regulator of JNK signaling in other developmental contexts. Therefore, we examined the expression of a JNK reporter and Mmp1, a downstream target of JNK signaling. We observed increased reporter expression and Mmp1 levels in the VNC and increased reporter expression and Mmp1-positive punctae along peripheral nerves upon raw knockdown in the presence of overexpressed dcr2. Supporting the hypothesis that increased JNK signaling can lead to defects in glial development, knockdown of puc, a negative feedback regulator of the JNK pathway, also resulted in a decrease in the number of glia along peripheral nerves. Finally, larvae with reduced raw levels had reduced crawling ability, suggesting that reduced glial number negatively impacts motor neuron function. These results suggest that Raw functions as a negative regulator of JNK signaling in the context of glial development, thereby affecting glial number and leading to morphological defects that impair nervous system function.