Abstract

Endoplasmic reticulum (ER) forms a dynamic network of sheets and tubules. It is central to Ca2+ and lipid homeostasis, and forms close contacts with other organelles. In neurons, it appears continuous through dendrites, cell body and axon, and is therefore termed a “neuron within a neuron” – potentially carrying local, regional, or long-range signals, independent of action potentials or physical transport, up to 1m in humans. The ubiquity and continuity of axon ER implies important roles for it, supported by the finding that several axon degeneration genes encode ER modelling proteins. In Drosophila we have developed tools to visualise axon ER, and shown that some ER-modelling proteins can affect the amount, continuity, or tubule dimensions of axon ER. Therefore our tools can help reveal how this underexplored compartment is assembled and regulated. We still understand little of how the axon ER network is assembled and regulated and responds to cell needs. While we see clear mutant phenotypes affecting axon ER, all phenotypes so far retain ER tubules in most parts of axons, implying that we have not yet fully disrupted the processes that localise it and maintain its continuity and local density. We will therefore screen for additional mutant phenotypes that affect axon ER, to understand their cellular and molecular basis, and the transport, synthesis, modelling, or regulatory processes that they reveal. We will first test fly homologs of axon degeneration genes that may model ER. We will next initiate a forward genetic screen for new mutations that cause ER defects, using MARCM to generate axons with labelled ER that are visible without dissection, and homozygous for new mutations on one chromosome arm. We will also screen for mutations in genes with some functional redundancy. We will identify affected genes in several new mutants by whole-genome sequencing, and select a small number for phenotypic study, to build a picture of the affected processes.