Abstract
Genes for endoplasmic reticulum (ER)-shaping proteins are among the most commonly mutated in hereditary spastic paraplegia (HSP). Mutation of these genes in model organisms can lead to disruption of the ER network. To investigate how the physiological roles of the ER might be affected by such disruption, we developed tools to interrogate its Ca2+ signaling function. We generated GAL4-driven Ca2+ sensors targeted to the ER lumen, to record ER Ca2+ fluxes in identified Drosophila neurons. Using GAL4 lines specific for Type Ib or Type Is larval motor neurons, we compared the responses of different lumenal indicators to electrical stimulation, in axons and presynaptic terminals. The most effective sensor, ER-GCaMP6-210, had a Ca2+ affinity close to the expected ER lumenal concentration. Repetitive nerve stimulation generally showed a transient increase of lumenal Ca2+ in both the axon and presynaptic terminals. Mutants lacking neuronal reticulon and REEP proteins, homologs of human HSP proteins, showed a larger ER lumenal evoked response compared to wild type; we propose mechanisms by which this phenotype could lead to neuronal dysfunction or degeneration. Our lines are useful additions to a Drosophila Ca2+ imaging toolkit, to explore the physiological roles of ER, and its pathophysiological roles in HSP and in axon degeneration more broadly.
Highlights
Hereditary spastic paraplegia (HSP) is a genetically heterogeneous disorder, with over 80 loci and 60 genes identified
Gene products that shape the tubular endoplasmic reticulum (ER) network are among the most commonly mutated in HSP, and mutation of these genes in model organisms leads to physical disruption of the ER network (O’Sullivan et al, 2012; Fowler and O’Sullivan, 2016; Summerville et al, 2016; Yalçın et al, 2017; Lindhout et al, 2019)
The ER Lumenal Ca2+ Evoked Response Is More Delayed and Sustained Compared to the Cytoplasmic Response
Summary
Hereditary spastic paraplegia (HSP) is a genetically heterogeneous disorder, with over 80 loci and 60 genes identified It shows phenotypic heterogeneity in manifestations such as age of onset and the presence of other symptoms (Blackstone, 2018). Gene products that shape the tubular ER network are among the most commonly mutated in HSP, and mutation of these genes in model organisms leads to physical disruption of the ER network (O’Sullivan et al, 2012; Fowler and O’Sullivan, 2016; Summerville et al, 2016; Yalçın et al, 2017; Lindhout et al, 2019). It is imperative to understand the roles of ER architecture and the consequences of disrupting it, to understand how this might result in a neurodegenerative cascade
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