Identification of raw as a regulator of glial development

Opposing roles of Fos, Raw, and SARM1 in the regulation of axonal degeneration and synaptic structure

Introduction

The degeneration of injured axons is driven by conserved molecules, including the sterile armadillo TIR domain-containing protein SARM1, the cJun N-terminal kinase JNK, and regulators of these proteins. These molecules are also implicated in the regulation of synapse development though the mechanistic relationship of their functions in degeneration vs. development is poorly understood.

Results and discussion

Here, we uncover disparate functional relationships between SARM1 and the transmembrane protein Raw in the regulation of Wallerian degeneration and synaptic growth in motoneurons of Drosophila melanogaster. Our genetic data suggest that Raw antagonizes the downstream output MAP kinase signaling mediated by Drosophila SARM1 (dSarm). This relationship is revealed by dramatic synaptic overgrowth phenotypes at the larval neuromuscular junction when motoneurons are depleted for Raw or overexpress dSarm. While Raw antagonizes the downstream output of dSarm to regulate synaptic growth, it shows an opposite functional relationship with dSarm for axonal degeneration. Loss of Raw leads to decreased levels of dSarm in axons and delayed axonal degeneration that is rescued by overexpression of dSarm, supporting a model that Raw promotes the activation of dSarm in axons. However, inhibiting Fos also decreases dSarm levels in axons but has the opposite outcome of enabling Wallerian degeneration. The combined genetic data suggest that Raw, dSarm, and Fos influence each other's functions through multiple points of regulation to control the structure of synaptic terminals and the resilience of axons to degeneration.

CG11426 gene product negatively regulates glial population size in the Drosophila eye imaginal disc

Glial cells play essential roles in the nervous system. Although glial populations are tightly regulated, the mechanisms regulating the population size remain poorly understood. Since Drosophila glial cells are similar to the human counterparts in their functions and shapes, rendering them an excellent model system to understand the human glia biology. Lipid phosphate phosphatases (LPPs) are important for regulating bioactive lipids. In Drosophila, there are three known LPP-encoding genes: wunen, wunen-2, and lazaro. The wunens are important for germ cell migration and survival and septate junction formation during tracheal development. Lazaro is involved in phototransduction. In the present study, we characterized a novel Drosophila LPP-encoding gene, CG11426. Suppression of CG11426 increased glial cell number in the eye imaginal disc during larval development, while ectopic CG11426 expression decreased it. Both types of mutation also caused defects in axon projection to the optic lobe in larval eye-brain complexes. Moreover, CG11426 promoted apoptosis via inhibiting ERK signaling in the eye imaginal disc. Taken together, these findings demonstrated that CG11426 gene product negatively regulates ERK signaling to promote apoptosis for proper maintenance of the glial population in the developing eye disc.

The membrane protein Raw regulates dendrite pruning via the secretory pathway

Neuronal pruning is essential for proper wiring of the nervous systems in invertebrates and vertebrates. Drosophila ddaC sensory neurons selectively prune their larval dendrites to sculpt the nervous system during early metamorphosis. However, the molecular mechanisms underlying ddaC dendrite pruning remain elusive. Here, we identify an important and cell-autonomous role of the membrane protein Raw in dendrite pruning of ddaC neurons. Raw appears to regulate dendrite pruning via a novel mechanism, which is independent of JNK signaling. Importantly, we show that Raw promotes endocytosis and downregulation of the conserved L1-type cell-adhesion molecule Neuroglian (Nrg) prior to dendrite pruning. Moreover, Raw is required to modulate the secretory pathway by regulating the integrity of secretory organelles and efficient protein secretion. Mechanistically, Raw facilitates Nrg downregulation and dendrite pruning in part through regulation of the secretory pathway. Thus, this study reveals a JNK-independent role of Raw in regulating the secretory pathway and thereby promoting dendrite pruning.

Peripheral axonal ensheathment is regulated by RalA GTPase and the exocyst complex

Axon ensheathment is fundamental for fast impulse conduction and the normal physiological functioning of the nervous system. Defects in axonal insulation lead to debilitating conditions, but, despite its importance, the molecular players responsible are poorly defined. Here, we identify RalA GTPase as a key player in axon ensheathment in Drosophila larval peripheral nerves. We demonstrate through genetic analysis that RalA action through the exocyst complex is required in wrapping glial cells to regulate their growth and development. We suggest that the RalA-exocyst pathway controls the targeting of secretory vesicles for membrane growth or for the secretion of a wrapping glia-derived factor that itself regulates growth. In summary, our findings provide a new molecular understanding of the process by which axons are ensheathed in vivo, a process that is crucial for normal neuronal function.

Degeneration of Injured Axons and Dendrites Requires Restraint of a Protective JNK Signaling Pathway by the Transmembrane Protein Raw

The degeneration of injured axons involves a self-destruction pathway whose components and mechanism are not fully understood. Here, we report a new regulator of axonal resilience. The transmembrane protein Raw is cell autonomously required for the degeneration of injured axons, dendrites, and synapses in Drosophila melanogaster In both male and female raw hypomorphic mutant or knock-down larvae, the degeneration of injured axons, dendrites, and synapses from motoneurons and sensory neurons is strongly inhibited. This protection is insensitive to reduction in the levels of the NAD+ synthesis enzyme Nmnat (nicotinamide mononucleotide adenylyl transferase), but requires the c-Jun N-terminal kinase (JNK) mitogen-activated protein (MAP) kinase and the transcription factors Fos and Jun (AP-1). Although these factors were previously known to function in axonal injury signaling and regeneration, Raw's function can be genetically separated from other axonal injury responses: Raw does not modulate JNK-dependent axonal injury signaling and regenerative responses, but instead restrains a protective pathway that inhibits the degeneration of axons, dendrites, and synapses. Although protection in raw mutants requires JNK, Fos, and Jun, JNK also promotes axonal degeneration. These findings suggest the existence of multiple independent pathways that share modulation by JNK, Fos, and Jun that influence how axons respond to stress and injury.SIGNIFICANCE STATEMENT Axonal degeneration is a major feature of neuropathies and nerve injuries and occurs via a cell autonomous self-destruction pathway whose mechanism is poorly understood. This study reports the identification of a new regulator of axonal degeneration: the transmembrane protein Raw. Raw regulates a cell autonomous nuclear signaling pathway whose yet unknown downstream effectors protect injured axons, dendrites, and synapses from degenerating. These findings imply that the susceptibility of axons to degeneration is strongly regulated in neurons. Future understanding of the cellular pathway regulated by Raw, which engages the c-Jun N-terminal kinase (JNK) mitogen-activated protein (MAP) kinase and Fos and Jun transcription factors, may suggest new strategies to increase the resiliency of axons in debilitating neuropathies.